Merge branch 'bpf-fix-bpf_d_path-helper-prototype'

Shuran Liu says:

====================
bpf: fix bpf_d_path() helper prototype

Hi,

This series fixes a verifier issue with bpf_d_path() and adds a
regression test to cover its use within a hook function.

Patch 1 updates the bpf_d_path() helper prototype so that the second
argument is marked as MEM_WRITE. This makes it explicit to the verifier
that the helper writes into the provided buffer.

Patch 2 extends the existing d_path selftest to cover incorrect verifier
assumptions caused by an incorrect function prototype. The test program calls
bpf_d_path() and checks if the first character of the path can be read.
It ensures the verifier does not assume the buffer remains unwritten.

Changelog
=========

v5:
  - Moved the temporary file for the fallocate test from /tmp to /dev/shm
    Since bpf CI's 9P filesystem under /tmp does not support fallocate.

v4:
  - Use the fallocate hook instead of an LSM hook to simplify the selftest,
    as suggested by Matt and Alexei.
  - Add a utility function in test_d_path.c to load the BPF program,
    improving code reuse.

v3:
  - Switch the pathname prefix loop to use bpf_for() instead of
    #pragma unroll, as suggested by Matt.
  - Remove /tmp/bpf_d_path_test in the test cleanup path.
  - Add the missing Reviewed-by tags.

v2:
  - Merge the new test into the existing d_path selftest rather than
  creating new files.
  - Add PID filtering in the LSM program to avoid nondeterministic failures
  due to unrelated processes triggering bprm_check_security.
  - Synchronize child execution using a pipe to ensure deterministic
  updates to the PID.

Thanks for your time and reviews.
====================

Link: https://patch.msgid.link/20251206141210.3148-1-electronlsr@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>