Hey Linus,
/* Summary */
This contains updates for pidfs:
- Allow retrieving exit information after a process has been reaped
through pidfds via the new PIDFD_INTO_EXIT extension for the
PIDFD_GET_INFO ioctl. Various tools need access to information about a
process/task even after it has already been reaped.
Pidfd polling allows waiting on either task exit or for a task to have
been reaped. The contract for PIDFD_INFO_EXIT is simply that EPOLLHUP
must be observed before exit information can be retrieved, i.e., exit
information is only provided once the task has been reaped and then
can be retrieved as long as the pidfd is open.
- Add PIDFD_SELF_{THREAD,THREAD_GROUP} sentinels allowing userspace to forgo
allocating a file descriptor for their own process. This is useful in
scenarios where users want to act on their own process through pidfds and is
akin to AT_FDCWD.
- Improve premature thread-group leader and subthread exec behavior when
polling on pidfds:
(1) During a multi-threaded exec by a subthread, i.e., non-thread-group
leader thread, all other threads in the thread-group including the
thread-group leader are killed and the struct pid of the
thread-group leader will be taken over by the subthread that called
exec. IOW, two tasks change their TIDs.
(2) A premature thread-group leader exit means that the thread-group
leader exited before all of the other subthreads in the thread-group
have exited.
Both cases lead to inconsistencies for pidfd polling with PIDFD_THREAD.
Any caller that holds a PIDFD_THREAD pidfd to the current thread-group
leader may or may not see an exit notification on the file descriptor
depending on when poll is performed. If the poll is performed before the
exec of the subthread has concluded an exit notification is generated
for the old thread-group leader. If the poll is performed after the exec
of the subthread has concluded no exit notification is generated for the
old thread-group leader.
The correct behavior is to simply not generate an exit notification on
the struct pid of a subhthread exec because the struct pid is taken
over by the subthread and thus remains alive.
But this is difficult to handle because a thread-group may exit
premature as mentioned in (2). In that case an exit notification is
reliably generated but the subthreads may continue to run for an
indeterminate amount of time and thus also may exec at some point.
After this pull no exit notifications will be generated for a
PIDFD_THREAD pidfd for a thread-group leader until all subthreads have
been reaped. If a subthread should exec before no exit notification
will be generated until that task exits or it creates subthreads and
repeates the cycle.
This means an exit notification indicates the ability for the father
to reap the child.
/* Testing */
gcc version 14.2.0 (Debian 14.2.0-6)
Debian clang version 16.0.6 (27+b1)
No build failures or warnings were observed.
/* Conflicts */
Merge conflicts with mainline
=============================
No known conflicts.
Merge conflicts with other trees
================================
No known conflicts.
The following changes since commit 2014c95afecee3e76ca4a56956a936e23283f05b:
Linux 6.14-rc1 (2025-02-02 15:39:26 -0800)
are available in the Git repository at:
git@xxxxxxxxxxxxxxxxxxx:pub/scm/linux/kernel/git/vfs/vfs tags/vfs-6.15-rc1.pidfs
for you to fetch changes up to d40dc30c7b7c80db2100b73ac26d39c362643a39:
Merge patch series "pidfs: handle multi-threaded exec and premature thread-group leader exit" (2025-03-20 15:32:51 +0100)
Please consider pulling these changes from the signed vfs-6.15-rc1.pidfs tag.
Thanks!
Christian
----------------------------------------------------------------
vfs-6.15-rc1.pidfs
----------------------------------------------------------------
Christian Brauner (25):
selftests/pidfd: add new PIDFD_SELF* defines
Merge patch series "introduce PIDFD_SELF* sentinels"
pidfs: switch to copy_struct_to_user()
pidfd: rely on automatic cleanup in __pidfd_prepare()
pidfs: move setting flags into pidfs_alloc_file()
pidfs: use private inode slab cache
pidfs: record exit code and cgroupid at exit
pidfs: allow to retrieve exit information
selftests/pidfd: fix header inclusion
pidfs/selftests: ensure correct headers for ioctl handling
selftests/pidfd: expand common pidfd header
selftests/pidfd: add first PIDFD_INFO_EXIT selftest
selftests/pidfd: add second PIDFD_INFO_EXIT selftest
selftests/pidfd: add third PIDFD_INFO_EXIT selftest
selftests/pidfd: add fourth PIDFD_INFO_EXIT selftest
selftests/pidfd: add fifth PIDFD_INFO_EXIT selftest
selftests/pidfd: add sixth PIDFD_INFO_EXIT selftest
selftests/pidfd: add seventh PIDFD_INFO_EXIT selftest
Merge patch series "pidfs: provide information after task has been reaped"
pidfs: ensure that PIDFS_INFO_EXIT is available
pidfs: improve multi-threaded exec and premature thread-group leader exit polling
selftests/pidfd: first test for multi-threaded exec polling
selftests/pidfd: second test for multi-threaded exec polling
selftests/pidfd: third test for multi-threaded exec polling
Merge patch series "pidfs: handle multi-threaded exec and premature thread-group leader exit"
Lorenzo Stoakes (3):
pidfd: add PIDFD_SELF* sentinels to refer to own thread/process
selftests/pidfd: add tests for PIDFD_SELF_*
selftests/mm: use PIDFD_SELF in guard pages test
fs/internal.h | 1 +
fs/libfs.c | 4 +-
fs/pidfs.c | 247 +++++++-
include/linux/pidfs.h | 1 +
include/uapi/linux/pidfd.h | 31 +-
kernel/exit.c | 8 +-
kernel/fork.c | 22 +-
kernel/pid.c | 24 +-
kernel/signal.c | 108 ++--
tools/testing/selftests/mm/guard-pages.c | 16 +-
tools/testing/selftests/pidfd/.gitignore | 2 +
tools/testing/selftests/pidfd/Makefile | 4 +-
tools/testing/selftests/pidfd/pidfd.h | 109 ++++
tools/testing/selftests/pidfd/pidfd_exec_helper.c | 12 +
tools/testing/selftests/pidfd/pidfd_fdinfo_test.c | 1 +
tools/testing/selftests/pidfd/pidfd_info_test.c | 692 ++++++++++++++++++++++
tools/testing/selftests/pidfd/pidfd_open_test.c | 30 +-
tools/testing/selftests/pidfd/pidfd_setns_test.c | 45 --
tools/testing/selftests/pidfd/pidfd_test.c | 76 ++-
19 files changed, 1241 insertions(+), 192 deletions(-)
create mode 100644 tools/testing/selftests/pidfd/pidfd_exec_helper.c
create mode 100644 tools/testing/selftests/pidfd/pidfd_info_test.c
