# 7. `ch-image`

Build and manage images; completely unprivileged.

## 7.1. Synopsis

    $ ch-image [...] build [-t TAG] [-f DOCKERFILE] [...] CONTEXT
    $ ch-image [...] build-cache [...]
    $ ch-image [...] delete IMAGE_GLOB [IMAGE_GLOB ...]
    $ ch-image [...] gestalt [SELECTOR]
    $ ch-image [...] import PATH IMAGE_REF
    $ ch-image [...] list [-l] [IMAGE_REF]
    $ ch-image [...] pull [...] IMAGE_REF [DEST_REF]
    $ ch-image [...] push [--image DIR] IMAGE_REF [DEST_REF]
    $ ch-image [...] reset
    $ ch-image [...] undelete IMAGE_REF
    $ ch-image { --help | --version }

## 7.2. Description

`ch-image` is a tool for building and manipulating container images, but not running them (for that you want `ch-run`). It is completely unprivileged, with no setuid/setgid/setcap helpers. Many operations can use caching for speed. The action to take is specified by a sub-command.

Options that print brief information and then exit:

> `-h`, `--help`

>   Print help and exit successfully. If specified before the sub-command, print general help and list of sub-commands; if after the sub-command, print help specific to that sub-command.

> `--version`

>   Print version number and exit successfully.

Common options placed before or after the sub-command:

> `-a`, `--arch ARCH`

>   Use `ARCH` for architecture-aware registry operations. (See section “Architecture” below for details.)

> `--always-download`

>   Download all files when pulling, even if they are already in builder storage. Note that `ch-image pull` will always retrieve the most up-to-date image; this option is mostly for debugging.

> `--auth`

>   Authenticate with the remote repository, then (if successful) make all subsequent requests in authenticated mode. For most subcommands, the default is to never authenticate, i.e., make all requests anonymously. The exception is `push`, which implies `--auth`.

> `--break MODULE:LINE`

>   Set a PDB breakpoint at line number `LINE` of module named `MODULE` (typically the filename with `.py` removed, or `__main__` for `ch-image` itself). That is, a PDB debugger shell will open before executing the specified line.

>   This is accomplished by re-parsing the module, injecting `import pdb; pdb.set_trace()` into the parse tree, re-compiling the tree, and replacing the module’s code with the result. This has various gotchas, including (1) module-level code in the target module is executed twice, (2) the option is parsed with bespoke early code so command line argument parsing itself can be debugged, (3) breakpoints on function definition will trigger while the module is being re-executed, not when the function is called (break on the first line of the function body instead), and (4) other weirdness we haven’t yet characterized.

> `--cache`

>   Enable build cache. Default if a sufficiently new Git is available. See section Build cache for details.

> `--cache-large SIZE`

>   Set the cache’s large file threshold to `SIZE` MiB, or `0` for no large files, which is the default. Values greater than zero can speed up many builds but can also cause performance degradation. Experimental. See section Large file threshold for details.

> `--debug`

>   Add a stack trace to fatal error hints. This can also be done by setting the environment variable `CH_IMAGE_DEBUG`.

> `--no-cache`

>   Disable build cache. Default if a sufficiently new Git is not available. This option turns off the cache completely; if you want to re-execute a Dockerfile and store the new results in cache, use `--rebuild` instead.

> `--no-lock`

>   Disable storage directory locking. This lets you run as many concurrent `ch-image` instances as you want against the same storage directory, which risks corruption but may be OK for some workloads.

> `--no-xattrs`

>   Enforce default handling of xattrs, i.e. do not save them in the build cache or restore them on rebuild. This is the default, but the option is provided to override the `$CH_XATTRS` environment variable.

> `--password-many`

>   Re-prompt the user every time a registry password is needed.

> `--profile`

>   Dump profile to files `/tmp/chofile.p` (`cProfile` dump format) and `/tmp/chofile.txt` (text summary). You can convert the former to a PDF call graph with `gprof2dot -f pstats /tmp/chofile.p | dot -Tpdf -o /tmp/chofile.pdf`. This excludes time spend in subprocesses. Profile data should still be written on fatal errors, but not if the program crashes.

> `-q, --quiet`

>   Be quieter; can be repeated. Incompatible with `-v` and suppresses `--debug` regardless of option order. See the FAQ entry on verbosity for details.

> `--rebuild`

>   Execute all instructions, even if they are build cache hits, except for `FROM` which is retrieved from cache on hit.

> `-s`, `--storage DIR`

>   Set the storage directory (see below for important details).

> `--tls-no-verify`

>   Don’t verify TLS certificates of the repository. (Do not use this option unless you understand the risks.)

> `-v`, `--verbose`

>   Print extra chatter; can be repeated. See the FAQ entry on verbosity for details.

> `--xattrs`

>   Save xattrs and ACLs in the build cache, and restore them when rebuilding from the cache.

## 7.3. Architecture

Charliecloud provides the option `--arch ARCH` to specify the architecture for architecture-aware registry operations. The argument `ARCH` can be: (1) `yolo`, to bypass architecture-aware code and use the registry’s default architecture; (2) `host`, to use the host’s architecture, obtained with the equivalent of `uname -m` (default if `--arch` not specified); or (3) an architecture name. If the specified architecture is not available, the error message will list which ones are.

Notes:

1. `ch-image` is limited to one image per image reference in builder storage at a time, regardless of architecture. For example, if you say `ch-image pull --arch=foo baz` and then `ch-image pull --arch=bar baz`, builder storage will contain one image called “baz”, with architecture “bar”.

2. Images’ default architecture is usually `amd64`, so this is usually what you get with `--arch=yolo`. Similarly, if a registry image is architecture-unaware, it will still be pulled with `--arch=amd64` and `--arch=host` on x86-64 hosts (other host architectures must specify `--arch=yolo` to pull architecture-unaware images).

3. `uname -m` and image registries often use different names for the same architecture. For example, what `uname -m` reports as “x86_64” is known to registries as “amd64”. `--arch=host` should translate if needed, but it’s useful to know this is happening. Directly specified architecture names are passed to the registry without translation.

4. Registries treat architecture as a pair of items, architecture and sometimes variant (e.g., “arm” and “v7”). Charliecloud treats architecture as a simple string and converts to/from the registry view transparently.

## 7.4. Authentication

Charliecloud does not have configuration files; thus, it has no separate `login` subcommand to store secrets. Instead, Charliecloud will prompt for a username and password when authentication is needed. Note that some repositories refer to the secret as something other than a “password”; e.g., GitLab calls it a “personal access token (PAT)”, Quay calls it an “application token”, and nVidia NGC calls it an “API token”.

For non-interactive authentication, you can use environment variables `CH_IMAGE_USERNAME` and `CH_IMAGE_PASSWORD`. Only do this if you fully understand the implications for your specific use case, because it is difficult to securely store secrets in environment variables.

By default for most subcommands, all registry access is anonymous. To instead use authenticated access for everything, specify `--auth` or set the environment variable `$CH_IMAGE_AUTH=yes`. The exception is `push`, which always runs in authenticated mode. Even for pulling public images, it can be useful to authenticate for registries that have per-user rate limits, such as Docker Hub. (Older versions of Charliecloud started with anonymous access, then tried to upgrade to authenticated if it seemed necessary. However, this turned out to be brittle; see issue #1318.)

The username and password are remembered for the life of the process and silently re-offered to the registry if needed. One case when this happens is on push to a private registry: many registries will first offer a read-only token when `ch-image` checks if something exists, then re-authenticate when upgrading the token to read-write for upload. If your site uses one-time passwords such as provided by a security device, you can specify `--password-many` to provide a new secret each time.

These values are not saved persistently, e.g. in a file. Note that we do use normal Python variables for this information, without pinning them into physical RAM with mlock(2) or any other special treatment, so we cannot guarantee they will never reach non-volatile storage.

Technical details

Most registries use something called Bearer authentication, where the client (e.g., Charliecloud) includes a token in the headers of every HTTP request.

The authorization dance is different from the typical UNIX approach, where there is a separate login sequence before any content requests are made. The client starts by simply making the HTTP request it wants (e.g., to `GET` an image manifest), and if the registry doesn’t like the client’s token (or if there is no token because the client doesn’t have one yet), it replies with HTTP 401 Unauthorized, but crucially it also provides instructions in the response header on how to get a token. The client then follows those instructions, obtains a token, re-tries the request, and (hopefully) all is well. This approach also allows a client to upgrade a token if needed, e.g. when transitioning from asking if a layer exists to uploading its content.

The distinction between Charliecloud’s anonymous mode and authenticated modes is that it will only ask for anonymous tokens in anonymous mode and authenticated tokens in authenticated mode. That is, anonymous mode does involve an authentication procedure to obtain a token, but this “authentication” is done anonymously. (Yes, it’s confusing.)

Registries also often reply HTTP 401 when an image does not exist, rather than the seemingly more correct HTTP 404 Not Found. This is to avoid information leakage about the existence of images the client is not allowed to pull, and it’s why Charliecloud never says an image simply does not exist.

## 7.5. Storage directory

`ch-image` maintains state using normal files and directories located in its storage directory; contents include various caches and temporary images used for building.

In descending order of priority, this directory is located at:

> `-s`, `--storage DIR`

>   Command line option.

> `$CH_IMAGE_STORAGE`

>   Environment variable. The path must be absolute, because the variable is likely set in a very different context than when it’s used, which seems error-prone on what a relative path is relative to.

> `/var/tmp/$USER.ch`

>   Default. (Previously, the default was `/var/tmp/$USER/ch-image`. If a valid storage directory is found at the old default path, `ch-image` tries to move it to the new default path.)

Unlike many container implementations, there is no notion of storage drivers, graph drivers, etc., to select and/or configure.

The storage directory can reside on any single filesystem (i.e., it cannot be split across multiple filesystems). However, it contains lots of small files and metadata traffic can be intense. For example, the Charliecloud test suite uses approximately 400,000 files and directories in the storage directory as of this writing. Place it on a filesystem appropriate for this; tmpfs’es such as `/var/tmp` are a good choice if you have enough RAM (`/tmp` is not recommended because `ch-run` bind-mounts it into containers by default).

While you can currently poke around in the storage directory and find unpacked images runnable with `ch-run`, this is not a supported use case. The supported workflow uses `ch-convert` to obtain a packed image; see the tutorial for details.

The storage directory format changes on no particular schedule. `ch-image` is normally able to upgrade directories produced by a given Charliecloud version up to one year after that version’s release. Upgrades outside this window and downgrades are not supported. In these cases, `ch-image` will refuse to run until you delete and re-initialize the storage directory with `ch-image reset`.

Warning

Network filesystems, especially Lustre, are typically bad choices for the storage directory. This is a site-specific question and your local support will likely have strong opinions.

## 7.6. Build cache

### 7.6.1. Overview

Subcommands that create images, such as `build` and `pull`, can use a build cache to speed repeated operations. That is, an image is created by starting from the empty image and executing a sequence of instructions, largely Dockerfile instructions but also some others like “pull” and “import”. Some instructions are expensive to execute (e.g., `RUN wget http://slow.example.com/bigfile` or transferring data billed by the byte), so it’s often cheaper to retrieve their results from cache instead.

The build cache uses a relatively new Git under the hood; see the installation instructions for version requirements. Charliecloud implements workarounds for Git’s various storage limitations, so things like file metadata and Git repositories within the image should work. Important exception: No files named `.git*` or other Git metadata are permitted in the image’s root directory.

Extended attributes (xattrs) are ignored by the build cache by default. Cache support for xattrs belonging to unprivileged xattr namespaces (e.g. `user`) can be enabled by specifying the `--xattrs` option or by setting the `CH_XATTRS` environment variable. If `CH_XATTRS` is set, you override it with `--no-xattrs`. Note that extended attributes in privileged xattr namespaces (e.g. :code:`trusted`) cannot be read by :code:`ch-image` and will always be lost without warning.

The cache has three modes: enabled, disabled, and a hybrid mode called rebuild where the cache is fully enabled for `FROM` instructions, but all other operations re-execute and re-cache their results. The purpose of rebuild is to do a clean rebuild of a Dockerfile atop a known-good base image.

Enabled mode is selected with `--cache` or setting `$CH_IMAGE_CACHE` to `enabled`, disabled mode with `--no-cache` or `disabled`, and rebuild mode with `--rebuild` or `rebuild`. The default mode is enabled if an appropriate Git is installed, otherwise disabled.

### 7.6.2. Compared to other implementations

Note

This section is a lightly edited excerpt from our paper “Charliecloud’s layer-free, Git-based container build cache”.

Existing tools such as Docker and Podman implement their build cache with a layered (union) filesystem such as OverlayFS or FUSE-OverlayFS and tar archives to represent the content of each layer; this approach is standardized by OCI. The layered cache works, but it has drawbacks in three critical areas:

1. Diff format. The tar format is poorly standardized and not designed for diffs. Notably, tar cannot represent file deletion. The workaround used for OCI layers is specially named whiteout files, which means the tar archives cannot be unpacked by standard UNIX tools and require special container-specific processing.

2. Cache overhead. Each time a Dockerfile instruction is started, a new overlay filesystem is mounted atop the existing layer stack. File metadata operations in the instruction then start at the top layer and descend the stack until the layer containing the desired file is reached. The cost of these operations is therefore proportional to the number of layers, i.e., the number of instructions between the empty root image and the instruction being executed. This results in a best practice of large, complex instructions to minimize their number, which can conflict with simpler, more numerous instructions the user might prefer.

3. De-duplication. Identical files on layers with an ancestry relationship (i.e., instruction A precedes B in a build) are stored only once. However, identical files on layers without this relationship are stored multiple times. For example, if instructions B and B’ both follow A — perhaps because B was modified and the image rebuilt — then any files created by both B and B’ will be stored twice.

   Also, similar files are never de-duplicated, regardless of ancestry. For example, if instruction A creates a file and subsequently instruction B modifies a single bit in that file, both versions are stored in their entirety.

Our Git-based cache addresses the three drawbacks: (1) Git is purpose-built to store changing directory trees, (2) cache overhead is imposed only at instruction commit time, and (3) Git de-duplicates both identical and similar files. Also, it is based on an extremely widely used tool that enjoys development support from well-resourced actors, in particular on scaling (e.g., Microsoft’s large-repository accelerator Scalar was recently merged into Git).

In addition to these structural advantages, performance experiments reported in our paper above show that the Git-based approach is as good as (and sometimes better than) overlay-based caches. On build time, the two approaches are broadly similar, with one or the other being faster depending on context. Both had performance problems on NFS. Notably, however, the Git-based cache was much faster for a 129-instruction Dockerfile. On disk usage, the winner depended on the condition. For example, we saw the layered cache storing large sibling layers redundantly; on the other hand, the Git-based cache has some obvious redundancies as well, and one must compact it for full de-duplication benefit. However, Git’s de-duplication was quite effective in some conditions and we suspect will prove even better in more realistic scenarios.

That is, we believe our results show that the Git-based build cache is highly competitive with the layered approach, with no obvious inferiority so far and hints that it may be superior on important dimensions. We have ongoing work to explore these questions in more detail.

### 7.6.3. De-duplication and garbage collection

Charliecloud’s build cache takes advantage of Git’s file de-duplication features. This operates across the entire build cache, i.e., files are de-duplicated no matter where in the cache they are found or the relationship between their container images. Files are de-duplicated at different times depending on whether they are identical or merely similar.

Identical files are de-duplicated at `git add` time; in `ch-image build` terms, that’s upon committing a successful instruction. That is, it’s impossible to store two files with the same content in the build cache. If you try — say with `RUN yum install -y foo` in one Dockerfile and `RUN yum install -y foo bar` in another, which are different instructions but both install RPM `foo`’s files — the content is stored once and each copy gets its own metadata and a pointer to the content, much like filesystem hard links.

Similar files, however, are only de-duplicated during Git’s garbage collection process. When files are initially added to a Git repository (with `git add`), they are stored inside the repository as (possibly compressed) individual files, called objects in Git jargon. Upon garbage collection, which happens both automatically when certain parameters are met and explicitly with `git gc`, these files are archived and (re-)compressed together into a single file called a packfile. Also, existing packfiles may be re-written into the new one.

During this process, similar files are identified, and each set of similar files is stored as one base file plus diffs to recover the others. (Similarity detection seems to be based primarily on file size.) This delta process is agnostic to alignment, which is an advantage over alignment-sensitive block-level de-duplicating filesystems. Exception: “Large” files are not compressed or de-duplicated. We use the Git default threshold of 512 MiB (as of this writing).

Charliecloud runs Git garbage collection at two different times. First, a lighter-weight garbage pass runs automatically when the number of loose files (objects) grows beyond a limit. This limit is in flux as we learn more about build cache performance, but it’s quite a bit higher than the Git default. This garbage runs in the background and can continue after the build completes; you may see Git processes using a lot of CPU.

An important limitation of the automatic garbage is that large packfiles (again, this is in flux, but it’s several GiB) will not be re-packed, limiting the scope of similar file detection. To address this, a heavier garbage collection can be run manually with `ch-image build-cache --gc`. This will re-pack (and re-write) the entire build cache, de-duplicating all similar files. In both cases, garbage uses all available cores.

`git build-cache` prints the specific garbage collection parameters in use, and `-v` can be added for more detail.

### 7.6.4. Large file threshold

Because Git uses content-addressed storage, upon commit, it must read in full all files modified by an instruction. This I/O cost can be a significant fraction of build time for some images. To mitigate this, regular files larger than the experimental large file threshold are stored outside the Git repository, somewhat like Git Large File Storage.

`ch-image` copies large files in and out of images at each instruction commit. It tries to do this with a fast metadata-only copy-on-write operation called “reflink”, but that is only supported with the right Python version, Linux kernel version, and filesystem. If unsupported, Charliecloud falls back to an expensive standard copy, which is likely slower than letting Git deal with the files. See File copy performance for details.

Every version of a large file is stored verbatim and uncompressed (e.g., a large file with a one-byte change will be stored in full twice), so Git’s de-duplication does not apply. However, on filesystems with reflink support, files can share extents (e.g., each of the two files will have its own extent containing the changed byte, but the rest of the extents will remain shared). This provides de-duplication between large files images that share ancestry. Also, unused large files are deleted by `ch-image build-cache --gc`.

A final caveat: Large files in any image with the same path, mode, size, and mtime (to nanosecond precision if possible) are considered identical, even if their content is not actually identical (e.g., `touch(1)` shenanigans can corrupt an image).

Option `--cache-large` sets the threshold in MiB; if not set, environment variable `CH_IMAGE_CACHE_LARGE` is used; if that is not set either, the default value `0` indicates that no files are considered large.

(Note that Git has an unrelated setting called `core.bigFileThreshold`.)

### 7.6.5. Example

Suppose we have this Dockerfile:

    $ cat a.df
    FROM alpine:3.17
    RUN echo foo
    RUN echo bar

On our first build, we get:

    $ ch-image build -t foo -f a.df .
      1. FROM alpine:3.17
    [ ... pull chatter omitted ... ]
      2. RUN echo foo
    copying image ...
    foo
      3. RUN echo bar
    bar
    grown in 3 instructions: foo

Note the dot after each instruction’s line number. This means that the instruction was executed. You can also see this by the output of the two `echo` commands.

But on our second build, we get:

    $ ch-image build -t foo -f a.df .
      1* FROM alpine:3.17
      2* RUN echo foo
      3* RUN echo bar
    copying image ...
    grown in 3 instructions: foo

Here, instead of being executed, each instruction’s results were retrieved from cache. (Charliecloud uses lazy retrieval; nothing is actually retrieved until the end, as seen by the “copying image” message.) Cache hit for each instruction is indicated by an asterisk (`*`) after the line number. Even for such a small and short Dockerfile, this build is noticeably faster than the first.

We can also try a second, slightly different Dockerfile. Note that the first three instructions are the same, but the third is different:

    $ cat c.df
    FROM alpine:3.17
    RUN echo foo
    RUN echo qux
    $ ch-image build -t c -f c.df .
      1* FROM alpine:3.17
      2* RUN echo foo
      3. RUN echo qux
    copying image ...
    qux
    grown in 3 instructions: c

Here, the first two instructions are hits from the first Dockerfile, but the third is a miss, so Charliecloud retrieves that state and continues building.

We can also inspect the cache:

    $ ch-image build-cache --tree
    *  (c) RUN echo qux
    | *  (a) RUN echo bar
    |/
    *  RUN echo foo
    *  (alpine+3.9) PULL alpine:3.17
    *  (root) ROOT

    named images:     4
    state IDs:        5
    commits:          5
    files:          317
    disk used:        3 MiB

Here there are four named images: `a` and `c` that we built, the base image `alpine:3.17` (written as `alpine+3.9` because colon is not allowed in Git branch names), and the empty base of everything `root`. Also note how `a` and `c` diverge after the last common instruction `RUN echo foo`.

## 7.7. `build`

Build an image from a Dockerfile and put it in the storage directory.

### 7.7.1. Synopsis

    $ ch-image [...] build [-t TAG] [-f DOCKERFILE] [...] CONTEXT

### 7.7.2. Description

See below for differences with other Dockerfile interpreters. Charliecloud supports an extended instruction (`RSYNC`), a few other instructions behave slightly differently, and a few are ignored.

Note that `FROM` implicitly pulls the base image if needed, so you may want to read about the `pull` subcommand below as well.

Required argument:

> `CONTEXT`

>   Path to context directory. This is the root of `COPY` instructions in the Dockerfile. If a single hyphen (`-`) is specified: (a) read the Dockerfile from standard input, (b) specifying `--file` is an error, and (c) there is no context, so `COPY` will fail. (See `--file` for how to provide the Dockerfile on standard input while also having a context.)

Options:

> `-b`, `--bind SRC[:DST]`

>   For `RUN` instructions only, bind-mount `SRC` at guest `DST`. The default destination if not specified is to use the same path as the host; i.e., the default is equivalent to `--bind=SRC:SRC`. If `DST` does not exist, try to create it as an empty directory, though images do have ten directories `/mnt/[0-9]` already available as mount points. Can be repeated.

>   Note: See documentation for `ch-run --bind` for important caveats and gotchas.

>   Note: Other instructions that modify the image filesystem, e.g. `COPY`, can only access host files from the context directory, regardless of this option.

> `--build-arg KEY[=VALUE]`

>   Set build-time variable `KEY` defined by `ARG` instruction to `VALUE`. If `VALUE` not specified, use the value of environment variable `KEY`.

> `-f`, `--file DOCKERFILE`

>   Use `DOCKERFILE` instead of `CONTEXT/Dockerfile`. If a single hyphen (`-`) is specified, read the Dockerfile from standard input; like `docker build`, the context directory is still available in this case.

> `--force[=MODE]`

>   Use unprivileged build with root emulation mode `MODE`, which can be `fakeroot`, `seccomp` (the default), or `none`. See section “Privilege model” below for details on what this does and when you might need it.

> `--force-cmd=CMD,ARG1[,ARG2...]`

>   If command `CMD` is found in a `RUN` instruction, add the comma-separated `ARGs` to it. For example, `--force-cmd=foo,-a,--bar=baz` would transform `RUN foo -c` into `RUN foo -a --bar=baz -c`. This is intended to suppress validation that defeats `--force=seccomp` and implies that option. Can be repeated. If specified, replaces (does not extend) the default suppression options. Literal commas can be escaped with backslash; importantly however, backslash will need to be protected from the shell also. Section “Privilege model” below explains why you might need this.

> `-n`, `--dry-run`

>   Don’t actually execute any Dockerfile instructions.

> `--parse-only`

>   Stop after parsing the Dockerfile.

> `-t`, `--tag TAG`

>   Name of image to create. If not specified, infer the name:

>   1. If Dockerfile named `Dockerfile` with an extension: use the extension with invalid characters stripped, e.g. `Dockerfile.@FOO.bar` → `foo.bar`.

>   2. If Dockerfile has extension `df` or `dockerfile`: use the basename with the same transformation, e.g. `baz.@QUX.dockerfile` → `baz.qux`.

>   3. If the directory containing the Dockerfile is not `/`: use its name with the same transformation,

>   4. Otherwise (Dockerfile is in `/`): use `root`.

>   If no colon present in the name, append `:latest`.

Uses `ch-run -w -u0 -g0 --no-passwd --unsafe` to execute `RUN` instructions.

### 7.7.3. Privilege model

#### 7.7.3.1. Overview

`ch-image` is a fully unprivileged image builder. It does not use any setuid or setcap helper programs, and it does not use configuration files `/etc/subuid` or `/etc/subgid`. This contrasts with the “rootless” or “fakeroot” modes of some competing builders, which do require privileged supporting code or utilities.

Without root emulation, this approach does confuse programs that expect to have real root privileges, most notably distribution package installers. This subsection describes why that happens and what you can do about it.

`ch-image` executes all instructions as the normal user who invokes it. For `RUN`, this is accomplished with `ch-run` arguments including `-w --uid=0 --gid=0`. That is, your host EUID and EGID are both mapped to zero inside the container, and only one UID (zero) and GID (zero) are available inside the container. Under this arrangement, processes running in the container for each `RUN` appear to be running as root, but many privileged system calls will fail without the root emulation methods described below. This affects any fully unprivileged container build, not just Charliecloud.

The most common time to see this is installing packages. For example, here is RPM failing to `chown(2)` a file, which makes the package update fail:

      Updating   : 1:dbus-1.10.24-13.el7_6.x86_64                            2/4
    Error unpacking rpm package 1:dbus-1.10.24-13.el7_6.x86_64
    error: unpacking of archive failed on file /usr/libexec/dbus-1/dbus-daemon-launch-helper;5cffd726: cpio: chown
      Cleanup    : 1:dbus-libs-1.10.24-12.el7.x86_64                         3/4
    error: dbus-1:1.10.24-13.el7_6.x86_64: install failed

This one is (ironically) `apt-get` failing to drop privileges:

    E: setgroups 65534 failed - setgroups (1: Operation not permitted)
    E: setegid 65534 failed - setegid (22: Invalid argument)
    E: seteuid 100 failed - seteuid (22: Invalid argument)
    E: setgroups 0 failed - setgroups (1: Operation not permitted)

Charliecloud provides two different mechanisms to avoid these problems. Both involve lying to the containerized process about privileged system calls, but at very different levels of complexity.

#### 7.7.3.2. Root emulation mode `fakeroot`

This mode uses `fakeroot(1)` to maintain an elaborate web of deceit that is internally consistent. This program intercepts both privileged system calls (e.g., `setuid(2)`) as well as other system calls whose return values depend on those calls (e.g., `getuid(2)`), faking success for privileged system calls (perhaps making no system call at all) and altering return values to be consistent with earlier fake success. Charliecloud automatically installs the `fakeroot(1)` program inside the container and then wraps `RUN` instructions having known privilege needs with it. Thus, this mode is only available for certain distributions.

The advantage of this mode is its consistency; e.g., careful programs that check the new UID after attempting to change it will not notice anything amiss. Its disadvantage is complexity: detailed knowledge and procedures for multiple Linux distributions.

This mode has three basic steps:

> 1. After `FROM`, analyze the image to see what distribution it contains, which determines the specific workarounds.

> 2. Before the user command in the first `RUN` instruction where the injection seems needed, install `fakeroot(1)` in the image, if one is not already installed, as well as any other necessary initialization commands. For example, we turn off the `apt` sandbox (for Debian Buster) and configure EPEL but leave it disabled (for CentOS/RHEL).

> 3. Prepend `fakeroot` to `RUN` instructions that seem to need it, e.g. ones that contain `apt`, `apt-get`, `dpkg` for Debian derivatives and `dnf`, `rpm`, or `yum` for RPM-based distributions.

`RUN` instructions that do not seem to need modification are unaffected by this mode.

The details are specific to each distribution. `ch-image` analyzes image content (e.g., grepping `/etc/debian_version`) to select a configuration; see `lib/force.py` for details. `ch-image` prints exactly what it is doing.

Warning

Because of `fakeroot` mode’s complexity, we plan to remove it if `seccomp` mode performs well enough. If you have a situation where `fakeroot` mode works and `seccomp` does not, please let us know.

#### 7.7.3.3. Root emulation mode `seccomp` (default)

This mode uses the kernel’s `seccomp(2)` system call filtering to intercept certain privileged system calls, do absolutely nothing, and return success to the program.

Some system calls are quashed regardless of their arguments: `capset(2)`; `chown(2)` and friends; `kexec_load(2)` (used to validate the filter itself); ; and `setuid(2)`, `setgid(2)`, and `setgroups(2)` along with the other system calls that change user or group. `mknod(2)` and `mknodat(2)` are quashed if they try to create a device file (e.g., creating FIFOs works normally).

The advantages of this approach is that it’s much simpler, it’s faster, it’s completely agnostic to libc, and it’s mostly agnostic to distribution. The disadvantage is that it’s a very lazy liar; even the most cursory consistency checks will fail, e.g., `getuid(2)` after `setuid(2)`.

While this mode does not provide consistency, it does offer a hook to help prevent programs asking for consistency. For example, `apt-get -o APT::Sandbox::User=root` will prevent `apt-get` from attempting to drop privileges, which it verifies, exiting with failure if the correct IDs are not found (which they won’t be under this approach). This can be expressed with `--force-cmd=apt-get,-o,APT::Sandbox::User=root`, though this particular case is built-in and does not need to be specified. The full default configuration, which is applied regardless of the image distribution, can be examined in the source file `force.py`. If any `--force-cmd` are specified, this replaces (rather than extends) the default configuration.

Note that because the substitutions are a simple regex with no knowledge of shell syntax, they can cause unwanted modifications. For example, `RUN apt-get install -y apt-get` will be run as `/bin/sh -c "apt-get -o APT::Sandbox::User=root install -y apt-get -o APT::Sandbox::User=root"`. One workaround is to add escape syntax transparent to the shell; e.g., `RUN apt-get install -y apt-get`.

This mode executes all `RUN` instructions with the `seccomp(2)` filter and has no knowledge of which instructions actually used the intercepted system calls. Therefore, the printed “instructions modified” number is only a count of instructions with a hook applied as described above.

#### 7.7.3.4. `RUN` logging

In terminal output, image metadata, and the build cache, the `RUN` instruction is always logged as `RUN.S`, `RUN.F`, or `RUN.N`. The letter appended to the instruction reflects the root emulation mode used during the build in which the instruction was executed. `RUN.S` indicates `seccomp`, `RUN.F` indicates `fakeroot`, and `RUN.N` indicates that neither form of root emulation was used (`--force=none`).

### 7.7.4. Compatibility and behavior differences

`ch-image` is an independent implementation and shares no code with other Dockerfile interpreters. It uses a formal Dockerfile parsing grammar developed from the Dockerfile reference documentation and miscellaneous other sources, which you can examine in the source code.

We believe this independence is valuable for several reasons. First, it helps the community examine Dockerfile syntax and semantics critically, think rigorously about what is really needed, and build a more robust standard. Second, it yields disjoint sets of bugs (note that Podman, Buildah, and Docker all share the same Dockerfile parser). Third, because it is a much smaller code base, it illustrates how Dockerfiles work more clearly. Finally, it allows straightforward extensions if needed to support scientific computing.

`ch-image` tries hard to be compatible with Docker and other interpreters, though as an independent implementation, it is not bug-compatible.

The following subsections describe differences from the Dockerfile reference that we expect to be approximately permanent. For not-yet-implemented features and bugs in this area, see related issues on GitHub.

None of these are set in stone. We are very interested in feedback on our assessments and open questions. This helps us prioritize new features and revise our thinking about what is needed for HPC containers.

#### 7.7.4.1. Context directory

The context directory is bind-mounted into the build, rather than copied like Docker. Thus, the size of the context is immaterial, and the build reads directly from storage like any other local process would (i.e., it is reasonable use `/` for the context). However, you still can’t access anything outside the context directory.

#### 7.7.4.2. Variable substitution

Variable substitution happens for all instructions, not just the ones listed in the Dockerfile reference.

`ARG` and `ENV` cause cache misses upon definition, in contrast with Docker where these variables miss upon use, except for certain cache-excluded variables that never cause misses, listed below.

Note that `ARG` and `ENV` have different syntax despite very similar semantics.

`ch-image` passes the following proxy environment variables in to the build. Changes to these variables do not cause a cache miss. They do not require an `ARG` instruction, as documented in the Dockerfile reference. Unlike Docker, they are available if the same-named environment variable is defined; `--build-arg` is not required.

    HTTP_PROXY
    http_proxy
    HTTPS_PROXY
    https_proxy
    FTP_PROXY
    ftp_proxy
    NO_PROXY
    no_proxy

In addition to those listed in the Dockerfile reference, these environment variables are passed through in the same way:

    SSH_AUTH_SOCK
    USER

Finally, these variables are also pre-defined but are unrelated to the host environment:

    PATH=/ch/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
    TAR_OPTIONS=--no-same-owner

#### 7.7.4.3. `ARG`

Variables set with `ARG` are available anywhere in the Dockerfile, unlike Docker, where they only work in `FROM` instructions, and possibly in other `ARG` before the first `FROM`.

#### 7.7.4.4. `FROM`

The `FROM` instruction accepts option `--arg=NAME=VALUE`, which serves the same purpose as the `ARG` instruction. It can be repeated.

#### 7.7.4.5. `LABEL`

The `LABEL` instruction accepts `key=value` pairs to add metadata for an image. Unlike Docker, multiline values are not supported; see issue #1512. Can be repeated.

#### 7.7.4.6. `COPY`

Note

The behavior described here matches Docker’s now-deprecated legacy builder. Docker’s new builder, BuildKit, has different behavior in some cases, which we have not characterized.

Especially for people used to UNIX `cp(1)`, the semantics of the Dockerfile `COPY` instruction can be confusing.

Most notably, when a source of the copy is a directory, the contents of that directory, not the directory itself, are copied. This is documented, but it’s a real gotcha because that’s not what `cp(1)` does, and it means that many things you can do in one `cp(1)` command require multiple `COPY` instructions.

Also, the reference documentation is incomplete. In our experience, Docker also behaves as follows; `ch-image` does the same in an attempt to be bug-compatible.

1. You can use absolute paths in the source; the root is the context directory.

2. Destination directories are created if they don’t exist in the following situations:

   1. If the destination path ends in slash. (Documented.)

   2. If the number of sources is greater than 1, either by wildcard or explicitly, regardless of whether the destination ends in slash. (Not documented.)

   3. If there is a single source and it is a directory. (Not documented.)

3. Symbolic links behave differently depending on how deep in the copied tree they are. (Not documented.)

   1. Symlinks at the top level — i.e., named as the destination or the source, either explicitly or by wildcards — are dereferenced. They are followed, and whatever they point to is used as the destination or source, respectively.

   2. Symlinks at deeper levels are not dereferenced, i.e., the symlink itself is copied.

4. If a directory appears at the same path in source and destination, and is at the 2nd level or deeper, the source directory’s metadata (e.g., permissions) are copied to the destination directory. (Not documented.)

5. If an object (a) appears in both the source and destination, (b) is at the 2nd level or deeper, and (c) is different file types in source and destination, the source object will overwrite the destination object. (Not documented.)

We expect the following differences to be permanent:

- Wildcards use Python glob semantics, not the Go semantics.

- `COPY --chown` is ignored, because it doesn’t make sense in an unprivileged build.

#### 7.7.4.7. Features we do not plan to support

- Parser directives are not supported. We have not identified a need for any of them.

- `EXPOSE`: Charliecloud does not use the network namespace, so containerized processes can simply listen on a host port like other unprivileged processes.

- `HEALTHCHECK`: This instruction’s main use case is monitoring server processes rather than applications. Also, it requires a container supervisor daemon, which we have no plans to add.

- `MAINTAINER` is deprecated.

- `STOPSIGNAL` requires a container supervisor daemon process, which we have no plans to add.

- `USER` does not make sense for unprivileged builds.

- `VOLUME`: Charliecloud has good support for bind mounts; we anticipate that it will continue to focus on that and will not introduce the volume management features that Docker has.

### 7.7.5. `RSYNC` (Dockerfile extension)

Warning

This instruction is experimental and may change or be removed.

#### 7.7.5.1. Overview

Copying files is often simple but has numerous difficult corner cases, e.g. when dealing with symbolic or hard links. The standard instruction `COPY` deals with many of these corner cases differently from other UNIX utilities, lacks complete documentation, and behaves inconsistently between different Dockerfile interpreters (e.g., Docker’s legacy builder vs. BuildKit), as detailed above. On the other hand, `rsync(1)` is an extremely capable, widely used file copy tool, with detailed options to specify behavior and 25 years of history dealing with weirdness.

`RSYNC` (also spelled `NSYNC`) is a Charliecloud extension that gives copying behavior identical to `rsync(1)`. In fact, Charliecloud’s current implementation literally calls the host’s `rsync(1)` to do the copy, though this may change in the future. There is no list form of `RSYNC`.

The two key usage challenges are trailing slashes on paths and symlink handling. In particular, the default symlink handling seemed reasonable to us, but you may want something different. See the arguments and examples below. Importantly, `COPY` is not any less fraught, and you have no choice about what to do with symlinks.

#### 7.7.5.2. Arguments

`RSYNC` takes the same arguments as `rsync(1)`, so refer to its man page for a detailed explanation of all the options (with possible emphasis on its symlink options). Sources are relative to the context directory even if they look absolute with a leading slash. Any globbed sources are processed by `ch-image(1)` using Python rules, i.e., `rsync(1)` sees the expanded sources with no wildcards. Relative destinations are relative to the image’s current working directory, while absolute destinations refer to the image’s root.

For arguments that read input from a file (e.g. `--exclude-from` or `--files-from`), relative paths are relative to the context directory, absolute paths refer to the image root, and `-` (standard input) is an error.

For example,

    WORKDIR /foo
    RSYNC --foo src1 src2 dst

is translated to (the equivalent of):

    $ mkdir -p /foo
    $ rsync -@=-1 -AHSXpr --info=progress2 -l --safe-links \
            --foo /context/src1 /context/src2 /storage/imgroot/foo/dst2

Note the extensive default arguments to `rsync(1)`. `RSYNC` takes a single instruction option beginning with `+` (plus) that is shorthand for a group of `rsync(1)` options. This single option is one of:

> `+m`

>   Preserves metadata and directory structure. Symlinks are skipped with a warning. Equivalent to all of:

>   - `-@=-1`: use nanosecond precision when comparing timestamps.

>   - `-A`: preserve ACLs.

>   - `-H`: preserve hard link groups.

>   - `-S`: preserve file sparseness when possible.

>   - `-X`: preserve xattrs in `user.*` namespace.

>   - `-p`: preserve permissions.

>   - `-r`: recurse into directories.

>   - `--info=progress2` (only if stderr is a terminal): show progress meter (note subtleties in interpretation).

> `+l` (default)

>   Like `+m`, but silently skips “unsafe” symlinks whose target is outside the top-of-transfer directory. Preserves:

>   - Metadata.

>   - Directory structure.

>   - Symlinks, if a link’s target is within the “top-of-transfer directory”. This is not the context directory and often not the source either. Also, this creates broken symlinks if the target is not within the source but is within the top-of-transfer. See examples below.

>   Equivalent to the `rsync(1)` options listed for `+m` plus `--links` (copy symlinks as symlinks unless otherwise specified) and `--safe-links` (silently skip unsafe symlinks).

> `+u`

>   Like `+l`, but replaces with their target “unsafe” symlinks whose target is outside the top-of-transfer directory, and thus can copy data outside the context directory into the image. Preserves:

>   - Metadata.

>   - Directory structure.

>   - Symlinks, if a link’s target is within the “top-of-transfer directory”. This is not the context directory and often not the source either. Also, this creates broken symlinks if the target is not within the source but is within the top-of-transfer. See examples below.

>   Equivalent to the `rsync(1)` options listed for `+m` plus `--links` (copy symlinks as symlinks unless otherwise specified) and `--copy-unsafe-links` (copy the target of unsafe symlinks).

> `+z`

>   No default arguments. Directories will not be descended, no metadata will be preserved, and both hard and symbolic links will be ignored, except as otherwise specified by `rsync(1)` options starting with a hyphen. (Note that `-a`/`--archive` is discouraged because it omits some metadata and handles symlinks inappropriately for containers.)

Note

`rsync(1)` supports a configuration file `~/.popt` that alters its command line processing. Currently, this configuration is respected for `RSYNC` arguments, but that may change without notice.

#### 7.7.5.3. Disallowed `rsync(1)` features

A small number of `rsync(1)` features are actively disallowed:

> 1. `rsync:` and `ssh:` transports are an error. Charliecloud needs access to the entire input to compute cache hit or miss, and these transports make that impossible. It is possible these will become available in the future (please let us know if that is your use case!). For now, the workaround is to install `rsync(1)` in the image and use it in a `RUN` instruction, though only the instruction text will be considered for the cache.

> 2. Option arguments must be delimited with `=` (equals). For example, to set the block size to 4 MiB, you must say `--block-size=4M` or `-B=4M`. `-B4M` will be interpreted as the three arguments `-B`, `-4`, and `-M`; `--block-size 4M` will be interpreted as `--block-size` with no argument and a copy source named `4M`. This is so Charliecloud can process `rsync(1)` options without knowing which ones take an argument.

> 3. Invalid `rsync(1)` options:

>    `--daemon`

>      Running `rsync(1)` in daemon mode does not make sense for container build.

>    `-n`, `--dry-run`

>      This makes the copy a no-op, and Charliecloud may want to use it internally in the future.

>    `--remove-source-files`

>      This would let the instruction alter the context directory.

Note that there are likely other flags that don’t make sense and/or cause undesirable behavior. We have not characterized this problem.

#### 7.7.5.4. Build cache

The instruction is a cache hit if the metadata of all source files is unchanged (specifically: filename, file type and permissions, xattrs, size, and last modified time). Unlike Docker, Charliecloud does not use file contents. This has two implications. First, it is possible to fool the cache by manually restoring the last-modified time. Second, `RSYNC` is I/O-intensive even when it hits, because it must `stat(2)` every source file before checking the cache. However, this is still less I/O than reading the file content too.

Notably, Charliecloud’s cache ignores `rsync(1)`’s own internal notion of whether anything would be transferred (e.g., `rsync -ni`). This may change in the future.

#### 7.7.5.5. Examples and tutorial

All of these examples use the same input, whose content will be introduced gradually, using edited output of `ls -oghR` (which is like `ls -lhR` but omits user and group). Examples assume a umask of `0007`. The Dockerfile instructions listed also assume a preceding:

    FROM alpine:3.17
    RUN mkdir /dst

i.e., a simple base image containing a top-level directory `dst`.

Many additional examples are available in the source code in the file `test/build/50_rsync.bats`.

We begin by copying regular files. The context directory `ctx` contains, in part, two directories containing one regular file each. Note that one of these files (`file-basic1`) and one of the directories (`basic1`) have strange permissions.

    ./ctx:
    drwx---r-x 2  60 Oct 11 13:20 basic1
    drwxrwx--- 2  60 Oct 11 13:20 basic2

    ./ctx/basic1:
    -rw----r-- 1 12 Oct 11 13:20 file-basic1

    ./ctx/basic2:
    -rw-rw---- 1 12 Oct 11 13:20 file-basic2

The simplest form of `RSYNC` is to copy a single file into a specified directory:

    RSYNC /basic1/file-basic1 /dst

resulting in:

    $ ls -oghR dst
    dst:
    -rw----r-- 1 12 Oct 11 13:26 file-basic1

Note that `file-basic1`’s metadata — here its odd permissions — are preserved. `1` is the number of hard links to the file, and `12` is the file size.

One can also rename the destination by specifying a new file name, and with `+z`, not copy metadata (from here on the `ls` command is omitted for brevity):

    RSYNC +z /basic1/file-basic1 /dst/file-basic1_nom

    dst:
    -rw------- 1 12 Sep 21 15:51 file-basic1_nom

A trailing slash on the destination creates a new directory and places the source file within:

    RSYNC /basic1/file-basic1 /dst/new/

    dst:
    drwxrwx--- 1 22 Oct 11 13:26 new

    dst/new:
    -rw----r-- 1 12 Oct 11 13:26 file-basic1

With multiple source files, the destination trailing slash is optional:

    RSYNC /basic1/file-basic1 /basic2/file-basic2 /dst/newB

    dst:
    drwxrwx--- 1 44 Oct 11 13:26 newB

    dst/newB:
    -rw----r-- 1 12 Oct 11 13:26 file-basic1
    -rw-rw---- 1 12 Oct 11 13:26 file-basic2

For directory sources, the presence or absence of a trailing slash is highly significant. Without one, the directory itself is placed in the destination (recall that this would rename a source file):

    RSYNC /basic1 /dst/basic1_new

    dst:
    drwxrwx--- 1 12 Oct 11 13:28 basic1_new

    dst/basic1_new:
    drwx---r-x 1 22 Oct 11 13:28 basic1

    dst/basic1_new/basic1:
    -rw----r-- 1 12 Oct 11 13:28 file-basic1

A source trailing slash means copy the contents of a directory rather than the directory itself. Importantly, however, the directory’s metadata is copied to the destination directory.

    RSYNC /basic1/ /dst/basic1_renamed

    dst:
    drwx---r-x 1 22 Oct 11 13:28 basic1_renamed

    dst/basic1_renamed:
    -rw----r-- 1 12 Oct 11 13:28 file-basic1

One gotcha is that `RSYNC +z` is a no-op if the source is a directory:

    RSYNC +z /basic1 /dst/basic1_newC

    dst:

At least `-r` is needed with `+z` in this case:

    RSYNC +z -r /basic1/ /dst/basic1_newD

    dst:
    drwx------ 1 22 Oct 11 13:28 basic1_newD

    dst/basic1_newD:
    -rw------- 1 12 Oct 11 13:28 file-basic1

Multiple source directories can be specified, including with wildcards. This example also illustrates that copies files are by default merged with content already existing in the image.

    RUN mkdir /dst/dstC && echo file-dstC > /dst/dstC/file-dstC
    RSYNC /basic* /dst/dstC

    dst:
    drwxrwx--- 1 42 Oct 11 13:33 dstC

    dst/dstC:
    drwx---r-x 1 22 Oct 11 13:33 basic1
    drwxrwx--- 1 22 Oct 11 13:33 basic2
    -rw-rw---- 1 10 Oct 11 13:33 file-dstC

    dst/dstC/basic1:
    -rw----r-- 1 12 Oct 11 13:33 file-basic1

    dst/dstC/basic2:
    -rw-rw---- 1 12 Oct 11 13:33 file-basic2

Trailing slashes can be specified independently for each source:

    RUN mkdir /dst/dstF && echo file-dstF > /dst/dstF/file-dstF
    RSYNC /basic1 /basic2/ /dst/dstF

    dst:
    drwxrwx--- 1 52 Oct 11 13:33 dstF

    dst/dstF:
    drwx---r-x 1 22 Oct 11 13:33 basic1
    -rw-rw---- 1 12 Oct 11 13:33 file-basic2
    -rw-rw---- 1 10 Oct 11 13:33 file-dstF

    dst/dstF/basic1:
    -rw----r-- 1 12 Oct 11 13:33 file-basic1

Bare `/` (i.e., the entire context directory) is considered to have a trailing slash:

    RSYNC / /dst

    dst:
    drwx---r-x 1  22 Oct 11 13:33 basic1
    drwxrwx--- 1  22 Oct 11 13:33 basic2

    dst/basic1:
    -rw----r-- 1 12 Oct 11 13:33 file-basic1

    dst/basic2:
    -rw-rw---- 1 12 Oct 11 13:33 file-basic2

To replace (rather than merge with) existing content, use `--delete`. Note also that wildcards can be combined with trailing slashes and that the directory gets the metadata of the first slashed directory.

    RUN mkdir /dst/dstG && echo file-dstG > /dst/dstG/file-dstG
    RSYNC --delete /basic*/ /dst/dstG

    dst:
    drwx---r-x 1 44 Oct 11 14:00 dstG

    dst/dstG:
    -rw----r-- 1 12 Oct 11 14:00 file-basic1
    -rw-rw---- 1 12 Oct 11 14:00 file-basic2

Symbolic links in the source(s) add significant complexity. Like `rsync(1)`, `RSYNC` can do one of three things with a given symlink:

1. Ignore it, silently or with a warning.

2. Preserve it: copy as a symlink, with the same target.

3. Dereference it: copy the target instead.

These actions are selected independently for safe symlinks and unsafe symlinks. Safe symlinks are those which point to a target within the top of transfer, which is the deepest directory in the source path with a trailing slash. For example, `/foo/bar`’s top-of-transfer is `/foo` (regardless of whether `bar` is a directory or file), while `/foo/bar/`’s top-of-transfer is `/foo/bar`.

For the symlink examples, the context contains two sub-directories with a variety of symlinks, as well as a sibling file and directory outside the context. All of these links are valid on the host. In this listing, the absolute path to the parent of the context directory is replaced with `/...`.

    .:
    drwxrwx--- 9 200 Oct 11 14:00 ctx
    drwxrwx--- 2  60 Oct 11 14:00 dir-out
    -rw-rw---- 1   9 Oct 11 14:00 file-out

    ./ctx:
    drwxrwx--- 3 320 Oct 11 14:00 sym1

    ./ctx/sym1:
    lrwxrwxrwx 1 13 Oct 11 14:00 dir-out_rel -> ../../dir-out
    drwxrwx--- 2 60 Oct 11 14:00 dir-sym1
    lrwxrwxrwx 1  8 Oct 11 14:00 dir-sym1_direct -> dir-sym1
    lrwxrwxrwx 1 10 Oct 11 14:00 dir-top_rel -> ../dir-top
    lrwxrwxrwx 1 47 Oct 11 14:00 file-out_abs -> /.../file-out
    lrwxrwxrwx 1 14 Oct 11 14:00 file-out_rel -> ../../file-out
    -rw-rw---- 1 10 Oct 11 14:00 file-sym1
    lrwxrwxrwx 1 57 Oct 11 14:00 file-sym1_abs -> /.../ctx/sym1/file-sym1
    lrwxrwxrwx 1  9 Oct 11 14:00 file-sym1_direct -> file-sym1
    lrwxrwxrwx 1 17 Oct 11 14:00 file-sym1_upover -> ../sym1/file-sym1
    lrwxrwxrwx 1 51 Oct 11 14:00 file-top_abs -> /.../ctx/file-top
    lrwxrwxrwx 1 11 Oct 11 14:00 file-top_rel -> ../file-top

    ./ctx/sym1/dir-sym1:
    -rw-rw---- 1 14 Oct 11 14:00 dir-sym1.file

    ./dir-out:
    -rw-rw---- 1 13 Oct 11 14:00 dir-out.file

By default, safe symlinks are preserved while unsafe symlinks are silently ignored:

    RSYNC /sym1 /dst

    dst:
    drwxrwx--- 1 206 Oct 11 17:10 sym1

    dst/sym1:
    drwxrwx--- 1 26 Oct 11 17:10 dir-sym1
    lrwxrwxrwx 1  8 Oct 11 17:10 dir-sym1_direct -> dir-sym1
    lrwxrwxrwx 1 10 Oct 11 17:10 dir-top_rel -> ../dir-top
    -rw-rw---- 1 10 Oct 11 17:10 file-sym1
    lrwxrwxrwx 1  9 Oct 11 17:10 file-sym1_direct -> file-sym1
    lrwxrwxrwx 1 17 Oct 11 17:10 file-sym1_upover -> ../sym1/file-sym1
    lrwxrwxrwx 1 17 Oct 11 17:10 file-sym2_upover -> ../sym2/file-sym2
    lrwxrwxrwx 1 11 Oct 11 17:10 file-top_rel -> ../file-top

    dst/sym1/dir-sym1:
    -rw-rw---- 1 14 Oct 11 17:10 dir-sym1.file

The source files have four rough fates:

1. Regular files and directories (`file-sym1` and `dir-sym1`). These are copied into the image unchanged, including metadata.

2. Safe symlinks, now broken. This is one of the gotchas of `RSYNC`’s top-of-transfer directory (here host path `./ctx`, image path `/`) differing from the source directory (`./ctx/sym1`, `/sym1`), because the latter lacks a trailing slash. `dir-top_rel`, `file-sym2_upover`, and `file-top_rel` all ascend only as high as `./ctx` (host path, `/` image) before re-descending. This is within the top-of-transfer, so the symlinks are safe and thus copied unchanged, but their targets were not included in the copy.

3. Safe symlinks, still valid.

   1. `dir-sym1_direct` and `file-sym1_direct` point directly to files in the same directory.

   2. `dir-sym1_upover` and `file-sym1_upover` point to files in the same directory, but by first ascending into their parent — within the top-of-transfer, so they are safe — and then re-descending. If `sym1` were renamed during the copy, these links would break.

4. Unsafe symlinks, which are ignored by the copy and do not appear in the image.

   1. Absolute symlinks are always unsafe (`*_abs`).

   2. `dir-out_rel` and `file-out_rel` are relative symlinks that ascend above the top-of-transfer, in this case to targets outside the context, and are thus unsafe.

The top-of-transfer can be changed to `sym1` with a trailing slash. This also adds `sym1` to the destination so the resulting directory structure is the same.

    RSYNC /sym1/ /dst/sym1

    dst:
    drwxrwx--- 1 96 Oct 11 17:10 sym1

    dst/sym1:
    drwxrwx--- 1 26 Oct 11 17:10 dir-sym1
    lrwxrwxrwx 1  8 Oct 11 17:10 dir-sym1_direct -> dir-sym1
    -rw-rw---- 1 10 Oct 11 17:10 file-sym1
    lrwxrwxrwx 1  9 Oct 11 17:10 file-sym1_direct -> file-sym1

    dst/sym1/dir-sym1:
    -rw-rw---- 1 14 Oct 11 17:10 dir-sym1.file

`*_upover` and `*-out_rel` are now unsafe and replaced with their targets.

Another common use case is to follow unsafe symlinks and copy their targets in place of the links. This is accomplished with `+u`:

    RSYNC +u /sym1/ /dst/sym1

    dst:
    drwxrwx--- 1 352 Oct 11 17:10 sym1

    dst/sym1:
    drwxrwx--- 1 24 Oct 11 17:10 dir-out_rel
    drwxrwx--- 1 26 Oct 11 17:10 dir-sym1
    lrwxrwxrwx 1  8 Oct 11 17:10 dir-sym1_direct -> dir-sym1
    drwxrwx--- 1 24 Oct 11 17:10 dir-top_rel
    -rw-rw---- 1  9 Oct 11 17:10 file-out_abs
    -rw-rw---- 1  9 Oct 11 17:10 file-out_rel
    -rw-rw---- 1 10 Oct 11 17:10 file-sym1
    -rw-rw---- 1 10 Oct 11 17:10 file-sym1_abs
    lrwxrwxrwx 1  9 Oct 11 17:10 file-sym1_direct -> file-sym1
    -rw-rw---- 1 10 Oct 11 17:10 file-sym1_upover
    -rw-rw---- 1 10 Oct 11 17:10 file-sym2_abs
    -rw-rw---- 1 10 Oct 11 17:10 file-sym2_upover
    -rw-rw---- 1  9 Oct 11 17:10 file-top_abs
    -rw-rw---- 1  9 Oct 11 17:10 file-top_rel

    dst/sym1/dir-out_rel:
    -rw-rw---- 1 13 Oct 11 17:10 dir-out.file

    dst/sym1/dir-sym1:
    -rw-rw---- 1 14 Oct 11 17:10 dir-sym1.file

    dst/sym1/dir-top_rel:
    -rw-rw---- 1 13 Oct 11 17:10 dir-top.file

Now all the unsafe symlinks noted above are present in the image, but they have changed to the normal files and directories pointed to.

Warning

This feature lets you copy files outside the context into the image, unlike other container builders where `COPY` can never access anything outside the context.

The sources themselves, if symlinks, do not get special treatment:

    RSYNC /sym1/file-sym1_direct /sym1/file-sym1_upover /dst

    dst:
    lrwxrwxrwx 1 9 Oct 11 17:10 file-sym1_direct -> file-sym1

Note that `file-sym1_upover` does not appear in the image, despite being named explicitly in the instruction, because it is an unsafe symlink.

If the destination is a symlink to a file, and the source is a file, the link is replaced and the target is unchanged. (If the source is a directory, that is an error.)

    RUN touch /dst/file-dst && ln -s file-dst /dst/file-dst_direct
    RSYNC /file-top /dst/file-dst_direct

    dst:
    -rw-rw---- 1 0 Oct 11 17:42 file-dst
    -rw-rw---- 1 9 Oct 11 17:42 file-dst_direct

If the destination is a symlink to a directory, the link is followed:

    RUN mkdir /dst/dir-dst && ln -s dir-dst /dst/dir-dst_direct
    RSYNC /file-top /dst/dir-dst_direct

    dst:
    drwxrwx--- 1 16 Oct 11 17:50 dir-dst
    lrwxrwxrwx 1  7 Oct 11 17:50 dir-dst_direct -> dir-dst

    dst/dir-dst:
    -rw-rw---- 1 9 Oct 11 17:50 file-top

### 7.7.6. Examples

Build image `bar` using `./foo/bar/Dockerfile` and context directory `./foo/bar`:

    $ ch-image build -t bar -f ./foo/bar/Dockerfile ./foo/bar
    [...]
    grown in 4 instructions: bar

Same, but infer the image name and Dockerfile from the context directory path:

    $ ch-image build ./foo/bar
    [...]
    grown in 4 instructions: bar

Build using humongous vendor compilers you want to bind-mount instead of installing into the image:

    $ ch-image build --bind /opt/bigvendor:/opt .
    $ cat Dockerfile
    FROM centos:7

    RUN /opt/bin/cc hello.c
    #COPY /opt/lib/*.so /usr/local/lib   # fail: COPY doesn’t bind mount
    RUN cp /opt/lib/*.so /usr/local/lib  # possible workaround
    RUN ldconfig

## 7.8. `build-cache`

    $ ch-image [...] build-cache [...]

Print basic information about the cache. If `-v` is given, also print some Git statistics and the Git repository configuration.

If any of the following options are given, do the corresponding operation before printing. Multiple options can be given, in which case they happen in this order.

> `--dot`

>   Create a DOT export of the tree named `./build-cache.dot` and a PDF rendering `./build-cache.pdf`. Requires `graphviz` and `git2dot`.

> `--gc`

>   Run Git garbage collection on the cache, including full de-duplication of similar files. This will immediately remove all cache entries not currently reachable from a named branch (which is likely to cause corruption if the build cache is being accessed concurrently by another process). The operation can take a long time on large caches.

> `--reset`

>   Clear and re-initialize the build cache.

> `--tree`

>   Print a text tree of the cache using Git’s `git log --graph` feature. If `-v` is also given, the tree has more detail.

## 7.9. `delete`

    $ ch-image [...] delete IMAGE_GLOB [IMAGE_GLOB ... ]

Delete the image(s) described by each `IMAGE_GLOB` from the storage directory (including all build stages).

`IMAGE_GLOB` can be either a plain image reference or an image reference with glob characters to match multiple images. For example, `ch-image delete 'foo*'` will delete all images whose names start with `foo`. Multiple images and/or globs can also be given in a single command line.

Importantly, this sub-command does not also remove the image from the build cache. Therefore, it can be used to reduce the size of the storage directory, trading off the time needed to retrieve an image from cache.

Warning

Glob characters must be quoted or otherwise protected from the shell, which also desires to interpret them and will do so incorrectly.

## 7.10. `gestalt`

    $ ch-image [...] gestalt [SELECTOR]

Provide information about the configuration and available features of `ch-image`. End users generally will not need this; it is intended for testing and debugging.

`SELECTOR` is one of:

> - `bucache`. Exit successfully if the build cache is available, unsuccessfully with an error message otherwise. With `-v`, also print version information about dependencies.

> - `bucache-dot`. Exit successfully if build cache DOT trees can be written, unsuccessfully with an error message otherwise. With `-v`, also print version information about dependencies.

> - `dependencies`. Report dependency problems on standard output, if any, and exit. If all is well, there is no output and the exit is successful; in case of problems, the exit is unsuccessful.

> - `python-path`. Print the path to the Python interpreter in use and exit successfully.

> - `storage-path`. Print the storage directory path and exit successfully.

## 7.11. `list`

Print information about images. If no argument given, list the images in builder storage.

### 7.11.1. Synopsis

    $ ch-image [...] list [-l] [IMAGE_REF]

### 7.11.2. Description

Optional argument:

> `-l`, `--long`

>   Use long format (name, last change timestamp) when listing images.

> `-u`, `--undeletable`

>   List images that can be undeleted. Can also be spelled `--undeleteable`.

> `IMAGE_REF`

>   Print details of what’s known about `IMAGE_REF`, both locally and in the remote registry, if any.

### 7.11.3. Examples

List images in builder storage:

    $ ch-image list
    alpine:3.17 (amd64)
    alpine:latest (amd64)
    debian:buster (amd64)

Print details about Debian Buster image:

    $ ch-image list debian:buster
    details of image:    debian:buster
    in local storage:    no
    full remote ref:     registry-1.docker.io:443/library/debian:buster
    available remotely:  yes
    remote arch-aware:   yes
    host architecture:   amd64
    archs available:     386       bae2738ed83
                         amd64     98285d32477
                         arm/v7    97247fd4822
                         arm64/v8  122a0342878

For remotely available images like Debian Buster, the associated digest is listed beside each available architecture. Importantly, this feature does not provide the hash of the local image, which is only calculated on push.

## 7.12. `import`

    $ ch-image [...] import PATH IMAGE_REF

Copy the image at `PATH` into builder storage with name `IMAGE_REF`. `PATH` can be:

- an image directory

- a tarball with no top-level directory (a.k.a. a “tarbomb”)

- a standard tarball with one top-level directory

If the imported image contains Charliecloud metadata, that will be imported unchanged, i.e., images exported from `ch-image` builder storage will be functionally identical when re-imported.

Warning

Descendant images (i.e., `FROM` the imported `IMAGE_REF`) are linked using `IMAGE_REF` only. If a new image is imported under a new `IMAGE_REF`, all instructions descending from that `IMAGE_REF` will still hit, even if the new image is different.

## 7.13. `pull`

Pull the image described by the image reference `IMAGE_REF` from a repository to the local filesystem.

### 7.13.1. Synopsis

    $ ch-image [...] pull [...] IMAGE_REF [DEST_REF]

See the FAQ for the gory details on specifying image references.

### 7.13.2. Description

Destination:

> `DEST_REF`

>   If specified, use this as the destination image reference, rather than `IMAGE_REF`. This lets you pull an image with a complicated reference while storing it locally with a simpler one.

Options:

> `--last-layer N`

>   Unpack only `N` layers, leaving an incomplete image. This option is intended for debugging.

> `--parse-only`

>   Parse `IMAGE_REF`, print a parse report, and exit successfully without talking to the internet or touching the storage directory.

This script does a fair amount of validation and fixing of the layer tarballs before flattening in order to support unprivileged use despite image problems we frequently see in the wild. For example, device files are ignored, and file and directory permissions are increased to a minimum of `rwx------` and `rw-------` respectively. Note, however, that symlinks pointing outside the image are permitted, because they are not resolved until runtime within a container.

The following metadata in the pulled image is retained; all other metadata is currently ignored. (If you have a need for additional metadata, please let us know!)

> - Current working directory set with `WORKDIR` is effective in downstream Dockerfiles.

> - Environment variables set with `ENV` are effective in downstream Dockerfiles and also written to `/ch/environment` for use in `ch-run --set-env`.

> - Mount point directories specified with `VOLUME` are created in the image if they don’t exist, but no other action is taken.

Note that some images (e.g., those with a “version 1 manifest”) do not contain metadata. A warning is printed in this case.

### 7.13.3. Examples

Download the Debian Buster image matching the host’s architecture and place it in the storage directory:

    $ uname -m
    aarch32
    pulling image:    debian:buster
    requesting arch:  arm64/v8
    manifest list: downloading
    manifest: downloading
    config: downloading
    layer 1/1: c54d940: downloading
    flattening image
    layer 1/1: c54d940: listing
    validating tarball members
    resolving whiteouts
    layer 1/1: c54d940: extracting
    image arch: arm64
    done

Same, specifying the architecture explicitly:

    $ ch-image --arch=arm/v7 pull debian:buster
    pulling image:    debian:buster
    requesting arch:  arm/v7
    manifest list: downloading
    manifest: downloading
    config: downloading
    layer 1/1: 8947560: downloading
    flattening image
    layer 1/1: 8947560: listing
    validating tarball members
    resolving whiteouts
    layer 1/1: 8947560: extracting
    image arch: arm (may not match host arm64/v8)

## 7.14. `push`

Push the image described by the image reference `IMAGE_REF` from the local filesystem to a repository.

### 7.14.1. Synopsis

    $ ch-image [...] push [--image DIR] IMAGE_REF [DEST_REF]

See the FAQ for the gory details on specifying image references.

### 7.14.2. Description

Destination:

> `DEST_REF`

>   If specified, use this as the destination image reference, rather than `IMAGE_REF`. This lets you push to a repository without permanently adding a tag to the image.

Options:

> `--image DIR`

>   Use the unpacked image located at `DIR` rather than an image in the storage directory named `IMAGE_REF`.

Because Charliecloud is fully unprivileged, the owner and group of files in its images are not meaningful in the broader ecosystem. Thus, when pushed, everything in the image is flattened to user:group `root:root`. Also, setuid/setgid bits are removed, to avoid surprises if the image is pulled by a privileged container implementation.

### 7.14.3. Examples

Push a local image to the registry `example.com:5000` at path `/foo/bar` with tag `latest`. Note that in this form, the local image must be named to match that remote reference.

    $ ch-image push example.com:5000/foo/bar:latest
    pushing image:   example.com:5000/foo/bar:latest
    layer 1/1: gathering
    layer 1/1: preparing
    preparing metadata
    starting upload
    layer 1/1: a1664c4: checking if already in repository
    layer 1/1: a1664c4: not present, uploading
    config: 89315a2: checking if already in repository
    config: 89315a2: not present, uploading
    manifest: uploading
    cleaning up
    done

Same, except use local image `alpine:3.17`. In this form, the local image name does not have to match the destination reference.

    $ ch-image push alpine:3.17 example.com:5000/foo/bar:latest
    pushing image:   alpine:3.17
    destination:     example.com:5000/foo/bar:latest
    layer 1/1: gathering
    layer 1/1: preparing
    preparing metadata
    starting upload
    layer 1/1: a1664c4: checking if already in repository
    layer 1/1: a1664c4: not present, uploading
    config: 89315a2: checking if already in repository
    config: 89315a2: not present, uploading
    manifest: uploading
    cleaning up
    done

Same, except use unpacked image located at `/var/tmp/image` rather than an image in `ch-image` storage. (Also, the sole layer is already present in the remote registry, so we don’t upload it again.)

    $ ch-image push --image /var/tmp/image example.com:5000/foo/bar:latest
    pushing image:   example.com:5000/foo/bar:latest
    image path:      /var/tmp/image
    layer 1/1: gathering
    layer 1/1: preparing
    preparing metadata
    starting upload
    layer 1/1: 892e38d: checking if already in repository
    layer 1/1: 892e38d: already present
    config: 546f447: checking if already in repository
    config: 546f447: not present, uploading
    manifest: uploading
    cleaning up
    done

## 7.15. `reset`

    $ ch-image [...] reset

Delete all images and cache from ch-image builder storage.

## 7.16. `modify`

Modify an image with shell commands, possibly interactively.

### 7.16.1. Synopsis

    $ ch-image [...] shell [...] SOURCE DEST [SCRIPT]

### 7.16.2. Description

This subcommand modifies `SOURCE` using shell commands to create `DEST`. These commands can be provided either interactively (discouraged) or non-interactively. In the non-interactive case, the commands are repackaged internally into a Dockerfile.

Options:

> `-c CMD`

>   Run `CMD` as though specified by a `RUN` instruction. Can be repeated to run multiple commands sequentially.

> `-i`

>   Execute the shell in interactive mode (by specifying `-i` to it) even if standard input is not a TTY.

> `-S`, `--shell SHELL`

>   Use `SHELL` instead of the default `/bin/sh`.

The following options are shared with `ch-image build`. For more details about these options, see the section on build.

> `-b`, `--bind SRC[:DST]`

>   For `RUN` instructions only, bind-mount `SRC` at guest `DST`.

> `--build-arg KEY[=VALUE]`

>   Set build-time variable `KEY` defined by `ARG` instruction to `VALUE`.

> `--force[=MODE]`

>   Use unprivileged build with root emulation mode `MODE`.

> `--force-cmd=CMD,ARG1[,ARG2...]`

>   If command `CMD` is found in a `RUN` instruction, add the comma-separated `ARGs` to it.

`ch-image modify` operates in one of the following three modes. If the mode desired is ambiguous, that is an error.

### 7.16.3. Non-interactive mode, commands specified with `-c`

The following are equivalent:

    $ ch-image modify -S /bin/ash -c 'echo hello' -c 'echo world' foo bar

and:

    $ ch-image build -t bar <<'EOF'
    FROM foo
    SHELL /bin/ash
    RUN echo hello
    RUN echo world
    EOF

`ch-image` simply builds a Dockerfile internally that uses `foo` as a base image, starts with an appropriate `SHELL` if `-S` was given, converts each `-c` to a `RUN` command, and executes this Dockerfile to produce image `bar`. As with regular Dockerfiles, if any command fails, the build fails and no further commands are attempted.

This mode provides detailed image provenance just like a Dockerfile.

### 7.16.4. Non-interactive mode using a shell script

The following are equivalent:

    $ ch-image modify foo bar /baz/qux.sh

and:

    $ ch-image build -t bar -f - / <<'EOF'
    FROM foo
    COPY /baz/qux.sh /ch/script.sh
    RUN /bin/sh /ch/script.sh

That is, `ch-image` uses `COPY` to put the script inside the image, then runs it.

If `SCRIPT` is not provided, standard input is not a TTY, and `-i` is not specified, a script is read from stdin instead. In this case, standard input is copied in full to a file in a temporary directory, which is used as the context. The file’s modification time is set to 1993-10-21T10:00:00Z and its name to a hash of the content, so the cache hits if the content is the same and misses if not. That is, the following are equivalent:

    $ ch-image modify foo bar <<'EOF'
    echo hello world
    EOF

and:

    $ ctx=$(mktemp -d)
    $ cat > $ctx/foo <<'EOF'
    echo hello world
    EOF
    $ hash=$(md5sum $ctx/foo)
    $ mv $ctx/foo $ctx/$hash
    $ touch -d 1993-10-21T10:00:00Z $ctx/$hash
    $ ch-image build -t bar -f - $ctx <<EOF
    FROM foo
    COPY $hash /ch/script.sh   # note: substituted by shell, not ch-image(1)
    RUN /bin/sh /ch/script.sh

Importantly:

1. Any shebang line is ignored.

2. The script receives no arguments.

3. Cache semantics are the same as a normal `COPY` instruction, i.e., if the script’s name, permissions, size, or modification time change, then the `COPY` is re-executed. Be cognizant in particular of mtime when creating the file automatically (this is why a script is also accepted on standard input).

If the shell exits successfully (i.e., exit code zero), the image changes are saved; if unsuccessfully, they are discarded. Thus, judicious use of `set -e`, explicit `exit`, etc. are recommended.

This mode does work with the cache but provides a looser and less transparent provenance than a Dockerfile.

### 7.16.5. Interactive mode

Warning

Interactive mode is strongly discouraged because it gives no record of how an image was created and interacts poorly with the build cache. Even when iterating a container, it rarely saves time over editing a Dockerfile or shell script. Only use it if you really know what you are doing.

If `SCRIPT` is not provided and standard input is a TTY, or if `-i` is specified `ch-image modify` opens an interactive shell. That is, the following are roughly equivalent (assuming a terminal):

    $ ch-image modify foo bar

and:

    $ ch-convert foo /tmp/foo
    $ ch-run -w /tmp/foo -- /bin/sh -i
    [... interactive session ...]
    $ ch-convert /tmp/foo bar

except that the parent of image `bar` is `foo` rather than the empty root image.

This mode largely defeats the cache. While images descending from `DEST` will use the latest version (as `FROM` normally behaves), no other operations can re-use the results with a cache hit, and `ch-image modify` cannot have a cache hit itself. (This is implemented with a random state ID.) We reasoned that making an interactive session cache-aware would be too difficult both conceptually and to implement. The mode is much like `ch-image import` in this regard.

For example, starting with a non-empty cache:

    $ ch-image build-cache --tree
     *  (bar) IMPORT bar
     | *  (foo) IMPORT foo
     |/
     *  (root) ROOT

     named images:       3
     state IDs:          3
     large files:        0
     commits:            3
     internal files:     7 K
     disk used:        227 MiB
     $ ch-image modify foo baz
     copying image from cache ...
     [...]
     > exit
     $ ch-image build-cache --tree
     *  (baz) MODIFY interactive
     *  (foo) IMPORT foo
     | *  (bar) IMPORT bar
     |/
     *  (root) ROOT

     named images:       4
     state IDs:          4
     large files:        0
     commits:            4
     internal files:     7 K
     disk used:        227 MiB

## 7.17. `undelete`

    $ ch-image [...] undelete IMAGE_REF

If `IMAGE_REF` has been deleted but is in the build cache, recover it from the cache. Only available when the cache is enabled, and will not overwrite `IMAGE_REF` if it exists.

## 7.18. Environment variables

`CH_IMAGE_USERNAME`, `CH_IMAGE_PASSWORD`

  Username and password for registry authentication. See important caveats in section “Authentication” above.

`CH_LOG_FILE`

  If set, append log chatter to this file, rather than standard error. This is useful for debugging situations where standard error is consumed or lost.

  Also sets verbose mode if not already set (equivalent to `--verbose`).

`CH_LOG_FESTOON`

  If set, prepend PID and timestamp to logged chatter.

`CH_XATTRS`

  If set, save xattrs in the build cache and restore them when rebuilding from the cache (equivalent to `--xattrs`).