Re: [PATCH] Linux: Add execveat system call wrapper

[Adhemerval Zanella <adhemerval.zanella@linaro.org>]

On 28/04/2020 09:20, Alexandra Hájková via Libc-alpha wrote:

> +/* Execute the file FD refers to, overlaying the running program image.
> +   ARGV and ENVP are passed to the new program, as for `execve'.  */
> +int
> +execveat (int dirfd, const char *path, char *const argv[], char *const envp[],
> +          int flags)
> +{
> +  /* Avoid implicit array coercion in syscall macros.  */
> +  INLINE_SYSCALL_CALL (execveat, dirfd, path, &argv[0], &envp[0], flags);
> +#ifndef __ASSUME_EXECVEAT
> +  if (errno != ENOSYS)
> +    return -1;
> +
> +  int fd = openat (dirfd, path, flags | O_CLOEXEC);
> +  if (fd < 0)
> +    return -1;
> +

Is this subject to a TOCTOU where a thread might close the fd by explicitly
reading /proc/self/fd/?


> +  struct fd_to_filename fdfilename;
> +  const char *gfilename
> +    = path != NULL ? path : __fd_to_filename (fd, &fdfilename);
> +> +  /* We do not need the return value.  */
> +  __	execve (gfilename, argv, envp);
> +
> +  int save = errno;
> +
> +  /* We come here only if the 'execve' call fails.  Determine whether
> +     /proc is mounted.  If not we return ENOSYS.  */
> +  struct stat st;
> +  if (stat ("/proc/self/fd", &st) != 0 && errno == ENOENT)
> +    save = ENOSYS;
> +
> +  close (fd);
> +  __set_errno (save);
> +#endif
> +
> +  return -1;
> +}