linux-kernel - Re: [PATCH bpf-next v2 1/7] bpf: Implement bpf_probe_read_kernel

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Message-ID:
 <MEYP282MB23123214D4F50F0F622C4697C6EF2@MEYP282MB2312.AUSP282.PROD.OUTLOOK.COM>
Date: Tue, 28 Jan 2025 19:13:12 +0800
From: Levi Zim <rsworktech@...look.com>
To: Alexei Starovoitov <alexei.starovoitov@...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>,
 Mykola Lysenko <mykolal@...com>, Shuah Khan <shuah@...nel.org>,
 bpf <bpf@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>,
 linux-trace-kernel <linux-trace-kernel@...r.kernel.org>,
 "open list:KERNEL SELFTEST FRAMEWORK" <linux-kselftest@...r.kernel.org>,
 Andrii Nakryiko <andrii.nakryiko@...il.com>
Subject: Re: [PATCH bpf-next v2 1/7] bpf: Implement
 bpf_probe_read_kernel_dynptr helper

On 2025/1/28 06:04, Alexei Starovoitov wrote:
> On Sat, Jan 25, 2025 at 5:05 PM Levi Zim <rsworktech@...look.com> wrote:
>> On 2025/1/26 00:58, Alexei Starovoitov wrote:
>>   > On Sat, Jan 25, 2025 at 12:30 AM Levi Zim via B4 Relay
>>   > <devnull+rsworktech.outlook.com@...nel.org> wrote:
>>   >> From: Levi Zim <rsworktech@...look.com>
>>   >>
>>   >> This patch add a helper function bpf_probe_read_kernel_dynptr:
>>   >>
>>   >> long bpf_probe_read_kernel_dynptr(const struct bpf_dynptr *dst,
>>   >>          u32 offset, u32 size, const void *unsafe_ptr, u64 flags);
>>   > We stopped adding helpers years ago.
>>   > Only new kfuncs are allowed.
>>
>> Sorry, I didn't know that. Just asking, is there any
>> documentation/discussion
>> about stopping adding helpers?
>>
>> I will switch the implementation to kfuncs in v3.
>>
>>   > This particular one doesn't look useful as-is.
>>   > The same logic can be expressed with
>>   > - create dynptr
>>   > - dynptr_slice
>>   > - copy_from_kernel
>>
>> By copy_from_kernel I assume you mean bpf_probe_read_kernel. The problem
>> with dynptr_slice_rdwr and probe_read_kernel is that they only support a
>> compile-time constant size [1].
>>
>> But in order to best utilize the space on a BPF ringbuf, it is possible
>> to reserve a
>> variable length of space as dynptr on a ringbuf with
>> bpf_ringbuf_reserve_dynptr.
> That makes sense. The commit log didn't call it out.
> Please spell out the motivation clearly.
Thanks for the advice! I will include it in v3.
> Also why bpf_probe_read_kernel_common ?
> Do we need to memset() it on failure?
Since the current patch is basically a thin wrapper around 
bpf_probe_read_kernel,
I think we'd better not deviate from the wrapped function.