[PATCH bpf v2 0/2] bpf: fix bpf_d_path() helper prototype

[Shuran Liu]

Hi,

this series fixes a verifier regression for bpf_d_path() introduced by
commit 37cce22dbd51 ("bpf: verifier: Refactor helper access type
tracking") and adds a small selftest to exercise the helper from an
LSM program.

Commit 37cce22dbd51 started distinguishing read vs write accesses
performed by helpers. bpf_d_path()'s buffer argument was left as
ARG_PTR_TO_MEM without MEM_WRITE, so the verifier could incorrectly
assume that the buffer contents are unchanged across the helper call
and base its optimizations on this wrong assumption.

In practice this showed up as a misbehaving LSM BPF program that calls
bpf_d_path() and then does a simple prefix comparison on the returned
path: the program would sometimes take the "mismatch" branch even
though both bytes being compared were actually equal.

Patch 1 fixes bpf_d_path()'s helper prototype by marking the buffer
argument as ARG_PTR_TO_MEM | MEM_WRITE, so that the verifier correctly
models the write to the caller-provided buffer.

Patch 2 adds a regression test that exercises bpf_d_path() from an LSM
program attached to bprm_check_security. The test verifies that pathname
prefix comparisons behave correctly with the fix applied.

Changes in 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,
Shuran Liu

Shuran Liu (2):
  bpf: mark bpf_d_path() buffer as writeable
  selftests/bpf: fix and consolidate d_path LSM regression test

 kernel/trace/bpf_trace.c                      |  2 +-
 .../testing/selftests/bpf/prog_tests/d_path.c | 64 +++++++++++++++++++
 .../testing/selftests/bpf/progs/test_d_path.c | 33 ++++++++++
 3 files changed, 98 insertions(+), 1 deletion(-)

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2.52.0