[PATCH RFC 0/3] pidfd: add task path ioctls

[Chen Linxuan via B4 Relay <devnull+me.black-desk.cn@kernel.org>]

Obtaining a target task's executable, working directory, or root
currently requires walking procfs symlinks such as /proc/<pid>/exe,
/proc/<pid>/cwd, and /proc/<pid>/root.  That makes the operation depend
on procfs being mounted and visible to the caller, even when it already
holds a pidfd for the target.

This series adds PIDFD_GET_EXE, PIDFD_GET_CWD, and PIDFD_GET_ROOT. Each
ioctl takes no argument and returns a close-on-exec O_PATH file
descriptor referencing the corresponding task path.  The new ioctls use
the same ptrace permission check and nonzero-argument rejection as the
existing pidfd namespace ioctls.

The target task is sampled while holding its exec_update_lock. This
keeps the access decision and the task-state read in the same exec
critical section, preventing a concurrent execve() from changing the
credentials or target state between the check and the use.

The first patch factors out helpers for acquiring referenced task paths
and reuses them in procfs and AppArmor.  The second patch introduces
scoped cleanup for privileged pidfd task access and separates namespace
lookup from namespace fd creation.  The final patch uses these pieces to
implement the three new ioctls.

Signed-off-by: Chen Linxuan <me@black-desk.cn>
---
Chen Linxuan (3):
      fs: Introduce task path helpers
      pidfd: Use scoped cleanup for task access
      pidfd: Add task path ioctls

 fs/fs_struct.c             |  44 +++++++++++++
 fs/pidfs.c                 | 161 ++++++++++++++++++++++++++++++++-------------
 fs/proc/base.c             |  34 +---------
 include/linux/fs_struct.h  |   3 +
 include/linux/mm.h         |   1 +
 include/uapi/linux/pidfd.h |   7 ++
 kernel/fork.c              |  21 ++++++
 security/apparmor/task.c   |  12 +---
 8 files changed, 196 insertions(+), 87 deletions(-)
---
base-commit: a4ff2be345d0abc943da8dd8da98151843b750dc
change-id: 20260819-pidfd-get-paths-59640d8cd1ee

Best regards,
--  
Chen Linxuan <me@black-desk.cn>