[PATCH v3 00/10] net: support thread-specific pidfds for send and connect

[Andrew Halaney <andrew@amutable.com>]

SO_PASSPIDFD/SCM_PIDFD and SO_PEERPIDFD allow to retrieve a pidfd for
the thread-group leader. The coredump server using the coredump socket
cannot get a handle on the task that took the signal and is writing the
coredump easily. Workloads interested in per-thread authentification
have similar problems.

Add SO_PASSPIDFD_THREAD/SCM_PIDFD_THREAD and SO_PEERPIDFD_THREAD. We
record the sending and the connecting thread in addition to the
thread-group leader.

SO_PASSPIDFD_THREAD functions like SO_PASSPIDFD but instead of an
SCM_PIDFD message for the thread-group leader, SCM_PIDFD_THREAD sends a
pidfd for the specific thread. SO_PASSPIDFD / SO_PASSPIDFD_THREAD are
independent, allowing callers to get both styles of pidfd back in their
cmsg.

SO_PEERPIDFD_THREAD allows to retrieve a pidfd for the specific thread
that called connect(), listen(), or socketpair().

pidfs_coredump() now also stamps the dumping thread's struct pid so a
pidfd for that thread reports the coredump like the pidfd of the
thread-group leader does.

Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>
Signed-off-by: Andrew Halaney <andrew@amutable.com>

---
Changes in v3:
 - I picked this series up from Christian at his request (thanks!)
 - Make SO_PASSPIDFD and SO_PASSPIDFD_THREAD completely independent (and
   in doing so add SCM_PIDFD_THREAD) instead of mutually exclusive. This goes
   against Kuniyuki's suggestion for a separate flag option, which I
   argued the mutually exclusive nature of the prior implementation +
   that would be confusing in v2 thread. I think with it being
   independent things are more clear, and useful, and it matches the
   SO_PEERPIDFD_THREAD counterpart nicely. For that reason I dropped
   Reviewed-by tags present on the implementation / selftests patches
   related to that.
 - Address grammar, reverse xmas tree, sk_is_unix() feedback from
   Kuniyuki Iwashima
 - Collect tags
 - Rebase on v7.3-rc5 for Christian to make shared branch for net to
   pull from
 - Link to v2: https://lore.kernel.org/r/20260909-work-unix-passpidfd-v2-0-7bd342abb2d1@kernel.org

Changes in v2:
- Fix a few documentation issues.
- Link to v1: https://patch.msgid.link/20260831-work-unix-passpidfd-v1-0-70cbfda0c7ba@kernel.org

---
Christian Brauner (10):
      pid: add helpers to operate on a struct pid array
      af_unix: record the pid of the sending thread
      net: add SO_PASSPIDFD_THREAD to get a thread-specific SCM_PIDFD
      selftests/net: SO_PASSPIDFD_THREAD
      net: turn sk_peer_pid into an array indexed by pid type
      af_unix: record the pid of the connecting thread
      net: add SO_PEERPIDFD_THREAD to get a thread-specific pidfd
      selftests/net: SO_PEERPIDFD_THREAD
      pidfs: record the coredump on the dumping thread's pid too
      selftests/coredump: check the dumping thread's pidfd

 arch/alpha/include/uapi/asm/socket.h               |   4 +
 arch/mips/include/uapi/asm/socket.h                |   4 +
 arch/parisc/include/uapi/asm/socket.h              |   4 +
 arch/sparc/include/uapi/asm/socket.h               |   4 +
 fs/coredump.c                                      |  22 +-
 fs/pidfs.c                                         |  27 +-
 include/linux/coredump.h                           |   4 +-
 include/linux/pid.h                                |  53 ++
 include/linux/pid_types.h                          |   8 +
 include/linux/pidfs.h                              |   5 +
 include/linux/sched/signal.h                       |  18 +
 include/linux/socket.h                             |   9 +-
 include/net/scm.h                                  |   7 +-
 include/net/sock.h                                 |  18 +-
 include/trace/events/landlock.h                    |   2 +-
 include/uapi/asm-generic/socket.h                  |   4 +
 net/bluetooth/af_bluetooth.c                       |   6 +-
 net/bluetooth/hci_sock.c                           |   8 +-
 net/bluetooth/l2cap_sock.c                         |   2 +-
 net/core/scm.c                                     |  44 +-
 net/core/sock.c                                    | 108 ++--
 net/unix/af_unix.c                                 |  68 +--
 net/unix/af_unix.h                                 |   3 +-
 tools/lib/python/kdoc/xforms_lists.py              |   1 +
 .../selftests/coredump/coredump_socket_test.c      | 175 ++++++
 tools/testing/selftests/coredump/coredump_test.h   |   2 +
 .../selftests/coredump/coredump_test_helpers.c     |  49 ++
 tools/testing/selftests/net/af_unix/Makefile       |   2 +
 tools/testing/selftests/net/af_unix/scm_pidfd.c    | 602 +++++++++++++++++++++
 29 files changed, 1149 insertions(+), 114 deletions(-)
---
base-commit: 72d3fcf802c45d00b300f25b848a93c3a2bd7c7e
change-id: 20260825-work-unix-passpidfd-086a1ec09e8a

Best regards,
-- 
Andrew Halaney <andrew@amutable.com>