seccomp/seccomp_deny_syscall.c

This is seccomp/seccomp_deny_syscall.c, an example to accompany the book, The Linux Programming Interface.

This file is not printed in the book; it demonstrates Linux features that are not described in the book (typically features that have appeared since the book was published).

In the listing below, the names of Linux system calls and C library functions are hyperlinked to manual pages from the Linux man-pages project, and the names of functions implemented in the book are hyperlinked to the implementations of those functions.

Download seccomp/seccomp_deny_syscall.c

Cover of The Linux Programming Interface

Distribution version

Function list (Bold in this list means a function is not static)

seccomp()
install_filter()
main()

/* seccomp_deny_syscall.c

   Usage: ./seccomp_deny_syscall <syscall-num> <command> <arg>...

   A simple seccomp filter example. Install a filter that executes
   the specified command after first installing a seccomp filter
   that will kill the program if it makes a call to the system call
   whose number is specified in the first command-line argument.
*/
#define _GNU_SOURCE
#include <stddef.h>
#include <sys/syscall.h>
#include <linux/filter.h>
#include <linux/seccomp.h>
#include <sys/prctl.h>
#include "tlpi_hdr.h"

/* The following is a hack to allow for systems (pre-Linux 4.14) that don't
   provide SECCOMP_RET_KILL_PROCESS, which kills (all threads in) a process.
   On those systems, define SECCOMP_RET_KILL_PROCESS as SECCOMP_RET_KILL
   (which simply kills the calling thread). */

#ifndef SECCOMP_RET_KILL_PROCESS
#define SECCOMP_RET_KILL_PROCESS SECCOMP_RET_KILL
#endif

static int
seccomp(unsigned int operation, unsigned int flags, void *args)
{
    return syscall(__NR_seccomp, operation, flags, args);
}

static void
install_filter(int syscallNum)
{
    struct sock_filter filter[] = {
        /* Architecture check deliberately omitted to keep things simple;
           a real-world application should always include the check. */

        /* Load system call number */

        BPF_STMT(BPF_LD | BPF_W | BPF_ABS,
                 offsetof(struct seccomp_data, nr)),

        /* Allow system calls other than the one with the specified number */

        BPF_JUMP(BPF_JMP | BPF_JEQ | BPF_K, syscallNum, 1, 0),
        BPF_STMT(BPF_RET | BPF_K,           SECCOMP_RET_ALLOW),
        BPF_STMT(BPF_RET | BPF_K,           SECCOMP_RET_KILL_PROCESS)

    };

    struct sock_fprog prog = {
        .len = sizeof(filter) / sizeof(filter[0]),
        .filter = filter,
    };

    if (seccomp(SECCOMP_SET_MODE_FILTER, 0, &prog) == -1)
        errExit("seccomp");
}

int
main(int argc, char *argv[])
{
    if (argc < 3) {
        fprintf(stderr, "Usage: %s <syscall-num> <command> <arg>...\n",
                argv[0]);
    }

    if (prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0))
        errExit("prctl");

    install_filter(atoi(argv[1]));

    execvp(argv[2], &argv[2]);
    errExit("execve");
    exit(EXIT_SUCCESS);
}

Download seccomp/seccomp_deny_syscall.c

Note that, in most cases, the programs rendered in these web pages are not free standing: you'll typically also need a few other source files (mostly in the lib/ subdirectory) as well. Generally, it's easier to just download the entire source tarball and build the programs with make(1). By hovering your mouse over the various hyperlinked include files and function calls above, you can see which other source files this file depends on.