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

[Shuran Liu <electronlsr@gmail.com>]

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 minimal selftest under tools/testing/selftests/bpf that
hooks bprm_check_security, calls bpf_d_path() on a binary under /tmp/,
and verifies that the prefix comparison on the returned path keeps
working.

On my local setup, tools/testing/selftests/bpf does not build fully
due to unrelated tests using newer helpers. I validated this series by
manually reproducing the issue with a small LSM program and by
building and running only the new d_path_lsm test on kernels with and
without patch 1 applied.

Thanks,
Shuran Liu

Shuran Liu (2):
  bpf: mark bpf_d_path() buffer as writeable
  selftests/bpf: add regression test for bpf_d_path()

 kernel/trace/bpf_trace.c                      |  2 +-
 .../selftests/bpf/prog_tests/d_path_lsm.c     | 27 ++++++++++++
 .../selftests/bpf/progs/d_path_lsm.bpf.c      | 43 +++++++++++++++++++
 3 files changed, 71 insertions(+), 1 deletion(-)
 create mode 100644 tools/testing/selftests/bpf/prog_tests/d_path_lsm.c
 create mode 100644 tools/testing/selftests/bpf/progs/d_path_lsm.bpf.c

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2.52.0