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

[Matt Bobrowski <mattbobrowski@google.com>]

On Mon, Dec 01, 2025 at 10:38:11PM +0800, Shuran Liu wrote:
> 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.

FTR, I strongly encourage any new BPF LSM implementation to consider
using the newer BPF kfunc alternative instead, being
bpf_path_d_path().

> 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.

This is the correct thing to do, appreciate you sending through the
fix.

> 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.

Makes sense to add a test for this regression, but please also see my
comments against this patch.

> 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
> 
> -- 
> 2.52.0
>