netdev - Re: Re: [PATCH v9 tip 3/9] tracing: attach BPF programs to kprobes

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Message-ID: <550E9421.7030507@hitachi.com>
Date:	Sun, 22 Mar 2015 19:06:25 +0900
From:	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>
To:	Alexei Starovoitov <ast@...mgrid.com>
CC:	Ingo Molnar <mingo@...nel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Namhyung Kim <namhyung@...nel.org>,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	Jiri Olsa <jolsa@...hat.com>,
	"David S. Miller" <davem@...emloft.net>,
	Daniel Borkmann <daniel@...earbox.net>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	linux-api@...r.kernel.org, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: Re: [PATCH v9 tip 3/9] tracing: attach BPF programs to kprobes

(2015/03/22 1:02), Alexei Starovoitov wrote:
> On 3/21/15 5:14 AM, Masami Hiramatsu wrote:
>> (2015/03/21 8:30), Alexei Starovoitov wrote:
>>>
>>> Note, kprobes are _not_ a stable kernel ABI, so bpf programs attached to
>>> kprobes must be recompiled for every kernel version and user must supply correct
>>> LINUX_VERSION_CODE in attr.kern_version during bpf_prog_load() call.
>>>
>>
>> Would you mean that the ABI of kprobe-based BPF programs? Kprobe API/ABIs
>> (register_kprobe() etc.) are stable, but the code who use kprobes certainly
>> depends the kernel binary by design. So, if you meant it, BPF programs must
>> be recompiled for every kernel binaries (including configuration changes,
>> not only its version).
> 
> yes. I mainly meant that bpf+kprobe programs must be recompiled
> for every kernel binary.

Hmm, if so, as we do in perf (and systemtap too), you'd better check
kernel's build-id instead of the kernel version when loading the
BPF program. It is safer than the KERNEL_VERSION_CODE.

> But you're incorrect saying that register_kprobe API is stable.
> It's equally kernel dependent.
> register_kprobe(struct kprobe *p) is export_gpl, but it takes
> kernel internal 'struct kprobe' and it's not declared in uapi header.
> Prototype of kprobe_handler_t is also kernel internal, so whoever
> is using kprobes must recompile their code every time.
> If we want, we can change register_kprobe function name to something
> else. Just like kernel modules cannot expect that exported symbols
> will stay around from version to version. We don't care when we
> break out of tree modules.

Indeed the register_kprobe (and other kprobe related functions)
should not be treated as a stable userland exported ABI/API. :)

Thank you,

-- 
Masami HIRAMATSU
Software Platform Research Dept. Linux Technology Research Center
Hitachi, Ltd., Yokohama Research Laboratory
E-mail: masami.hiramatsu.pt@...achi.com


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