linux-kernel - Re: [PATCH bpf 1/2] bpf: mark bpf_d

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Message-ID: <aS3jARS7a-gh9UCa@google.com>
Date: Mon, 1 Dec 2025 18:48:33 +0000
From: Matt Bobrowski <mattbobrowski@...gle.com>
To: Shuran Liu <electronlsr@...il.com>
Cc: song@...nel.org, bpf@...r.kernel.org, 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,
	haoluo@...gle.com, jolsa@...nel.org, rostedt@...dmis.org,
	mhiramat@...nel.org, mathieu.desnoyers@...icios.com,
	linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org,
	Zesen Liu <ftyg@...e.com>, Peili Gao <gplhust955@...il.com>,
	Haoran Ni <haoran.ni.cs@...il.com>
Subject: Re: [PATCH bpf 1/2] bpf: mark bpf_d_path() buffer as writeable

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@...e.com>
> Signed-off-by: Zesen Liu <ftyg@...e.com>
> Co-developed-by: Peili Gao <gplhust955@...il.com>
> Signed-off-by: Peili Gao <gplhust955@...il.com>
> Co-developed-by: Haoran Ni <haoran.ni.cs@...il.com>
> Signed-off-by: Haoran Ni <haoran.ni.cs@...il.com>
> Signed-off-by: Shuran Liu <electronlsr@...il.com>

Reviewed-by: Matt Bobrowski <mattbobrowski@...gle.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
>