Re: [PATCH 2/2] pidfd: add pidfdfs

[Jiri Slaby <jirislaby@kernel.org>]

On 13. 02. 24, 17:45, Christian Brauner wrote:
> This moves pidfds from the anonymous inode infrastructure to a tiny
> pseudo filesystem. This has been on my todo for quite a while as it will
> unblock further work that we weren't able to do simply because of the
> very justified limitations of anonymous inodes. Moving pidfds to a tiny
> pseudo filesystem allows:
> 
> * statx() on pidfds becomes useful for the first time.
> * pidfds can be compared simply via statx() and then comparing inode
>    numbers.
> * pidfds have unique inode numbers for the system lifetime.
> * struct pid is now stashed in inode->i_private instead of
>    file->private_data. This means it is now possible to introduce
>    concepts that operate on a process once all file descriptors have been
>    closed. A concrete example is kill-on-last-close.
> * file->private_data is freed up for per-file options for pidfds.
> * Each struct pid will refer to a different inode but the same struct
>    pid will refer to the same inode if it's opened multiple times. In
>    contrast to now where each struct pid refers to the same inode. Even
>    if we were to move to anon_inode_create_getfile() which creates new
>    inodes we'd still be associating the same struct pid with multiple
>    different inodes.
> * Pidfds now go through the regular dentry_open() path which means that
>    all security hooks are called unblocking proper LSM management for
>    pidfds. In addition fsnotify hooks are called and allow for listening
>    to open events on pidfds.
> 
> The tiny pseudo filesystem is not visible anywhere in userspace exactly
> like e.g., pipefs and sockfs. There's no lookup, there's no complex
> inode operations, nothing. Dentries and inodes are always deleted when
> the last pidfd is closed.

This breaks lsof and util-linux.

Without the commit, lsof shows:
systemd      ... 59 [pidfd:899]


With the commit:
systemd      ... 1187 pidfd


And that user-visible change breaks a lot of stuff, incl. lsof tests.

For util-linux, its test fail with:

> [  125s] --- tests/expected/lsfd/column-name-pidfd	2024-05-06 07:20:54.655845940 +0000
> [  125s] +++ tests/output/lsfd/column-name-pidfd	2024-05-15 01:04:15.406666666 +0000
> [  125s] @@ -1,2 +1,2 @@
> [  125s] -3 anon_inode:[pidfd] pid=1 comm= nspid=1
> [  125s] +3 pidfd:[INODENUM] pidfd:[INODENUM]
> [  125s]  pidfd:ASSOC,KNAME,NAME: 0
> [  125s]          lsfd: NAME and KNAME column: [02] pidfd             ... FAILED (lsfd/column-name-pidfd)

And:
> [  125s] --- tests/expected/lsfd/column-type-pidfd	2024-05-06 07:20:54.655845940 +0000
> [  125s] +++ tests/output/lsfd/column-type-pidfd	2024-05-15 01:04:15.573333333 +0000
> [  125s] @@ -1,2 +1,2 @@
> [  125s] -3 UNKN pidfd
> [  125s] +3 REG REG
> [  125s]  pidfd:ASSOC,STTYPE,TYPE: 0
> [  125s]          lsfd: TYPE and STTYPE column: [02] pidfd            ... FAILED (lsfd/column-type-pidfd)

Any ideas?

thanks,
-- 
js
suse labs