linux-kernel - Re: [PATCH bpf-next] bpf: Add bpf_read_raw

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Date:   Fri, 26 Aug 2022 20:52:13 +0000
From:   Song Liu <songliubraving@...com>
To:     Namhyung Kim <namhyung@...nel.org>
CC:     Song Liu <song@...nel.org>,
        Andrii Nakryiko <andrii.nakryiko@...il.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>, Martin Lau <kafai@...com>,
        Yonghong Song <yhs@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>,
        Stanislav Fomichev <sdf@...gle.com>,
        Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        "bpf@...r.kernel.org" <bpf@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH bpf-next] bpf: Add bpf_read_raw_record() helper



> On Aug 26, 2022, at 12:21 PM, Namhyung Kim <namhyung@...nel.org> wrote:
> 
> On Fri, Aug 26, 2022 at 11:09 AM Song Liu <songliubraving@...com> wrote:
>> 
>> 
>> 
>>> On Aug 26, 2022, at 9:33 AM, Namhyung Kim <namhyung@...nel.org> wrote:
>>> 
>>> On Thu, Aug 25, 2022 at 10:53 PM Song Liu <song@...nel.org> wrote:
>>>> 
>>>> On Thu, Aug 25, 2022 at 10:22 PM Namhyung Kim <namhyung@...nel.org> wrote:
>>>>> 
>>>>> On Thu, Aug 25, 2022 at 7:35 PM Song Liu <songliubraving@...com> wrote:
>>>>>> Actually, since we are on this, can we make it more generic, and handle
>>>>>> all possible PERF_SAMPLE_* (in enum perf_event_sample_format)? Something
>>>>>> like:
>>>>>> 
>>>>>> long bpf_perf_event_read_sample(void *ctx, void *buf, u64 size, u64 flags);
>>>>>> 
>>>>>> WDYT Namhyung?
>>>>> 
>>>>> Do you mean reading the whole sample data at once?
>>>>> Then it needs to parse the sample data format properly
>>>>> which is non trivial due to a number of variable length
>>>>> fields like callchains and branch stack, etc.
>>>>> 
>>>>> Also I'm afraid I might need event configuration info
>>>>> other than sample data like attr.type, attr.config,
>>>>> attr.sample_type and so on.
>>>>> 
>>>>> Hmm.. maybe we can add it to the ctx directly like ctx.attr_type?
>>>> 
>>>> The user should have access to the perf_event_attr used to
>>>> create the event. This is also available in ctx->event->attr.
>>> 
>>> Do you mean from BPF?  I'd like to have a generic BPF program
>>> that can handle various filtering according to the command line
>>> arguments.  I'm not sure but it might do something differently
>>> for each event based on the attr settings.
>> 
>> Yeah, we can access perf_event_attr from BPF program. Note that
>> the ctx for perf_event bpf program is struct bpf_perf_event_data_kern:
>> 
>> SEC("perf_event")
>> int perf_e(struct bpf_perf_event_data_kern *ctx)
>> {
>>        ...
>> }
>> 
>> struct bpf_perf_event_data_kern {
>>        bpf_user_pt_regs_t *regs;
>>        struct perf_sample_data *data;
>>        struct perf_event *event;
>> };
> 
> I didn't know that it's allowed to access the kernel data directly.
> For some reason, I thought it should use fields in bpf_event_event_data
> only, like sample_period and addr.  And the verifier will convert the
> access to them according to pe_prog_convert_ctx_access().

We can bypass pe_prog_convert_ctx_access() with something like:

	struct perf_event *event;
	u64 config;

        bpf_probe_read_kernel(&event, sizeof(void *), &ctx->event);
        bpf_probe_read_kernel(&config, sizeof(u64), &event->attr.config);

Thanks,
Song