Re: [PATCH] pidfs: preserve thread pidfds reopened by file handle

[Christian Brauner]

On 2026-07-16 13:11 +0200, Jan Kara wrote:
> On Thu 16-07-26 13:27:22, Li Chen wrote:
> > PIDFD_THREAD shares O_EXCL. do_dentry_open() clears O_EXCL after
> > pidfs_export_open() validates the flags, so open_by_handle_at()
> > silently turns a thread pidfd into a process pidfd.
> > 
> > Restore PIDFD_THREAD on the opened file, matching pidfs_alloc_file(),
> > and cover the restored flag with F_GETFL.
> > 
> > Signed-off-by: Li Chen <me@linux.beauty>
> 
> Makes sense. Just looking at this code, what about PIDFD_AUTOKILL (aka
> O_TRUNC?)? Reading the commit introducing it 07c3ef58223e it appears it was
> never meant to be directly requested by userspace so probably
> pidfs_export_open() needs to reject it. But that's unrelated issue to your
> patch so feel free to add:

Good point. But pidfs_export_open() is guarded by
pidfs_export_permission() which does:

#define VALID_FILE_HANDLE_OPEN_FLAGS \
	(O_RDONLY | O_WRONLY | O_RDWR | O_NONBLOCK | O_CLOEXEC | O_EXCL)

	if (oflags & ~(VALID_FILE_HANDLE_OPEN_FLAGS | O_LARGEFILE))
		return -EINVAL;

and so is safe. But I've added the following patch on top which also
splats if that is passed!