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seccomp_notify_alloc(3) libseccomp Documentation seccomp_notify_alloc(3)


seccomp_notify_alloc, seccomp_notify_free, seccomp_notify_receive, seccomp_notify_respond, seccomp_notify_id_valid, seccomp_notify_fd - Manage seccomp notifications


#include <seccomp.h>

int seccomp_notify_alloc(struct seccomp_notif **req, struct seccomp_notif_resp **resp)
void seccomp_notify_free(struct seccomp_notif *req, struct seccomp_notif_resp *resp)
int seccomp_notify_receive(int fd, struct seccomp_notif *req)
int seccomp_notify_respond(int fd, struct seccomp_notif_resp *resp)
int seccomp_notify_id_valid(int fd, uint64_t id)
int seccomp_notify_fd(const scmp_filter_ctx ctx)

Link with -lseccomp.


The seccomp_notify_alloc() function dynamically allocates enough memory for a seccomp notification and response. Note that one should always use these functions and not depend on the structure sizes in headers, since the size can vary depending on the kernel version. This function takes care to ask the kernel how big each structure should be, and allocates the right amount of memory. The seccomp_notify_free() function frees memory allocated by seccomp_notify_alloc().

The seccomp_notify_receive() function receives a notification from a seccomp notify fd (obtained from seccomp_notify_fd()).

The seccomp_notify_respond() function sends a response to a particular notification. The id field should be the same as the id from the request, so that the kernel knows which request this response corresponds to.

The seccomp_notify_id_valid() function checks to see if the syscall from a particular notification request is still valid, i.e. if the task is still alive. See NOTES below for details on race conditions.

The seccomp_notify_fd() returns the notification fd of a filter after it has been loaded.


The seccomp_notify_fd() returns the notification fd of the loaded filter, -1 if a notification fd has not yet been created, and -EINVAL if the filter context is invalid.

The seccomp_notify_id_valid() returns 0 if the id is valid, and -ENOENT if it is not.

The seccomp_notify_alloc(), seccomp_notify_receive(), and seccomp_notify_respond() functions return zero on success, or one of the following error codes on failure:

There was a system failure beyond the control of the library, check the errno value for more information.
Internal libseccomp failure.
The library was unable to allocate enough memory.
The library doesn't support the particular operation.


Care should be taken to avoid two different time of check/time of use errors. First, after opening any resources relevant to the pid for a notification (e.g. /proc/pid/mem for reading tracee memory to make policy decisions), applications should call seccomp_notify_id_valid() to make sure that the resources the application has opened correspond to the right pid, i.e. that the pid didn't die and a different task take its place.

Second, the classic time of check/time of use issue with seccomp memory should also be avoided: applications should copy any memory they wish to use to make decisions from the tracee into its own address space before applying any policy decisions, since a multi-threaded tracee may edit the memory at any time, including after it's used to make a policy decision.

A complete example of how to avoid these two races is available in the Linux Kernel source tree at /samples/seccomp/user-trap.c.


Tycho Andersen <>

30 May 2020