/*************************************************************************\
*                  Copyright (C) Michael Kerrisk, 2026.                   *
*                                                                         *
* This program is free software. You may use, modify, and redistribute it *
* under the terms of the GNU General Public License as published by the   *
* Free Software Foundation, either version 3 or (at your option) any      *
* later version. This program is distributed without any warranty.  See   *
* the file COPYING.gpl-v3 for details.                                    *
\*************************************************************************/

/* seccomp_deny_open.c

   A simple seccomp filter example. Install a filter that kills the process
   if open() or openat() is called.
*/
#define _GNU_SOURCE
#include <stddef.h>
#include <fcntl.h>
#include <linux/audit.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(void)
{
    struct sock_filter filter[] = {

        /* Perform the architecture check and load the system call number
           into the accumulator */

#if defined __x86_64__          /* Architecture checking code for x86-64 */

        /* Load architecture */

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

        /* Kill the process if the architecture is not what we expect */

        BPF_JUMP(BPF_JMP | BPF_JEQ | BPF_K, AUDIT_ARCH_X86_64, 0, 2),

        /* Load system call number */

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

        /* Kill the process if this is an x32 system call (bit 30 is set) */

#define X32_SYSCALL_BIT 0x40000000
        BPF_JUMP(BPF_JMP | BPF_JGE | BPF_K, X32_SYSCALL_BIT, 0, 1),
        BPF_STMT(BPF_RET | BPF_K, SECCOMP_RET_KILL_PROCESS),

#elif defined __aarch64__       /* Architecture checking code for AArch64 */

        /* Load architecture */

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

        /* Kill the process if the architecture is not what we expect */

        BPF_JUMP(BPF_JMP | BPF_JEQ | BPF_K, AUDIT_ARCH_AARCH64, 1, 0),
        BPF_STMT(BPF_RET | BPF_K, SECCOMP_RET_KILL_PROCESS),

        /* Load system call number */

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

#else
# error Unknown architecture
#endif

        /* Allow system calls other than open() and openat() */

#ifdef __NR_open    /* Some architectures (e.g., AArch64) don't have open() */
        BPF_JUMP(BPF_JMP | BPF_JEQ | BPF_K, __NR_open, 2, 0),
#endif
        BPF_JUMP(BPF_JMP | BPF_JEQ | BPF_K, __NR_openat, 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 (prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0))
        errExit("prctl");

    install_filter();

    if (open("/tmp/a", O_RDONLY) == -1)
        errExit("open");

    printf("We shouldn't see this message\n");

    exit(EXIT_SUCCESS);
}