Commit *pidfd: prevent creation of pidfds for kthreads* causes `Failed to remount '/' read-only: Device or resource busy`

[Paul Menzel]

#regzbot introduced: 3b5bbe798b24

Dear Christian,


Commit 3b5bbe798b24 (pidfd: prevent creation of pidfds for kthreads) 
causes a regression. On the Dell XPS 13 9360 with Debian sid/unstable 
and *systemd* 256.5-1, systemd is not able to unmount the root partition 
(LUKS encrypted) anymore.

```
[…]
Aug 19 22:45:45 abreu systemd[1]: Shutting down.
Aug 19 22:45:45 abreu systemd[1]: Using hardware watchdog 'iTCO_wdt', 
version 4, device /dev/watchdog0
Aug 19 22:45:45 abreu systemd[1]: Watchdog running with a hardware 
timeout of 10min.
Aug 19 22:45:45 abreu kernel: watchdog: watchdog0: watchdog did not stop!
Aug 19 22:45:45 abreu systemd-shutdown[1]: Using hardware watchdog 
'iTCO_wdt', version 4, device /dev/watchdog0
Aug 19 22:45:45 abreu systemd-shutdown[1]: Watchdog running with a 
hardware timeout of 10min.
Aug 19 22:45:45 abreu systemd-shutdown[1]: Syncing filesystems and block 
devices.
Aug 19 22:45:46 abreu systemd-shutdown[1]: Sending SIGTERM to remaining 
processes...
Aug 19 22:45:46 abreu systemd-shutdown[1]: Failed to enumerate /proc/: 
Invalid argument
Aug 19 22:45:46 abreu systemd-shutdown[1]: Sending SIGKILL to remaining 
processes...
Aug 19 22:45:46 abreu systemd-shutdown[1]: Failed to enumerate /proc/: 
Invalid argument
Aug 19 22:45:46 abreu systemd-shutdown[1]: Unmounting file systems.
Aug 19 22:45:46 abreu (sd-umount)[1585]: Unmounting 
'/run/credentials/systemd-journald.service'.
Aug 19 22:45:46 abreu (sd-remount)[1586]: Remounting '/' read-only with 
options 'errors=remount-ro'.
Aug 19 22:45:46 abreu (sd-remount)[1586]: Failed to remount '/' 
read-only: Device or resource busy
Aug 19 22:45:46 abreu (sd-remount)[1587]: Remounting '/' read-only with 
options 'errors=remount-ro'.
Aug 19 22:45:46 abreu (sd-remount)[1587]: Failed to remount '/' 
read-only: Device or resource busy
Aug 19 22:45:46 abreu systemd-shutdown[1]: Not all file systems 
unmounted, 1 left.
Aug 19 22:45:46 abreu systemd-shutdown[1]: Deactivating swaps.
Aug 19 22:45:46 abreu systemd-shutdown[1]: All swaps deactivated.
Aug 19 22:45:46 abreu systemd-shutdown[1]: Detaching loop devices.
Aug 19 22:45:46 abreu systemd-shutdown[1]: All loop devices detached.
Aug 19 22:45:46 abreu systemd-shutdown[1]: Stopping MD devices.
Aug 19 22:45:46 abreu systemd-shutdown[1]: All MD devices stopped.
Aug 19 22:45:46 abreu systemd-shutdown[1]: Detaching DM devices.
Aug 19 22:45:46 abreu systemd-shutdown[1]: Not all DM devices detached, 
1 left.
Aug 19 22:45:46 abreu systemd-shutdown[1]: Unmounting file systems.
Aug 19 22:45:46 abreu (sd-remount)[1588]: Remounting '/' read-only with 
options 'errors=remount-ro'.
Aug 19 22:45:46 abreu (sd-remount)[1588]: Failed to remount '/' 
read-only: Device or resource busy
Aug 19 22:45:46 abreu systemd-shutdown[1]: Not all file systems 
unmounted, 1 left.
Aug 19 22:45:46 abreu systemd-shutdown[1]: Detaching DM devices.
Aug 19 22:45:46 abreu systemd-shutdown[1]: Not all DM devices detached, 
1 left.
Aug 19 22:45:46 abreu systemd-shutdown[1]: Unmounting file systems.
Aug 19 22:45:46 abreu (sd-remount)[1589]: Remounting '/' read-only with 
options 'errors=remount-ro'.
Aug 19 22:45:46 abreu (sd-remount)[1589]: Failed to remount '/' 
read-only: Device or resource busy
Aug 19 22:45:46 abreu systemd-shutdown[1]: Not all file systems 
unmounted, 1 left.
Aug 19 22:45:46 abreu systemd-shutdown[1]: Detaching DM devices.
Aug 19 22:45:46 abreu systemd-shutdown[1]: Not all DM devices detached, 
1 left.
Aug 19 22:45:46 abreu systemd-shutdown[1]: Cannot finalize remaining 
file systems, DM devices, continuing.
Aug 19 22:45:46 abreu kernel: watchdog: watchdog0: watchdog did not stop!
```

On the next boot, the journal needs to be recovered.


Kind regards,

Paul


PS: For the record:

$ git bisect bad
3b5bbe798b2451820e74243b738268f51901e7d0 is the first bad commit
commit 3b5bbe798b2451820e74243b738268f51901e7d0 (HEAD)
Author: Christian Brauner <brauner@xxxxxxxxxx>
Date:   Wed Jul 31 12:01:12 2024 +0200

    pidfd: prevent creation of pidfds for kthreads

    It's currently possible to create pidfds for kthreads but it is unclear
    what that is supposed to mean. Until we have use-cases for it and we
    figured out what behavior we want block the creation of pidfds for
    kthreads.

    Link: 
https://lore.kernel.org/r/20240731-gleis-mehreinnahmen-6bbadd128383@brauner
    Fixes: 32fcb426ec00 ("pid: add pidfd_open()")
    Cc: stable@xxxxxxxxxxxxxxx
    Signed-off-by: Christian Brauner <brauner@xxxxxxxxxx>

 kernel/fork.c | 25 ++++++++++++++++++++++---
 1 file changed, 22 insertions(+), 3 deletions(-)
$ git bisect log
git bisect start
# status: waiting for both good and bad commits
# bad: [df6cbc62cc9b3bcf593d13400dd58cd339a0f56d] Merge tag 'scsi-fixes' 
of git://git.kernel.org/pub/scm/linux/kernel/git/jejb/scsi
git bisect bad df6cbc62cc9b3bcf593d13400dd58cd339a0f56d
# status: waiting for good commit(s), bad commit known
# good: [d74da846046aeec9333e802f5918bd3261fb5509] Merge tag 
'platform-drivers-x86-v6.11-3' of 
git://git.kernel.org/pub/scm/linux/kernel/git/pdx86/platform-drivers-x86
git bisect good d74da846046aeec9333e802f5918bd3261fb5509
# bad: [d7a5aa4b3c007fae50405ca75c40258d90300e96] Merge tag 
'perf-tools-fixes-for-v6.11-2024-08-15' of 
git://git.kernel.org/pub/scm/linux/kernel/git/perf/perf-tools
git bisect bad d7a5aa4b3c007fae50405ca75c40258d90300e96
# good: [9c5af2d7dfe18e3a36f85fad8204cd2442ecd82b] Merge tag 
'nf-24-08-15' of git://git.kernel.org/pub/scm/linux/kernel/git/netfilter/nf
git bisect good 9c5af2d7dfe18e3a36f85fad8204cd2442ecd82b
# bad: [d07b43284ab356daf7ec5ae1858a16c1c7b6adab] Merge tag 'for-linus' 
of git://git.kernel.org/pub/scm/virt/kvm/kvm
git bisect bad d07b43284ab356daf7ec5ae1858a16c1c7b6adab
# bad: [4ac0f08f44b62e59a389c7ed87c89087d9fefe29] Merge tag 
'vfs-6.11-rc4.fixes' of 
git://git.kernel.org/pub/scm/linux/kernel/git/vfs/vfs
git bisect bad 4ac0f08f44b62e59a389c7ed87c89087d9fefe29
# bad: [e3786b29c54cdae3490b07180a54e2461f42144c] 9p: Fix DIO read 
through netfs
git bisect bad e3786b29c54cdae3490b07180a54e2461f42144c
# good: [889ced4c9388785952d78d20d338bda2df209bb5] netfs: clean up after 
renaming FSCACHE_DEBUG config
git bisect good 889ced4c9388785952d78d20d338bda2df209bb5
# bad: [8e5ced7804cb9184c4a23f8054551240562a8eda] netfs, ceph: Revert 
"netfs: Remove deprecated use of PG_private_2 as a second writeback flag"
git bisect bad 8e5ced7804cb9184c4a23f8054551240562a8eda
# skip: [86509e38a80da34d7800985fa2be183475242c8c] file: fix typo in 
take_fd() comment
git bisect skip 86509e38a80da34d7800985fa2be183475242c8c
# bad: [3b5bbe798b2451820e74243b738268f51901e7d0] pidfd: prevent 
creation of pidfds for kthreads
git bisect bad 3b5bbe798b2451820e74243b738268f51901e7d0
# first bad commit: [3b5bbe798b2451820e74243b738268f51901e7d0] pidfd: 
prevent creation of pidfds for kthreads