KSNOOP(8)

NAME
KSNOOP - tool for tracing kernel function entry/return showing arguments/return values

SYNOPSIS
        ksnoop [OPTIONS] { COMMAND FUNC | help }

        OPTIONS := { { -V | --version } | { -h | --help } | { [-P | --pages] nr_pages} | { [-p | --pid] pid} | [{ -s | --stack }] | [{ -d | --debug }] }

        COMMAND := { trace | info }

        FUNC := { name | name(arg[,**arg]) }

DESCRIPTION
        ksnoop allows for inspection of arguments and return values associated with function entry/return.

            ksnoop info FUNC
                Show function description, arguments and return value types.

            ksnoop trace FUNC [FUNC]
                Trace function entry and return, showing arguments and return values. A function name can simply be specified, or a function name along with named arguments, return values. return is used to specify the return value.

        ksnoop requires the kernel to provide BTF for itself, and if tracing of module data is required, module BTF must be present also. Check /sys/kernel/btf to see if BTF is present.

        ksnoop requires CAP_BPF and CAP_TRACING capabilities.

OPTIONS

            -h, --help
                Show help information

            -V, --version
                Show version.

            -d, --debug
                Show debug output.

            -p, --pid
                Filter events by pid.

            -P, --pages
                Specify number of pages used per-CPU for perf event collection. Default is 8.

            -s, --stack
                Specified set of functions are traced if and only if they are encountered in the order specified.

EXAMPLES

# ksnoop info ip_send_skb

        int  ip_send_skb(struct net  * net, struct sk_buff  * skb);

Show function description.

# ksnoop trace ip_send_skb

                  TIME  CPU      PID FUNCTION/ARGS
        78101668506811    1     2813 ip_send_skb(
                                         net = *(0xffffffffb5959840)
                                          (struct net){
                                           .passive = (refcount_t){
                                            .refs = (atomic_t){
                                             .counter = (int)0x2,
                                            },
                                           },
                                           .dev_base_seq = (unsigned int)0x18,
                                           .ifindex = (int)0xf,
                                           .list = (struct list_head){
                                            .next = (struct list_head *)0xffff9895440dc120,
                                            .prev = (struct list_head *)0xffffffffb595a8d0,
                                           },
                                         ...

        79561322965250    1     2813 ip_send_skb(
                                         return =
                                          (int)0x0
                                     );

Show entry/return for ip_send_skb() with arguments, return values.

# ksnoop trace "ip_send_skb(skb)"

                 TIME  CPU      PID FUNCTION/ARGS
        78142420834537    1     2813 ip_send_skb(
                                         skb = *(0xffff989750797c00)
                                          (struct sk_buff){
                                           (union){
                                            .sk = (struct sock *)0xffff98966ce19200,
                                            .ip_defrag_offset = (int)0x6ce19200,
                                           },
                                           (union){
                                            (struct){
                                             ._skb_refdst = (long unsigned int)0xffff98981dde2d80,
                                             .destructor = (void (*)(struct sk_buff *))0xffffffffb3e1beb0,
                                            },
                                        ...

Show entry argument skb.

# ksnoop trace "ip_send_skb(return)"

                 TIME  CPU      PID FUNCTION/ARGS
        78178228354796    1     2813 ip_send_skb(
                                         return =
                                          (int)0x0
                                     );

Show return value from ip_send_skb().

# ksnoop trace "ip_send_skb(skb->sk)"

                  TIME  CPU      PID FUNCTION/ARGS
        78207649138829    2     2813 ip_send_skb(
                                         skb->sk = *(0xffff98966ce19200)
                                          (struct sock){
                                           .__sk_common = (struct sock_common){
                                            (union){
                                             .skc_addrpair = (__addrpair)0x1701a8c017d38f8d,
                                             (struct){
                                              .skc_daddr = (__be32)0x17d38f8d,
                                              .skc_rcv_saddr = (__be32)0x1701a8c0,
                                             },
                                            },
                                          ...

Trace member information associated with argument. Only one level of membership is supported.

# ksnoop -p 2813 "ip_rcv(dev)"

                  TIME  CPU      PID FUNCTION/ARGS
        78254803164920    1     2813 ip_rcv(
                                         dev = *(0xffff9895414cb000)
                                          (struct net_device){
                                           .name = (char[16])[
                                            'l',
                                            'o',
                                           ],
                                           .name_node = (struct netdev_name_node *)0xffff989541515ec0,
                                           .state = (long unsigned int)0x3,
                                         ...

Trace dev argument of ip_rcv(). Specify process id 2813 for events for that process only.

# ksnoop -s tcp_sendmsg __tcp_transmit_skb ip_output

                 TIME  CPU      PID FUNCTION/ARGS
        71827770952903    1     4777 __tcp_transmit_skb(
                                         sk = *(0xffff9852460a2300)
                                          (struct sock){
                                           .__sk_common = (struct sock_common){
                                            (union){
                                             .skc_addrpair = (__addrpair)0x61b2af0a35cbfe0a,

Trace entry/return of tcp_sendmsg, __tcp_transmit_skb and ip_output when tcp_sendmsg leads to a call to __tcp_transmit_skb and that in turn leads to a call to ip_output; i.e. with a call graph matching the order specified. The order does not have to be direct calls, i.e. function A can call another function that calls function B.

# ksnoop "ip_send_skb(skb->len > 100, skb)"

                  TIME  CPU      PID FUNCTION/ARGS
        39267395709745    1     2955 ip_send_skb(
                                         skb->len =
                                          (unsigned int)0x89,
                                         skb = *(0xffff89c8be81e500)
                                          (struct sk_buff){
                                           (union){
                                            .sk = (struct sock *)0xffff89c6c59e5580,
                                            .ip_defrag_offset = (int)0xc59e5580,
                                           },

Trace ip_send_skb() skbs which have len > 100.

SEE ALSO
        bpf(2),