fusl/src/unistd/faccessat.c

#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);
}