Re: ETXTBSY window in __fput

[Colin Walters]

On Tue, Aug 26, 2025, at 5:05 PM, Alexander Monakov wrote:
> Dear fs hackers,
>
> I suspect there's an unfortunate race window in __fput where file locks are
> dropped (locks_remove_file) prior to decreasing writer refcount
> (put_file_access). If I'm not mistaken, this window is observable and it
> breaks a solution to ETXTBSY problem on exec'ing a just-written file, explained
> in more detail below.
>
> The program demonstrating the problem is attached (a slightly modified version
> of the demo given by Russ Cox on the Go issue tracker, see URL in first line).
> It makes 20 threads, each executing an infinite loop doing the following:
>
> 1) open an fd for writing with O_CLOEXEC
> 2) write executable code into it
> 3) close it
> 4) fork
> 5) in the child, attempt to execve the just-written file
>
> If you compile it with -DNOWAIT, you'll see that execve often fails with
> ETXTBSY. This happens if another thread forked while we were holding an open fd
> between steps 1 and 3, our fd "leaked" in that child, and then we reached our
> step 5 before that child did execve (at which point the leaked fd would be
> closed thanks to O_CLOEXEC).

This one looks to be the same as what we hit in https://github.com/containers/composefs-rs/issues/106 but with fsverity.

I came to the conclusion that we want O_CLOFORK (ref https://www.austingroupbugs.net/view.php?id=1318 ).