linux-kernel - [PATCH bpf 0/2] bpf: fix bpf_d

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Message-ID: <20251201143813.5212-1-electronlsr@gmail.com>
Date: Mon,  1 Dec 2025 22:38:11 +0800
From: Shuran Liu <electronlsr@...il.com>
To: song@...nel.org,
	mattbobrowski@...gle.com,
	bpf@...r.kernel.org
Cc: ast@...nel.org,
	daniel@...earbox.net,
	andrii@...nel.org,
	martin.lau@...ux.dev,
	eddyz87@...il.com,
	yonghong.song@...ux.dev,
	john.fastabend@...il.com,
	kpsingh@...nel.org,
	sdf@...ichev.me,
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	mhiramat@...nel.org,
	mathieu.desnoyers@...icios.com,
	linux-kernel@...r.kernel.org,
	linux-trace-kernel@...r.kernel.org,
	electronlsr@...il.com
Subject: [PATCH bpf 0/2] bpf: fix bpf_d_path() helper prototype

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

-- 
2.52.0