[PATCH net-next v3 0/7] allow reaped pidfds receive in SCM_PIDFD

This is a logical continuation of a story from [1], where Christian
extented SO_PEERPIDFD to allow getting pidfds for a reaped tasks.

Git tree (based on vfs/vfs-6.17.pidfs):
v3: https://github.com/mihalicyn/linux/commits/scm_pidfd_stale.v3
current: https://github.com/mihalicyn/linux/commits/scm_pidfd_stale

Changelog for version 3:
 - rename __scm_replace_pid() to scm_replace_pid() [ as Kuniyuki suggested ]
 - ("af_unix/scm: fix whitespace errors") commit introduced [ as Kuniyuki suggested ]
 - don't stash pidfs dentry for netlink case [ as Kuniyuki suggested ]
 - splited whitespace changes [ as Kuniyuki suggested ]
 - removed unix_set_pid_to_skb() to simplify changes [ as Kuniyuki suggested ]

Changelog for version 2:
 - renamed __skb_set_pid() -> unix_set_pid_to_skb() [ as Kuniyuki suggested ]
 - get rid of extra helper (__scm_set_cred()) I've introduced before [ as Kuniyuki suggested ]
 - s/__inline__/inline/ for functions I touched [ as Kuniyuki suggested ]
 - get rid of chunk in unix_destruct_scm() with NULLifying UNIXCB(skb).pid [ as Kuniyuki suggested ]
 - added proper error handling in scm_send() for scm_set_cred() return value [ found by me during rework ]
 - don't do get_pid() in __scm_replace_pid() [ as Kuniyuki suggested ]
 - move __scm_replace_pid() from scm.h to scm.c [ as Kuniyuki suggested ]
 - fixed kdoc for unix_maybe_add_creds() [ thanks to Kuniyuki's review ]
 - added RWB tags from Christian and Kuniyuki

Links to previous versions:
v2: https://lore.kernel.org/netdev/20250701083922.97928-1-aleksandr.mikhalitsyn@canonical.com (local)
tree: https://github.com/mihalicyn/linux/commits/scm_pidfd_stale.v2
v1: https://lore.kernel.org/netdev/20250629214449.14462-1-aleksandr.mikhalitsyn@canonical.com (local)
tree: https://github.com/mihalicyn/linux/commits/scm_pidfd_stale.v1

/!\ Notice
Series based on https://git.kernel.org/pub/scm/linux/kernel/git/vfs/vfs.git/log/?h=vfs-6.17.pidfs
It does not use pidfs_get_pid()/pidfs_put_pid() API as these were removed in a scope of [2].
I've checked that net-next branch currently (still) has these obsolete functions, but it
will eventually include changes from [2], so it's not a big problem.

Link: https://lore.kernel.org/all/20250425-work-pidfs-net-v2-0-450a19461e75@kernel.org/ (local) [1]
Link: https://lore.kernel.org/all/20250618-work-pidfs-persistent-v2-0-98f3456fd552@kernel.org/ (local) [2]

Cc: linux-kernel@vger.kernel.org
Cc: netdev@vger.kernel.org
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: Paolo Abeni <pabeni@redhat.com>
Cc: Willem de Bruijn <willemb@google.com>
Cc: Leon Romanovsky <leon@kernel.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Kuniyuki Iwashima <kuniyu@google.com>
Cc: Lennart Poettering <redacted>
Cc: Luca Boccassi <redacted>
Cc: David Rheinsberg <david@readahead.eu>

Alexander Mikhalitsyn (7):
  af_unix: rework unix_maybe_add_creds() to allow sleep
  af_unix: introduce unix_skb_to_scm helper
  af_unix: introduce and use scm_replace_pid() helper
  af_unix/scm: fix whitespace errors
  af_unix: stash pidfs dentry when needed
  af_unix: enable handing out pidfds for reaped tasks in SCM_PIDFD
  selftests: net: extend SCM_PIDFD test to cover stale pidfds

 include/net/scm.h                             |   4 +-
 net/core/scm.c                                |  32 ++-
 net/unix/af_unix.c                            |  57 +++--
 .../testing/selftests/net/af_unix/scm_pidfd.c | 217 ++++++++++++++----
 4 files changed, 247 insertions(+), 63 deletions(-)

-- 
2.43.0