Subject: bug#61095: possible misuse of posix_spawn API on non-linux OSes

From: Josselin Poiret <[email-masked]>
Date: Tue, 28 Mar 2023 18:10:27 +0200

Hi Ludo,

Ludovic Courtès <[email-masked]> writes:

> -      posix_spawn_file_actions_addclose (actions, fd);
> +      /* Adding invalid file descriptors to an 'addclose' action leads
> +         to 'posix_spawn' failures on some operating systems:
> +         <https://bugs.gnu.org/61095>.  Hence the extra check.  */
> +      int flags = fcntl (max_fd, F_GETFD, NULL);
> +      if ((flags >= 0) && ((flags & FD_CLOEXEC) == 0))
> +        posix_spawn_file_actions_addclose (actions, max_fd);
>      }

I'm worried about TOCTOU in multi-threaded contexts here, which is why I
opted for the heavy handed-approach here.  In general I don't think we
can know in advance which fdes to close :/ It's a shame that the
posix_spawn actions fails on other kernels, since I don't really see a
way to mitigate this problem (apart from the new
posix_spawn_file_actions_addclosefrom_np as you mentioned).  I don't
know what we could do here.  Maybe not provide spawn?  Or provide it in
spite of the broken fd closing?

> -
> -  closedir (dirp);
>  }
>  
>  static pid_t
> @@ -1366,6 +1345,26 @@ do_spawn (char *exec_file, char **exec_argv, char **exec_env,
>    posix_spawn_file_actions_t actions;
>    posix_spawnattr_t *attrp = NULL;
>  
> +  posix_spawn_file_actions_init (&actions);
> +
> +  /* Duplicate IN, OUT, and ERR unconditionally to clear their
> +     FD_CLOEXEC flag, if any.  */
> +  posix_spawn_file_actions_adddup2 (&actions, in, STDIN_FILENO);
> +  posix_spawn_file_actions_adddup2 (&actions, out, STDOUT_FILENO);
> +  posix_spawn_file_actions_adddup2 (&actions, err, STDERR_FILENO);

This won't work, and actually this was one of the original logic bugs I
was trying to fix.  If err is equal to, let's say, STDIN_FILENO, then
the first call will overwrite the initial file descriptor at
STDIN_FILENO, and the second call won't do what the caller intended.
This is why I was moving them out of the way first, so that they would
not overwrite each other.

> +  /* TODO: Use 'closefrom' where available.  */
> +#if 0
> +  /* Version 2.34 of the GNU libc provides this function.  */
> +  posix_spawn_file_actions_addclosefrom_np (&actions, 3);
> +#else
> +  if (in > 2)
> +    posix_spawn_file_actions_addclose (&actions, in);
> +  if (out > 2 && out != in)
> +    posix_spawn_file_actions_addclose (&actions, out);
> +  if (err > 2 && err != out && err != in)
> +    posix_spawn_file_actions_addclose (&actions, err);

Isn't this unneeded given we call close_inherited_fds below?

> [...]
>
> Josselin: I simplified the ‘dup2’ logic somewhat.

See my comments above.

> Guile does that for file descriptors it opens internally, but
> applications using ‘open-file’ without the recently-added “e” flag, or
> ‘socket’ without ‘SOCK_CLOEXEC’, etc., end up with more file descriptors
> that need to be taken care of.
>
> I wish the default were close-on-exec, but we’re not there yet.

+1

Best,
-- 
Josselin Poiret