netdev - Re: [PATCH 23/33] x86/asm/bpf: Create stack frames in bpf

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Date:	Fri, 22 Jan 2016 08:36:38 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:	Josh Poimboeuf <jpoimboe@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
	linux-kernel@...r.kernel.org, live-patching@...r.kernel.org,
	Michal Marek <mmarek@...e.cz>,
	Peter Zijlstra <peterz@...radead.org>,
	Andy Lutomirski <luto@...nel.org>,
	Borislav Petkov <bp@...en8.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andi Kleen <andi@...stfloor.org>,
	Pedro Alves <palves@...hat.com>,
	Namhyung Kim <namhyung@...il.com>,
	Bernd Petrovitsch <bernd@...rovitsch.priv.at>,
	Chris J Arges <chris.j.arges@...onical.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jiri Slaby <jslaby@...e.cz>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Alexei Starovoitov <ast@...nel.org>, netdev@...r.kernel.org,
	daniel@...earbox.net
Subject: Re: [PATCH 23/33] x86/asm/bpf: Create stack frames in bpf_jit.S


* Alexei Starovoitov <alexei.starovoitov@...il.com> wrote:

> > > I could be missing something. I think either this patch is not need or you 
> > > need to teach the tool to ignore all JITed stuff. I don't think it's 
> > > practical to annotate everything. Different JITs do their own magic. s390 
> > > JIT is even more fancy.
> > 
> > Well, but the point of these patches isn't to make the tool happy.  It's 
> > really to make sure that runtime stack traces can be made reliable. Maybe I'm 
> > missing something but I don't see why JIT code can't honor 
> > CONFIG_FRAME_POINTER just like any other code.
> 
> It can if there is no performance cost added. I can speak for x64 JIT, but the 
> rest needs to be analyzed as well. My point was that may be it's easier to 
> ignore all JITed code and just say that such call stacks may be unreliable? 
> live-patching is not applicable to JITed code anyway or you want to livepatch 
> the callees of it?

So the rule is that if frame pointers are enabled all kernel code should have 
correct stack frames - in case an IRQ (or NMI) hits it or it crashes.

Thanks,

	Ingo