// SPDX-License-Identifier: GPL-2.0
/*
 * Naive system call dropper built on seccomp_filter.
 *
 * Copyright (c) 2012 The Chromium OS Authors <chromium-os-dev@chromium.org>
 * Author: Will Drewry <wad@chromium.org>
 *
 * The code may be used by anyone for any purpose,
 * and can serve as a starting point for developing
 * applications using prctl(PR_SET_SECCOMP, 2, ...).
 *
 * When run, returns the specified errno for the specified
 * system call number against the given architecture.
 *
 */

#include <errno.h>
#include <linux/audit.h>
#include <linux/filter.h>
#include <linux/seccomp.h>
#include <linux/unistd.h>
#include <stdio.h>
#include <stddef.h>
#include <stdlib.h>
#include <sys/prctl.h>
#include <unistd.h>

static int install_filter(int arch, int nr, int error)
{
        struct sock_filter filter[] = {
                BPF_STMT(BPF_LD+BPF_W+BPF_ABS,
                         (offsetof(struct seccomp_data, arch))),
                BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, arch, 0, 3),
                BPF_STMT(BPF_LD+BPF_W+BPF_ABS,
                         (offsetof(struct seccomp_data, nr))),
                BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, nr, 0, 1),
                BPF_STMT(BPF_RET+BPF_K,
                         SECCOMP_RET_ERRNO|(error & SECCOMP_RET_DATA)),
                BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_ALLOW),
        };
        struct sock_fprog prog = {
                .len = (unsigned short)(sizeof(filter)/sizeof(filter[0])),
                .filter = filter,
        };
        if (error == -1) {
                struct sock_filter kill = BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_KILL);
                filter[4] = kill;
        }
        if (prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0)) {
                perror("prctl(NO_NEW_PRIVS)");
                return 1;
        }
        if (prctl(PR_SET_SECCOMP, 2, &prog)) {
                perror("prctl(PR_SET_SECCOMP)");
                return 1;
        }
        return 0;
}

int main(int argc, char **argv)
{
        if (argc < 5) {
                fprintf(stderr, "Usage:\n"
                        "dropper <arch> <syscall_nr> <errno> <prog> [<args>]\n"
                        "Hint:  AUDIT_ARCH_I386: 0x%X\n"
                        "       AUDIT_ARCH_X86_64: 0x%X\n"
                        "       errno == -1 means SECCOMP_RET_KILL\n"
                        "\n", AUDIT_ARCH_I386, AUDIT_ARCH_X86_64);
                return 1;
        }
        if (install_filter(strtol(argv[1], NULL, 0), strtol(argv[2], NULL, 0),
                           strtol(argv[3], NULL, 0)))
                return 1;
        execv(argv[4], &argv[4]);
        printf("Failed to execv\n");
        return 255;
}