netdev - Re: [PATCH v3 perf, bpf-next 1/4] perf, bpf: Introduce PERF_RECORD_BPF

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Message-ID: <3FC2A5A4-9502-4B33-A944-BEB14B520E23@fb.com>
Date:   Thu, 13 Dec 2018 21:48:57 +0000
From:   Song Liu <songliubraving@...com>
To:     Peter Zijlstra <peterz@...radead.org>
CC:     Steven Rostedt <rostedt@...dmis.org>,
        lkml <linux-kernel@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "ast@...nel.org" <ast@...nel.org>,
        "daniel@...earbox.net" <daniel@...earbox.net>,
        Kernel Team <Kernel-team@...com>,
        "acme@...nel.org" <acme@...nel.org>
Subject: Re: [PATCH v3 perf, bpf-next 1/4] perf, bpf: Introduce
 PERF_RECORD_BPF_EVENT



> On Dec 13, 2018, at 10:45 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> 
> On Wed, Dec 12, 2018 at 01:33:20PM -0500, Steven Rostedt wrote:
>> On Wed, 12 Dec 2018 19:05:53 +0100
>> Peter Zijlstra <peterz@...radead.org> wrote:
>> 
>>> On Wed, Dec 12, 2018 at 05:09:17PM +0000, Song Liu wrote:
>>>>> And while this tracks the bpf kallsyms, it does not do all kallsyms.
>>>>> 
>>>>> .... Oooh, I see the problem, everybody is doing their own custom
>>>>> kallsym_{add,del}() thing, instead of having that in generic code :-(
>>>>> 
>>>>> This, for example, doesn't track module load/unload nor ftrace
>>>>> trampolines, even though both affect kallsyms.  
>>>> 
>>>> I think we can use PERF_RECORD_MMAP(or MMAP2) for module load/unload. 
>>>> That could be separate sets of patches.   
>>> 
>>> So I would actually like to move bpf_lock/bpf_kallsyms/bpf_tree +
>>> bpf_prog_kallsyms_*() + __bpf_address_lookup() into kernel/kallsyms.c
>>> and also have ftrace use that.
>>> 
>>> Because currently the ftrace stuff is otherwise invisible.
>>> 
>>> A generic kallsym register/unregister for any JIT.
>> 
>> That's if it needs to look up the symbols that were recorded when init
>> was unloaded.
>> 
>> The ftrace kallsyms is used to save the function names of init code
>> that was freed, but may have been recorded. With out the ftrace
>> kallsyms the functions traced at init time would just show up as hex
>> addresses (not very useful).
>> 
>> I'm not sure how BPF would need those symbols unless they were executed
>> during init (module or core) and needed to see what the symbols use to
>> be).
> 
> Aah, that sounds entirely dodgy and possibly quite broken. We freed that
> init code, so BPF or your trampolines (or a tiny module) could actually
> fit in there and insert their own kallsyms, and then we have overlapping
> symbols, which would be pretty bad.
> 
> I thought the ftrace kallsym stuff was for the trampolines, which would
> be fairly similar to what BPF is doing. And why I'm trying to get a
> generic dynamic kallsym thing sorted. There's bound the be other
> code-gen things at some point.

Hi Peter, 

I guess you are looking for something for all ksym add/delete events, like;

      /*
       * PERF_RECORD_KSYMBOL
       *
       * struct {
       *      struct perf_event_header header;
       *      u64                             addr;
       *      u32                             len;
       *      u16                             ksym_type;
       *      u16                             flags;
       *      char                            name[];
       *      struct sample_id                sample_id;
       * };
       */

We can use ksym_type to encode BPF_EVENT, trampolines, or other type of ksym.
We can use flags or header.misc to encode ksym add/delete. Is this right?

If we go this direction, shall we reserve a few more bytes in it for different
types to use, like:

      /*
       * PERF_RECORD_KSYMBOL
       *
       * struct {
       *      struct perf_event_header header;
       *      u64                             addr;
       *      u32                             len;
       *      u16                             ksym_type;
       *      u16                             flags;
       *      u64                             data[2];
       *      char                            name[];
       *      struct sample_id                sample_id;
       * };
       */

Thanks,
Song