From: Matt Bobrowski <mattbobrowski@google.com>
To: Shuran Liu <electronlsr@gmail.com>
Cc: song@kernel.org, bpf@vger.kernel.org, ast@kernel.org,
daniel@iogearbox.net, andrii@kernel.org, martin.lau@linux.dev,
eddyz87@gmail.com, yonghong.song@linux.dev,
john.fastabend@gmail.com, kpsingh@kernel.org, sdf@fomichev.me,
haoluo@google.com, jolsa@kernel.org, rostedt@goodmis.org,
mhiramat@kernel.org, mathieu.desnoyers@efficios.com,
linux-kernel@vger.kernel.org, linux-trace-kernel@vger.kernel.org,
Zesen Liu <ftyg@live.com>, Peili Gao <gplhust955@gmail.com>,
Haoran Ni <haoran.ni.cs@gmail.com>
Subject: Re: [PATCH bpf 1/2] bpf: mark bpf_d_path() buffer as writeable
Date: Mon, 1 Dec 2025 18:48:33 +0000 [thread overview]
Message-ID: <aS3jARS7a-gh9UCa@google.com> (raw)
In-Reply-To: <20251201143813.5212-2-electronlsr@gmail.com>
On Mon, Dec 01, 2025 at 10:38:12PM +0800, Shuran Liu wrote:
> Commit 37cce22dbd51 ("bpf: verifier: Refactor helper access type
> tracking") started distinguishing read vs write accesses performed by
> helpers.
>
> The second argument of bpf_d_path() is a pointer to a buffer that the
> helper fills with the resulting path. However, its prototype currently
> uses ARG_PTR_TO_MEM without MEM_WRITE.
>
> Before 37cce22dbd51, helper accesses were conservatively treated as
> potential writes, so this mismatch did not cause issues. Since that
> commit, the verifier may incorrectly assume that the buffer contents
> are unchanged across the helper call and base its optimizations on this
> wrong assumption. This can lead to misbehaviour in BPF programs that
> read back the buffer, such as prefix comparisons on the returned path.
>
> Fix this by marking the second argument of bpf_d_path() as
> ARG_PTR_TO_MEM | MEM_WRITE so that the verifier correctly models the
> write to the caller-provided buffer.
>
> Fixes: 37cce22dbd51 ("bpf: verifier: Refactor helper access type tracking")
> Co-developed-by: Zesen Liu <ftyg@live.com>
> Signed-off-by: Zesen Liu <ftyg@live.com>
> Co-developed-by: Peili Gao <gplhust955@gmail.com>
> Signed-off-by: Peili Gao <gplhust955@gmail.com>
> Co-developed-by: Haoran Ni <haoran.ni.cs@gmail.com>
> Signed-off-by: Haoran Ni <haoran.ni.cs@gmail.com>
> Signed-off-by: Shuran Liu <electronlsr@gmail.com>
Reviewed-by: Matt Bobrowski <mattbobrowski@google.com>
> ---
> kernel/trace/bpf_trace.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
> index 4f87c16d915a..49e0bdaa7a1b 100644
> --- a/kernel/trace/bpf_trace.c
> +++ b/kernel/trace/bpf_trace.c
> @@ -965,7 +965,7 @@ static const struct bpf_func_proto bpf_d_path_proto = {
> .ret_type = RET_INTEGER,
> .arg1_type = ARG_PTR_TO_BTF_ID,
> .arg1_btf_id = &bpf_d_path_btf_ids[0],
> - .arg2_type = ARG_PTR_TO_MEM,
> + .arg2_type = ARG_PTR_TO_MEM | MEM_WRITE,
> .arg3_type = ARG_CONST_SIZE_OR_ZERO,
> .allowed = bpf_d_path_allowed,
> };
> --
> 2.52.0
>
next prev parent reply other threads:[~2025-12-01 18:48 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2025-12-01 14:38 [PATCH bpf 0/2] bpf: fix bpf_d_path() helper prototype Shuran Liu
2025-12-01 14:38 ` [PATCH bpf 1/2] bpf: mark bpf_d_path() buffer as writeable Shuran Liu
2025-12-01 18:48 ` Matt Bobrowski [this message]
2025-12-01 14:38 ` [PATCH bpf 2/2] selftests/bpf: add regression test for bpf_d_path() Shuran Liu
2025-12-01 19:16 ` Matt Bobrowski
2025-12-01 19:22 ` [PATCH bpf 0/2] bpf: fix bpf_d_path() helper prototype Matt Bobrowski
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