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

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Date:   Thu, 13 Dec 2018 19:45:46 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Song Liu <songliubraving@...com>,
        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 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.