linux-kernel - Re: [PATCH bpf-next] bpf: clear user buf when bpf_d

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Message-ID: <6d0e5cd2-bb4e-45af-bf85-0b95d5d2ac18@linux.dev>
Date: Fri, 13 Jun 2025 10:37:38 +0800
From: Tao Chen <chen.dylane@...ux.dev>
To: Andrii Nakryiko <andrii.nakryiko@...il.com>,
 Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc: KP Singh <kpsingh@...nel.org>, Matt Bobrowski <mattbobrowski@...gle.com>,
 Alexei Starovoitov <ast@...nel.org>, Daniel Borkmann <daniel@...earbox.net>,
 Andrii Nakryiko <andrii@...nel.org>, Martin KaFai Lau
 <martin.lau@...ux.dev>, Eduard <eddyz87@...il.com>,
 Song Liu <song@...nel.org>, Yonghong Song <yonghong.song@...ux.dev>,
 John Fastabend <john.fastabend@...il.com>,
 Stanislav Fomichev <sdf@...ichev.me>, Hao Luo <haoluo@...gle.com>,
 Jiri Olsa <jolsa@...nel.org>, Steven Rostedt <rostedt@...dmis.org>,
 Masami Hiramatsu <mhiramat@...nel.org>,
 Mathieu Desnoyers <mathieu.desnoyers@...icios.com>, bpf
 <bpf@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>,
 linux-trace-kernel <linux-trace-kernel@...r.kernel.org>
Subject: Re: [PATCH bpf-next] bpf: clear user buf when bpf_d_path failed

在 2025/6/13 08:06, Andrii Nakryiko 写道:
> On Thu, Jun 12, 2025 at 4:56 PM Alexei Starovoitov
> <alexei.starovoitov@...il.com> wrote:
>>
>> On Thu, Jun 12, 2025 at 4:27 PM Andrii Nakryiko
>> <andrii.nakryiko@...il.com> wrote:
>>>
>>> On Thu, Jun 12, 2025 at 2:40 PM Alexei Starovoitov
>>> <alexei.starovoitov@...il.com> wrote:
>>>>
>>>> On Thu, Jun 12, 2025 at 2:29 PM Andrii Nakryiko
>>>> <andrii.nakryiko@...il.com> wrote:
>>>>>
>>>>> On Wed, Jun 11, 2025 at 8:49 AM Tao Chen <chen.dylane@...ux.dev> wrote:
>>>>>>
>>>>>> The bpf_d_path() function may fail. If it does,
>>>>>> clear the user buf, like bpf_probe_read etc.
>>>>>>
>>>>>
>>>>> But that doesn't mean we *have to* do memset(0) for bpf_d_path(),
>>>>> though. Especially given that path buffer can be pretty large (4KB).
>>>>>
>>>>> Is there an issue you are trying to address with this, or is it more
>>>>> of a consistency clean up? Note, that more or less recently we made
>>>>> this zero filling behavior an option with an extra flag
>>>>> (BPF_F_PAD_ZEROS) for newer APIs. And if anything, bpf_d_path() is
>>>>> more akin to variable-sized string probing APIs rather than
>>>>> fixed-sized bpf_probe_read* family.
>>>>
>>>> All old helpers had this BPF_F_PAD_ZEROS behavior
>>>> (or rather should have had).
>>>> So it makes sense to zero in this helper too for consistency.
>>>> I don't share performance concerns. This is an error path.
>>>
>>> It's just a bizarre behavior as it stands right now.
>>>
>>> On error, you'll have a zeroed out buffer, OK, good so far.
>>>
>>> On success, though, you'll have a buffer where first N bytes are
>>> filled out with good path information, but then the last sizeof(buf) -
>>> N bytes would be, effectively, garbage.
>>>
>>> All in all, you can't use that buffer as a key for hashmap looking
>>> (because of leftover non-zeroed bytes at the end), yet on error we
>>> still zero out bytes for no apparently useful reason.
>>>
>>> And then for the bpf_path_d_path(). What do we do about that one? It
>>> doesn't have zeroing out either in the error path, nor in the success
>>> path. So just more inconsistency all around.
>>
>> Consistency with bpf_path_d_path() kfunc is indeed missing.
>>
>> Ok, since you insist, dropped this patch, and force pushed.
> 
> Great, thank you!

The changes in this patch are relatively simple, but the discussion 
between the two of you is more meaningful to me. I agree with Andrii's 
point of view. Thank you both for discussing this patch.

-- 
Best Regards
Tao Chen