[PATCH v2 00/10] net: support thread-specific pidfds for send and connect

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 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 sends a pidfd
for the specific thread. SO_PASSPIDFD_THREAD is mutually exclusive with
SO_PASSPIDFD. The last one set takes precedence and disables the other
one. Both SO_PASSCRED and SO_PASSPIDFD receivers see one writer per
process as before.

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>
---
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/net/scm.h                                  |   7 +-
 include/net/sock.h                                 |  14 +-
 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                                     |  36 +-
 net/core/sock.c                                    | 120 ++++---
 net/unix/af_unix.c                                 |  67 ++--
 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    | 366 +++++++++++++++++++++
 28 files changed, 907 insertions(+), 110 deletions(-)
---
base-commit: cee9395acd8043be0644b25c34bfa86623f2b935
change-id: 20260825-work-unix-passpidfd-086a1ec09e8a