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

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Message-ID:
 <MEYP282MB2312A90273FF290ED5FC6F6AC6ED2@MEYP282MB2312.AUSP282.PROD.OUTLOOK.COM>
Date: Sun, 26 Jan 2025 09:05:34 +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 2025/1/26 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.

Then currently we have no way to read a variable length of kernel memory
into this dynptr, except doing it chunk by chunk[2], which is kinda awkward.
That's the problem the new helpers trying to solve.
And I am not the only one needing this kind of feature [3].

Andrii said it would be a straightforward addition as it is a super thin 
wrapper
around existing functionality (we are just avoiding fixed buffer size 
restrictions of
existing probe/copy_from APIs)

[1]: 
https://elixir.bootlin.com/linux/v6.12.6/source/kernel/bpf/helpers.c#L2600-L2601
[2]: 
https://github.com/libbpf/libbpf-bootstrap/commit/046fad60df3e39540937b5ec6ee86054f33d3f28
[3]: https://github.com/libbpf/libbpf-rs/issues/1041
[4]: 
https://lore.kernel.org/bpf/CAEf4BzZctXJsR+TwMhmXNWnR0_BV802-3KJw226ZZt8St4xNkw@mail.gmail.com/

 > pw-bot: cr