Re: [PATCH v2] pasta: Add --pass-fds option to skip closing file descriptors

[Stefano Brivio <sbrivio@redhat.com>]

On Mon, 26 Oct 2026 16:19:41 +0000
"Lawrence, Richard E" <rlawrence@tamu.edu> wrote:

> [...]
>
> I wanted to unconditionally relocate the fd_tap because I imagined
> the following bug: imagine if someone is using pasta, passes the tap
> on stdin, does NOT request to pass fds (tap | pasta -fd 0), and we
> choose not to relocate the tap. Since stdin is never closed, the tap
> would be passed to the spawned command on stdin, which would most
> likely causing the spawned command to have unexpected behavior.

Oh, you're right, I didn't consider that possibility.  I thought that
moving the tap file descriptor would only be relevant if we later close
file descriptors, but that's not actually the case.

I'll review your v3 in a bit.  I just had a glance and I couldn't
figure out right away why the second part of the change in
isolate_fds() isn't a simple:

	if (!c->pasta_pass_fds)
		return;

in the right place (after the --fd descriptor is moved, then), but I
guess I'll figure it out. :)

> Additionally, this choice is especially weird considering that the
> hypothetical user did not even request the tap to be passed to the
> spawned command at all. I would also like to point out that the code
> already unconditionally relocates the tap: I assume that someone must
> have foreseen some other bug (perhaps related to stdin in passt
> mode?) and I don't want to cause a regression in another workflow
> that I don't know anything about.

Your assumption is correct, exactly that, see:

  https://bugs.passt.top/show_bug.cgi?id=215
  https://github.com/libguestfs/libguestfs/issues/360#issuecomment-4939858744

as well as commits:

  c80d9db312df ("main: Ensure fds 0-2 are populated")
  fd5b0807f5d0 ("isolation: Move --fd descriptor to a number of our choosing")

-- 
Stefano