# Welcome to python-prctl’s documentation!

The linux prctl function allows you to control specific characteristics of a process’ behaviour. Usage of the function is fairly messy though, due to limitations in C and linux. This module provides a nice non-messy python(ic) interface. Most of the text in this documentation is based on text from the linux manpages prctl(2) and capabilities(7)

Besides prctl, this library also wraps libcap for complete capability handling and allows you to set the process name as seen in ps and top.

## Downloading and installing

Before you try to install python-prctl, you will need to install the following:

- gcc
- libc development headers
- libcap development headers

On Debian and Ubuntu, this is done as follows:

    $ sudo apt-get install build-essential libcap-dev

On Fedora and other RPM-based distributions:

    $ sudo yum install gcc glibc-devel libcap-devel

The latest stable version can be installed with distutils:

    $ sudo easy_install python-prctl

The latest development source for python-prctl can be downloaded from GitHub. Installing is again done with distutils.

    $ git clone http://github.com/seveas/python-prctl
    $ cd python-prctl
    $ python setup.py build
    $ sudo python setup.py install

The prctl module is now ready to use.

## `prctl` – Control process attributes

`prctl.``set_child_subreaper`(flag)

  When processes double-fork, they get implicitly re-parented to PID 1. Using this function, processes can mark themselves as service manager and will remain parent of any such processes they launch, becoming a sort of sub-init. They will then be responsible for handling `SIGCHLD` and calling `wait()` in them.

  This is only available in linux 3.4 and newer

`prctl.``get_child_subreaper`()

  Determine whether we are a sub-init.

  This is only available in linux 3.4 and newer

`prctl.``set_dumpable`(flag)

  Set the state of the flag determining whether core dumps are produced for this process upon delivery of a signal whose default behavior is to produce a core dump. (Normally this flag is set for a process by default, but it is cleared when a set-user-ID or set-group-ID program is executed and also by various system calls that manipulate process UIDs and GIDs).

`prctl.``get_dumpable`()

  Return the state of the dumpable flag.

`prctl.``set_endian`(endianness)

  Set the endian-ness of the calling process. Valid values are `ENDIAN_BIG`, `ENDIAN_LITTLE` and `ENDIAN_PPC_LITTLE` (PowerPC pseudo little endian).

  Note

  This function only works on PowerPC systems. An `OSError` is raised when called on other systems.

`prctl.``get_endian`()

  Return the endian-ness of the calling process, see `set_endian()`.

`prctl.``set_fpemu`(flag)

  Set floating-point emulation control flag. Pass `FPEMU_NOPRINT` to silently emulate fp operations accesses, or `FPEMU_SIGFPE` to not emulate fp operations and send `SIGFPE` instead.

  Note

  This function only works on ia64 systems. An `OSError` is raised when called on other systems.

`prctl.``get_fpemu`()

  Get floating-point emulation control flag. See `set_fpemu()`.

`prctl.``set_fpexc`(mode)

  Set floating-point exception mode. Pass `FP_EXC_SW_ENABLE` to use FPEXC for FP exception, `FP_EXC_DIV` for floating-point divide by zero, `FP_EXC_OVF` for floating-point overflow, `FP_EXC_UND` for floating-point underflow, `FP_EXC_RES` for floating-point inexact result, `FP_EXC_INV` for floating-point invalid operation, `FP_EXC_DISABLED` for FP exceptions disabled, `FP_EXC_NONRECOV` for async non-recoverable exception mode, `FP_EXC_ASYNC` for async recoverable exception mode, `FP_EXC_PRECISE` for precise exception mode. Modes can be combined with the `|` operator.

  Note

  This function only works on PowerPC systems. An `OSError` is raised when called on other systems.

`prctl.``get_fpexc`()

  Return the floating-point exception mode as a bitmap of enabled modes. See `set_fpexc()`.

`prctl.``set_keepcaps`(flag)

  Set the state of the thread’s “keep capabilities” flag, which determines whether the thread’s effective and permitted capability sets are cleared when a change is made to the thread’s user IDs such that the thread’s real UID, effective UID, and saved set-user-ID all become non-zero when at least one of them previously had the value 0. (By default, these credential sets are cleared). This value will be reset to `False` on subsequent calls to `execve()`.

`prctl.``get_keepcaps`()

  Return the current state of the calling thread’s “keep capabilities” flag.

`prctl.``set_mce_kill`(policy)

  Set the machine check memory corruption kill policy for the current thread. The policy can be early kill (`MCE_KILL_EARLY`), late kill (`MCE_KILL_LATE`), or the system-wide default (`MCE_KILL_DEFAULT`). Early kill means that the task receives a `SIGBUS` signal as soon as hardware memory corruption is detected inside its address space. In late kill mode, the process is only killed when it accesses a corrupted page. The policy is inherited by children. use the system-wide default. The system-wide default is defined by `/proc/sys/vm/memory_failure_early_kill`

  This is only available in linux 2.6.32 and newer

`prctl.``get_mce_kill`()

  Return the current per-process machine check kill policy.

  This is only available in linux 2.6.32 and newer

`prctl.``set_name`(name)

  Set the process name for the calling process, the name can be up to 16 bytes long. This name is displayed in the output of ps and top. The initial value is the name of the executable. For python applications this will likely be python.

  Note

  Use `set_proctitle()` to set the name that’s shown with `ps aux()` and `top -c()`

`prctl.``get_name`()

  Return the (first 16 bytes of) the name for the calling process.

`prctl.``set_no_new_privs`()

  Once this is set, no operation that can grant new privileges (such as execve’ing a setuid binary) will actually grant new privileges.

  This is only available in linux 3.5 and newer

`prctl.``get_no_new_privs`()

  Get whether new privileges can be granted to this pid.

  This is only available in linux 3.5 and newer

`prctl.``set_proctitle`(title)

  Set the process name for the calling process by overwriting the C-level `**argv` variable. The original value of `**argv` is then no longer visible. in ps, proc, or `/proc/self/cmdline`.

  Names longer that what fits in `**argv` will be silently truncated. To set a longer title, make your application accept bogus arguments and call the application with these arguments.

  Note

  This function is not actually part of the standard `pctrl()` syscall, but was added because it nicely complements `set_name()`.

`prctl.``set_pdeathsig`(signal)

  Set the parent death signal of the calling process (either a valid signal value from the `signal` module, or 0 to clear). This is the signal that the calling process will get when its parent dies. This value is cleared for the child of a `fork()`.

  Warning

  The “parent” in this case is considered to be the thread that created this process. In other words, the signal will be sent when that thread terminates (via, for example, `pthread_exit()`), rather than after all of the threads in the parent process terminate.

`prctl.``get_pdeathsig`()

  Return the current value of the parent process death signal. See `set_pdeathsig()`.

`prctl.``set_ptracer`(pid)

  Sets the top of the process tree that is allowed to use `PTRACE()` on the calling process, assuming other requirements are met (matching uid, wasn’t setuid, etc). Use pid 0 to disallow all processes. For more details, see `/etc/sysctl.d/10-ptrace.conf`.

  This is only available in linux 3.4 and newer

`prctl.``get_ptracer`(pid)

  Returns the top of the process tree that is allowed to use `PTRACE()` on the calling process. See `set_ptracer()`.

  This is only available in linux 3.4 and newer

`prctl.``set_seccomp`(mode)

  Set the secure computing mode for the calling thread. In the current implementation, mode must be `True`. After the secure computing mode has been set to `True`, the only system calls that the thread is permitted to make are `read()`, `write()`, `_exit()`, and `sigreturn()`. Other system calls result in the delivery of a `SIGKILL` signal. Secure computing mode is useful for number-crunching applications that may need to execute untrusted byte code, perhaps obtained by reading from a pipe or socket. This operation is only available if the kernel is configured with `CONFIG_SECCOMP` enabled.

`prctl.``get_seccomp`()

  Return the secure computing mode of the calling thread. Not very useful for the current implementation, but may be useful for other possible future modes: if the caller is not in secure computing mode, this operation returns False; if the caller is in secure computing mode, then the `prctl()` call will cause a `SIGKILL` signal to be sent to the process. This operation is only available if the kernel is configured with `CONFIG_SECCOMP` enabled.

`prctl.``get_tid_address`()

  Allows the process to obtain its own clear_tid_address, used when checkpointing/restoring processes.

  This is only available in linux 3.5 and newer

`prctl.``set_timerslack`()

  Control the default “rounding” in nanoseconds that is used by `select()`, `poll()` and friends.

  The default value of the slack is 50 microseconds; this is significantly less than the kernels average timing error but still allows the kernel to group timers somewhat to preserve power behavior.

  This is only available in linux 2.6.28 and newer

`prctl.``get_timerslack`(value)

  Return the current timing slack, see `get_timing_slack()`

  This is only available in linux 2.6.28 and newer

`prctl.``set_timing`(flag)

  Set whether to use (normal, traditional) statistical process timing or accurate timestamp based process timing, by passing `TIMING_STATISTICAL` or `PR_TIMING_TIMESTAMP`. `TIMING_TIMESTAMP` is not currently implemented (attempting to set this mode will cause an `OSError`).

`prctl.``get_timing`()

  Return which process timing method is currently in use.

`prctl.``set_tsc`(flag)

  Set the state of the flag determining whether the timestamp counter can be read by the process. Pass `TSC_ENABLE` to allow it to be read, or `TSC_SIGSEGV` to generate a `SIGSEGV` when the process tries to read the timestamp counter.

  Note

  This function only works on x86 systems. An `OSError` is raised when called on other systems.

`prctl.``get_tsc`()

  Return the state of the flag determining whether the timestamp counter can be read, see `set_tsc()`.

`prctl.``set_unalign`(flag)

  Set unaligned access control flag. Pass `UNALIGN_NOPRINT` to silently fix up unaligned user accesses, or `UNALIGN_SIGBUS` to generate `SIGBUS` on unaligned user access.

  Note

  This function only works on ia64, parisc, PowerPC and Alpha systems. An `OSError` is raised when called on other systems.

`prctl.``get_unalign`()

  Return unaligned access control bits, see `set_unalign()`.

`prctl.``set_securebits`(bitmap)

  Set the “securebits” flags of the calling thread.

  Note

  It is not recommended to use this function directly, use the `securebits` object instead.

`prctl.``get_securebits`()

  Get the “securebits” flags of the calling thread.

  Note

  As with `set_securebits()`, it is not recommended to use this function directly, use the `securebits` object instead.

`prctl.``capbset_read`(capability)

  Return whether the specified capability is in the calling thread’s capability bounding set. The capability bounding set dictates whether the process can receive the capability through a file’s permitted capability set on a subsequent call to `execve()`. An `OSError` will be raised when an invalid capability is specified.

  Note

  It is not recommended to use this function directly, use the `capbset` object instead.

`prctl.``capbset_drop`(capability)

  If the calling thread has the `CAP_SETPCAP` capability, then drop the specified capability specified by from the calling thread’s capability bounding set. Any children of the calling thread will inherit the newly reduced bounding set.

  An `OSError` will be raised if the calling thread does not have the `CAP_SETPCAP` capability or when the specified capability is invalid or when capabilities are not enabled in the kernel.

  Note

  As with `capbset_read()`, it is not recommended to use this function directly, use the `capbset` object instead.

## Capabilities and the capability bounding set

For the purpose of performing permission checks, traditional Unix implementations distinguish two categories of processes: privileged processes (whose effective user ID is 0, referred to as superuser or root), and unprivileged processes (whose effective UID is non-zero). Privileged processes bypass all kernel permission checks, while unprivileged processes are subject to full permission checking based on the process’s credentials (usually: effective UID, effective GID, and supplementary group list).

Starting with kernel 2.2, Linux divides the privileges traditionally associated with superuser into distinct units, known as capabilities, which can be independently enabled and disabled. Capabilities are a per-thread attribute.

Each thread has three capability sets containing zero or more of the capabilities described below

Permitted (the `cap_permitted` object):

  This is a limiting superset for the effective capabilities that the thread may assume. It is also a limiting superset for the capabilities that may be added to the inheritable set by a thread that does not have the `setpcap` capability in its effective set.

  If a thread drops a capability from its permitted set, it can never re-acquire that capability (unless it `execve()` s either a set-user-ID-root program, or a program whose associated file capabilities grant that capability).

Inheritable (the `cap_inheritable` object):
  This is a set of capabilities preserved across an `execve()`. It provides a mechanism for a process to assign capabilities to the permitted set of the new program during an `execve()`.
Effective (the `cap_effective` object):
  This is the set of capabilities used by the kernel to perform permission checks for the thread.

A child created via `fork()` inherits copies of its parent’s capability sets. See below for a discussion of the treatment of capabilities during `execve()`.

The `capbset` object represents the current capability bounding set of the process. The capability bounding set dictates whether the process can receive the capability through a file’s permitted capability set on a subsequent call to `execve()`. All attributes of `capbset` are `True` by default, unless a parent process already removed them from the bounding set.

These four objects have a number of attributes, all of which are properties. For the capability bounding set and the effective capabilities, these can only be set to `False`, this drops them from the corresponding set.

All details about capabilities and capability bounding sets can be found in the capabilities(7) manpage, on which most text below is based.

These are the attributes (`set` refers to each of the above objects):

`set.``audit_control`

  Enable and disable kernel auditing; change auditing filter rules; retrieve auditing status and filtering rules.

`set.``audit_write`

  Write records to kernel auditing log.

`set.``chown`

  Make arbitrary changes to file UIDs and GIDs (see chown(2)).

`set.``dac_override`

  Bypass file read, write, and execute permission checks. (DAC is an abbreviation of “discretionary access control”.)

`set.``dac_read_search`

  Bypass file read permission checks and directory read and execute permission checks.

`set.``fowner`
  - Bypass permission checks on operations that normally require the file system UID of the process to match the UID of the file (e.g., `chmod()`, `utime()`), excluding those operations covered by `dac_override` and `dac_read_search`.
  - Set extended file attributes (see chattr(1)) on arbitrary files.
  - Set Access Control Lists (ACLs) on arbitrary files.
  - Ignore directory sticky bit on file deletion.
  - Specify `O_NOATIME` for arbitrary files in `open()` and `fcntl()`.

`set.``fsetid`

  Don’t clear set-user-ID and set-group-ID permission bits when a file is modified; set the set-group-ID bit for a file whose GID does not match the file system or any of the supplementary GIDs of the calling process.

`set.``ipc_lock`

  Lock memory (`mlock()`, `mlockall()`, `mmap()`, `shmctl()`).

`set.``ipc_owner`

  Bypass permission checks for operations on System V IPC objects.

`set.``kill`

  Bypass permission checks for sending signals (see kill(2)). This includes use of the `ioctl()` `KDSIGACCEPT` operation.

`set.``lease`

  Establish leases on arbitrary files (see fcntl(2)).

`set.``linux_immutable`

  Set the `FS_APPEND_FL` and `FS_IMMUTABLE_FL` i-node flags (see chattr(1)).

`set.``mac_admin`

  Override Mandatory Access Control (MAC). Implemented for the Smack Linux Security Module (LSM).

`set.``mac_override`

  Allow MAC configuration or state changes. Implemented for the Smack LSM.

`set.``mknod`

  Create special files using `mknod()`.

`set.``net_admin`

  Perform various network-related operations (e.g., setting privileged socket options, enabling multicasting, interface configuration, modifying routing tables).

`set.``net_bind_service`

  Bind a socket to Internet domain privileged ports (port numbers less than 1024).

`set.``net_broadcast`

  (Unused) Make socket broadcasts, and listen to multicasts.

`set.``net_raw`

  Use `RAW` and `PACKET` sockets.

`set.``setgid`

  Make arbitrary manipulations of process GIDs and supplementary GID list; forge GID when passing socket credentials via Unix domain sockets.

`set.``setfcap`

  Set file capabilities.

`set.``setpcap`

  If file capabilities are not supported: grant or remove any capability in the caller’s permitted capability set to or from any other process. (This property of `setpcap` is not available when the kernel is configured to support file capabilities, since `setpcap` has entirely different semantics for such kernels.)

  If file capabilities are supported: add any capability from the calling thread’s bounding set to its inheritable set; drop capabilities from the bounding set (via `capbset_drop()`); make changes to the securebits flags.

`set.``setuid`

  Make arbitrary manipulations of process UIDs (`setuid()`, `setreuid()`, `setresuid()`, `setfsuid()`); make forged UID when passing socket credentials via Unix domain sockets.

`set.``syslog`

  Allow configuring the kernel’s syslog (printk behaviour). Before linux 2.6.38 the `sys_admin` capability was needed for this.

  This is only available in linux 2.6.38 and newer

`set.``sys_admin`
  - Perform a range of system administration operations including: `quotactl()`, func:mount, `umount()`, `swapon()`, `swapoff()`, `sethostname()`, and `setdomainname()`.
  - Perform `IPC_SET` and `IPC_RMID` operations on arbitrary System V IPC objects.
  - Perform operations on trusted and security Extended Attributes (see attr(5)).
  - Use `lookup_dcookie()`.
  - Use `ioprio_set()` to assign the `IOPRIO_CLASS_RT` scheduling class.
  - Forge UID when passing socket credentials.
  - Exceed `/proc/sys/fs/file-max`, the system-wide limit on the number of open files, in system calls that open files (e.g., `accept()`, `execve()`, `open()`, `pipe()`).
  - Employ `CLONE_NEWNS` flag with `clone()` and `unshare()`.
  - Perform `KEYCTL_CHOWN` and `KEYCTL_SETPERM` `keyctl()` operations.

`set.``sys_boot`

  Use `reboot()` and `kexec_load()`.

`set.``sys_chroot`

  Use `chroot()`.

`set.``sys_module`

  Load and unload kernel modules (see init_module(2) and delete_module(2)).

`set.``sys_nice`
  - Raise process nice value (`nice()`, `setpriority()`) and change the nice value for arbitrary processes.
  - Set real-time scheduling policies for calling process, and set scheduling policies and priorities for arbitrary processes (`sched_setscheduler()`, `sched_setparam()`).
  - Set CPU affinity for arbitrary processes (`sched_setaffinity()`)
  - Set I/O scheduling class and priority for arbitrary processes (`ioprio_set()`).
  - Apply `migrate_pages()` to arbitrary processes and allow processes to be migrated to arbitrary nodes.
  - Apply `move_pages()` to arbitrary processes.
  - Use the `MPOL_MF_MOVE_ALL` flag with `mbind()` and `move_pages()`.

`set.``sys_pacct`

  Use `acct()`.

`set.``sys_ptrace`

  Trace arbitrary processes using `ptrace()`.

`set.``sys_rawio`

  Perform I/O port operations (`iopl()` and `ioperm()`); access `/proc/kcore`.

`set.``sys_resource`
  - Use reserved space on ext2 file systems.
  - Make `ioctl()` calls controlling ext3 journaling.
  - Override disk quota limits.
  - Increase resource limits (see setrlimit(2)).
  - Override `RLIMIT_NPROC` resource limit.
  - Raise `msg_qbytes` limit for a System V message queue above the limit in `/proc/sys/kernel/msgmnb` (see msgop(2) and msgctl(2)).

`set.``sys_time`

  Set system clock (`settimeofday()`, `stime()`, `adjtimex()`); set real-time (hardware) clock.

`set.``sys_tty_config`

  Use `vhangup()`.

`set.``wake_alarm`

  Allow triggering something that will wake the system.

  This is only available in linux 3.0 and newer

The four capabilities objects also have two additional methods, to make dropping many capabilities at the same time easier:

`set.``drop`(cap[, ...])

  Drop all capabilities given as arguments from the set.

`set.``limit`(cap[, ...])

  Drop all but the given capabilities from the set.

These function accept both names of capabilities as given above and the `CAP_` constants as defined in `capabilities.h`. These constants are available as `prctl.CAP_SYS_ADMIN` et cetera.

## Capabilities and `execve()`

During an `execve()`, the kernel calculates the new capabilities of the process using the following algorithm:

- P’(permitted) = (P(inheritable) & F(inheritable)) | (F(permitted) & cap_bset)
- P’(effective) = F(effective) ? P’(permitted) : 0
- P’(inheritable) = P(inheritable) [i.e., unchanged]

Where:

- P denotes the value of a thread capability set before the `execve()`
- P’ denotes the value of a capability set after the `execve()`
- F denotes a file capability set
- cap_bset is the value of the capability bounding set

The downside of this is that you need to set file capabilities if you want to make applications capabilities-friendly via wrappers. For instance, to allow an http daemon to listen on port 80 without it needing root privileges, you could do the following:

    prctl.cap_inheritable.net_bind_service = True
    os.setuid(pwd.getpwnam('www-data').pw_uid)
    os.execve("/usr/sbin/httpd", ["/usr/sbin/httpd"], os.environ)

This only works if `/usr/sbin/httpd` has `CAP_NET_BIND_SOCK` in its inheritable and effective sets. You can do this with the setcap tool shipped with libcap.

    $ sudo setcap cap_net_bind_service=ie /usr/sbin/httpd
    $ getcap /usr/sbin/httpd
    /usr/sbin/httpd = cap_net_bind_service+ei

Note that it only sets the capability in the inheritable set, so this capability is only granted if the program calling execve has it in its inheritable set too. The effective set of file capabilities does not exist in linux, it is a single bit that specifies whether capabilities in the permitted set are automatically raised in the effective set upon `execve()`.

## Establishing a capabilities-only environment with securebits

With a kernel in which file capabilities are enabled, Linux implements a set of per-thread securebits flags that can be used to disable special handling of capabilities for UID 0 (root). The securebits flags are inherited by child processes. During an `execve()`, all of the flags are preserved, except `keep_caps` which is always cleared.

These capabilities are available via `get_securebits()`, but are easier accessed via the `securebits` object. This object has attributes tell you whether specific securebits are set, or unset.

The following attributes are available:

`securebits.``keep_caps`

  Setting this flag allows a thread that has one or more 0 UIDs to retain its capabilities when it switches all of its UIDs to a non-zero value. If this flag is not set, then such a UID switch causes the thread to lose all capabilities. This flag is always cleared on an `execve()`.

`securebits.``no_setuid_fixup`

  Setting this flag stops the kernel from adjusting capability sets when the thread’s effective and file system UIDs are switched between zero and non-zero values. (See the subsection Effect of User ID Changes on Capabilities in capabilities(7))

`securebits.``noroot`

  If this bit is set, then the kernel does not grant capabilities when a set-user-ID-root program is executed, or when a process with an effective or real UID of 0 calls `execve()`. (See the subsection Capabilities and execution of programs by root in capabilities(7))

`securebits.``keep_caps_locked`

  Like `keep_caps`, but irreversible

`securebits.``no_setuid_fixup_locked`

  Like `no_setuid_fixup`, but irreversible

`securebits.``noroot_locked`

  Like `noroot`, but irreversible

## `_prctl` – Basic C wrapper around prctl

This is the lower level C module that wraps the `prctl()` syscall in a way that it is easy to call from a python module. It should not be used directly, applications and other libraries should use the functionality provided by the `prctl` module.

This section of the documentation is meant for people who want to contribute to python-prctl.

static PyObject* `prctl_prctl`(PyObject *self, PyObject *args)

  This is the `prctl()` wrapper. It accepts as argument either one or two `int` variables or an `int` and a `str`.

  The mandatory first int must be one of the `PR_SET_*`, `PR_GET_*`, or `PR_CAPBSET_*` constants defined in `sys/prctl.h`. The accepted values of the second argument depend on the first argument, see prctl(2).

  The function validates arguments, calls `prctl()` in the argument-specific way and returns the proper value, whether `prctl()` returns it as return value or stores it in one of the parameters.

static PyObject* `prctl_set_proctitle`(PyObject *self, PyObject *args)

  Set the process title by mangling `**argv`. Mandatory argument is a `str`.

PyMODINIT_FUNC `init_prctl`(void)

  Create the module instance and add all the relevant constants to the module. That means all `PR_*`, `CAP_*` and `SECBIT_*` constants mentioned in prctl(2) and capabilities(7). To avoid repeating yourself all the time, use the `namedconstant` and `namedattribute` macros when adding new values.