[PATCH v3 0/4] pidfd: add task path ioctls

[Chen Linxuan via B4 Relay]

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.

Since pidfds can cross pid namespace boundaries, the first patch
restricts the existing namespace ioctls to targets visible in the
caller's active pid namespace, failing with -EREMOTE like
PIDFD_GET_INFO.  The second patch factors out helpers for acquiring
referenced task paths and reuses them in procfs and AppArmor.  The
third patch introduces scoped cleanup for privileged pidfd task
access, carries the visibility check into the shared task acquisition
helper, and separates namespace lookup from namespace fd creation.
The final patch uses these pieces to implement the three new ioctls.

---
Changes in v3:
- Restrict the pidfd task access ioctls to targets visible in the
  caller's pid namespace, failing with -EREMOTE consistent with
  PIDFD_GET_INFO, following review feedback from Jann Horn and
  Christian Brauner.
- Document that get_task_exe_path() leaves the output path unchanged
  on failure, as suggested by Jann Horn.
- Link to v2: https://patch.msgid.link/20260831-pidfd-get-paths-v2-0-c59ea6a21b72@black-desk.cn

Changes in v2:
- Make pidfd_get_task_locked() own the task reference until it is
  transferred to the pidfd_task_locked cleanup class.
- Link to v1: https://patch.msgid.link/20260820-pidfd-get-paths-v1-0-ac3eee4003d5@black-desk.cn

To: Alexander Viro <viro at zeniv.linux.org.uk>
To: Christian Brauner <brauner at kernel.org>
To: Jan Kara <jack at suse.cz>
To: Andrew Morton <akpm at linux-foundation.org>
To: David Hildenbrand <david at kernel.org>
To: Lorenzo Stoakes <ljs at kernel.org>
To: "Liam R. Howlett" <liam at infradead.org>
To: Vlastimil Babka <vbabka at kernel.org>
To: Mike Rapoport <rppt at kernel.org>
To: Suren Baghdasaryan <surenb at google.com>
To: Michal Hocko <mhocko at suse.com>
To: Ingo Molnar <mingo at redhat.com>
To: Peter Zijlstra <peterz at infradead.org>
To: Juri Lelli <juri.lelli at redhat.com>
To: Vincent Guittot <vincent.guittot at linaro.org>
To: Dietmar Eggemann <dietmar.eggemann at arm.com>
To: Steven Rostedt <rostedt at goodmis.org>
To: Ben Segall <bsegall at google.com>
To: Mel Gorman <mgorman at suse.de>
To: Valentin Schneider <vschneid at redhat.com>
To: K Prateek Nayak <kprateek.nayak at amd.com>
To: Kees Cook <kees at kernel.org>
To: John Johansen <john.johansen at canonical.com>
To: Georgia Garcia <georgia.garcia at canonical.com>
To: Paul Moore <paul at paul-moore.com>
To: James Morris <jmorris at namei.org>
To: "Serge E. Hallyn" <serge at hallyn.com>
To: Jann Horn <jannh at google.com>
Cc: linux-fsdevel at vger.kernel.org
Cc: linux-kernel at vger.kernel.org
Cc: linux-mm at kvack.org
Cc: apparmor at lists.ubuntu.com
Cc: linux-security-module at vger.kernel.org
Cc: linux-api at vger.kernel.org

---
Chen Linxuan (4):
      pidfd: restrict task access ioctls to the caller's pid namespace
      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                 | 158 +++++++++++++++++++++++++++++++++------------
 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              |  22 +++++++
 security/apparmor/task.c   |  12 +---
 8 files changed, 198 insertions(+), 83 deletions(-)
---
base-commit: cee9395acd8043be0644b25c34bfa86623f2b935
change-id: 20260819-pidfd-get-paths-59640d8cd1ee

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