隐式引用 seccomp

Question

问题： 我正在尝试使用 seccomp，但我不明白为什么 gcc 告诉我 seccomp() 函数调用具有隐式声明。

#define _GNU_SOURCE
#include <stddef.h> // offsetof
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <linux/audit.h> // arch
#include <linux/filter.h>
#include <linux/seccomp.h>
#include <sys/prctl.h>
#include <sys/syscall.h> // syscall numbers

struct sock_filter  bpfcode[] = {

    /* validate the architecture */
    BPF_STMT(BPF_LD+BPF_W+BPF_ABS, (offsetof(struct seccomp_data, arch))),
    BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, AUDIT_ARCH_X86_64, 0, 7),
    /* load syscall number in the accumulator */
    BPF_STMT(BPF_LD+BPF_W+BPF_ABS, (offsetof (struct seccomp_data, nr))),
    /* check if the syscall number is allowed */
    BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, SYS_nanosleep, 5, 0), // for sleep
    BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, SYS_exit, 4, 0),
    BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, SYS_exit_group, 3, 0),
    BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, SYS_write, 2, 0),
    BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, SYS_read, 1, 0),
    BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, SYS_rt_sigreturn, 0, 1),
    /* allow the sys call */
    BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_ALLOW),
    BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_TRAP)
};

struct sock_fprog  bpf = {
    .len = (unsigned short)( sizeof bpfcode / sizeof bpfcode[0] ),
    .filter = bpfcode 
};

int main(int argc, char **argv)
{
    if (prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0) == -1) {
        printf("prctl no_new_privs\n");
        _exit(EXIT_FAILURE);
    }
    if (seccomp(SECCOMP_SET_MODE_FILTER, 0, &bpf)) {
        printf("seccomp");
        exit(EXIT_FAILURE);
    } 
    sleep(2);    
    return 0;
}

警告/错误：这是我尝试编译程序时的 gcc 输出。

s.c: In function ‘main’:
    s.c:45:6: warning: implicit declaration of function ‘seccomp’ [-Wimplicit-function-declaration]
      if (seccomp(SECCOMP_SET_MODE_FILTER, 0, &bpf)) {
          ^~~~~~~
    /tmp/ccYo4APk.o: In function `main':
    s.c:(.text+0x65): undefined reference to `seccomp'
    collect2: error: ld returned 1 exit status

问题：我还应该包括什么才能让它发挥作用？

编辑：为什么这有效而第一个无效？ syscall(SYS_seccomp, SECCOMP_SET_MODE_FILTER, 0, &bpf)

Answer 1

它是a known issue：没有用于 seccomp 系统调用的 glibc 包装器。

您可能希望改用 prctl(2) 来加载 BPF 程序，原因有两个：

1. glibc 包装器可用于 prctl(2)
2. seccomp(2) 系统调用是only available on Linux v3.5+。

方法如下：

prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &bpf);