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

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Message-ID: <CABWLseu6=ZSYpQncaj=0EeVzKtP8vjPyOBJkWcFb-dxbm5OfVQ@mail.gmail.com>
Date: Mon, 27 Jan 2025 17:53:59 -0500
From: Andrei Matei <andreimatei1@...il.com>
To: Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc: Levi Zim <rsworktech@...look.com>, 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 Mon, Jan 27, 2025 at 5:04 PM Alexei Starovoitov
<alexei.starovoitov@...il.com> 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.

For our uprobes, we've run into similar issues around doing variable-sized
bpf_probe_read_user() into ring buffers for our debugger [1]. Our use case
is that we generate uprobes that recursively read data structures until we
fill up a buffer. The verifier's insistence on knowing statically that a read
fits into the buffer makes for awkward code, and makes it hard to pack the
buffer fully; we have to split our reads into a couple of static size classes.

Any chance there'd be interest in taking the opportunity to support
dynamically-sized reads from userspace too? :)

[1] https://side-eye.io

>
> 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?
>