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

From: Song Liu
Date: Wed Dec 12 2018 - 13:59:05 EST

> On Dec 12, 2018, at 10:33 AM, Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:
> On Wed, 12 Dec 2018 19:05:53 +0100
> Peter Zijlstra <peterz@xxxxxxxxxxxxx> 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).
> -- Steve

Currently, BPF programs are not executed during init.