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

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Message-ID: <CAADnVQJ7bw0Qa4UM_E0zb5bqt5P09f7rryFSe6faY8ibX0zWuA@mail.gmail.com>
Date: Mon, 27 Jan 2025 14:04:34 -0800
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: Levi Zim <rsworktech@...look.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 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.
Also why bpf_probe_read_kernel_common ?
Do we need to memset() it on failure?