[Libguestfs] [PATCH v3 libguestfs] launch: Implement a safer getumask.

[Eric Blake]

On 04/14/2016 03:59 AM, Richard W.M. Jones wrote:
> The current implementation of getumask involves writing a file with
> mode 0777 and then testing what mode was created by the kernel.  This
> doesn't work properly if the user set a per-mount umask (or fmask/
> dmask).
> 
> This alternative method was suggested by Josh Stone.  By forking, we
> can use the thread-unsafe method (calling umask) and pass the result
> back over a pipe.
> 
> This change also fixes another problem: mode_t is unsigned, so cannot
> be used to return an error indication (ie. -1).  Return a plain int
> instead.
> 
> Thanks: Josh Stone, Jiri Jaburek, Eric Blake.
> ---

> +guestfs_int_getumask (guestfs_h *g)
> +{

> +  if (pid == 0) {
> +    /* The child process must ONLY call async-safe functions. */
> +    close (fd[0]);
> +
> +    /* umask can't fail. */
> +    mask = umask (0);
> +
> +    if (write (fd[1], &mask, sizeof mask) != sizeof mask)
> +      _exit (EXIT_FAILURE);
> +    if (close (fd[1]) == -1)
> +      _exit (EXIT_FAILURE);
> +
> +    _exit (EXIT_SUCCESS);

Yay - the child looks good now.

> +  }
> +
> +  /* Parent. */
> +  close (fd[1]);
> +
> +  /* Read the umask. */
> +  if (read (fd[0], &mask, sizeof mask) != sizeof mask) {
> +    perrorf (g, "read");
> +    close (fd[0]);
> +    return -1;

Oops - this strands a child process.  You have to reap the child, even
if the read() failed.

> +  }
> +  close (fd[0]);
> +
> + again:
> +  if (waitpid (pid, &status, 0) == -1) {
> +    if (errno == EINTR) goto again;
> +    perrorf (g, "waitpid");
> +    return -1;
> +  }
> +  else if (!WIFEXITED (status) || WEXITSTATUS (status) != 0) {
> +    guestfs_int_external_command_failed (g, status, "umask", NULL);
> +    return -1;
> +  }

Getting closer.

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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