source: azure_iot_hub/trunk/musl-1.1.18/src/unistd/faccessat.c@ 388

#include <unistd.h>
#include <fcntl.h>
#include <errno.h>
#include <sys/wait.h>
#include "syscall.h"
#include "pthread_impl.h"

struct ctx {
        int fd;
        const char *filename;
        int amode;
};

static const int errors[] = {
        0, -EACCES, -ELOOP, -ENAMETOOLONG, -ENOENT, -ENOTDIR,
        -EROFS, -EBADF, -EINVAL, -ETXTBSY,
        -EFAULT, -EIO, -ENOMEM,
        -EBUSY
};

static int checker(void *p)
{
        struct ctx *c = p;
        int ret;
        int i;
        if (__syscall(SYS_setregid, __syscall(SYS_getegid), -1)
            || __syscall(SYS_setreuid, __syscall(SYS_geteuid), -1))
                __syscall(SYS_exit, 1);
        ret = __syscall(SYS_faccessat, c->fd, c->filename, c->amode, 0);
        for (i=0; i < sizeof errors/sizeof *errors - 1 && ret!=errors[i]; i++);
        return i;
}

int faccessat(int fd, const char *filename, int amode, int flag)
{
        if (!flag || (flag==AT_EACCESS && getuid()==geteuid() && getgid()==getegid()))
                return syscall(SYS_faccessat, fd, filename, amode, flag);

        if (flag != AT_EACCESS)
                return __syscall_ret(-EINVAL);

        char stack[1024];
        sigset_t set;
        pid_t pid;
        int ret = -EBUSY;
        struct ctx c = { .fd = fd, .filename = filename, .amode = amode };

        __block_all_sigs(&set);
        
        pid = __clone(checker, stack+sizeof stack, 0, &c);
        if (pid > 0) {
                int status;
                do {
                        __syscall(SYS_wait4, pid, &status, __WCLONE, 0);
                } while (!WIFEXITED(status) && !WIFSIGNALED(status));
                if (WIFEXITED(status))
                        ret = errors[WEXITSTATUS(status)];
        }

        __restore_sigs(&set);

        return __syscall_ret(ret);
}