linux-kernel - Re: [RFC bpf PATCH 0/2] bpf: Fix memory access tags in helper prototypes

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Message-Id: <991C3D59-65D5-4B31-B667-EDAF348F9F7F@gmail.com>
Date: Tue, 23 Dec 2025 11:15:46 +0800
From: Zesen Liu <ftyghome@...il.com>
To: Amery Hung <ameryhung@...il.com>
Cc: Alexei Starovoitov <ast@...nel.org>,
 Daniel Borkmann <daniel@...earbox.net>,
 Andrii Nakryiko <andrii@...nel.org>,
 Martin KaFai Lau <martin.lau@...ux.dev>,
 Eduard Zingerman <eddyz87@...il.com>,
 Song Liu <song@...nel.org>,
 Yonghong Song <yonghong.song@...ux.dev>,
 John Fastabend <john.fastabend@...il.com>,
 KP Singh <kpsingh@...nel.org>,
 Stanislav Fomichev <sdf@...ichev.me>,
 Hao Luo <haoluo@...gle.com>,
 Jiri Olsa <jolsa@...nel.org>,
 Matt Bobrowski <mattbobrowski@...gle.com>,
 Steven Rostedt <rostedt@...dmis.org>,
 Masami Hiramatsu <mhiramat@...nel.org>,
 Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
 "David S. Miller" <davem@...emloft.net>,
 Eric Dumazet <edumazet@...gle.com>,
 Jakub Kicinski <kuba@...nel.org>,
 Paolo Abeni <pabeni@...hat.com>,
 Simon Horman <horms@...nel.org>,
 Daniel Xu <dxu@...uu.xyz>,
 Shuah Khan <shuah@...nel.org>,
 bpf@...r.kernel.org,
 linux-kernel@...r.kernel.org,
 linux-trace-kernel@...r.kernel.org,
 netdev@...r.kernel.org,
 linux-kselftest@...r.kernel.org,
 Shuran Liu <electronlsr@...il.com>,
 Peili Gao <gplhust955@...il.com>,
 Haoran Ni <haoran.ni.cs@...il.com>
Subject: Re: [RFC bpf PATCH 0/2] bpf: Fix memory access tags in helper
 prototypes

Apologies for the resend due to an incorrect configuration in my mail client.

Thanks for your comment!

> On Dec 23, 2025, at 03:29, Amery Hung <ameryhung@...il.com> wrote:
> 
> Perhaps you are conflating one of your proposals here? This is fine
> currently. ARG_PTR_TO_MEM without any annotation is viewed as BPF_READ
> so passing a read-only buffer should work.

Actually, that is not the case. I tested this again, and ARG_PTR_TO_MEM
without MEM_RDONLY does reject read-only buffers. You can reproduce this
behavior with test_d_path_check_rdonly_mem in selftests by removing its
arg2’s MEM_WRITE tag.

> I think it is better to make the MEM_WRITE, MEM_RDONLY annotation
> explicit and check it in the verifier.
> 
> Flipping the default MEM_RDONLY semantic to MEM_WRITE does not prevent
> a similar bug in the future when we have helpers/optimizations/checks
> rely on an implicit semantic.

The current default semantic is in an inconsistent state: it implies
neither MEM_RDONLY nor MEM_WRITE. A naked ARG_PTR_TO_MEM rejects
read-only buffers, yet tells the verifier that the helper does not modify the memory.

I see two ways to resolve this ambiguity:
1) Enforce explicit memory access tags (disallow naked ARG_PTR_TO_MEM) as I
proposed eariler; or
2) Change ARG_PTR_TO_MEM semantics to behave exactly like
   ARG_PTR_TO_MEM | MEM_RDONLY.

I would appreciate your thoughts on this. :)

Thanks,
Zesen Liu