linux-kernel - Re: [PATCH v3 09/19] unwind: Introduce sframe user space unwinding

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Message-ID: <CAEf4Bza6QZt=N8=O7NU3saHpJ_XrXRdGn48gVJMN+kawurNP3g@mail.gmail.com>
Date: Fri, 1 Nov 2024 12:44:13 -0700
From: Andrii Nakryiko <andrii.nakryiko@...il.com>
To: Josh Poimboeuf <jpoimboe@...nel.org>
Cc: x86@...nel.org, Peter Zijlstra <peterz@...radead.org>, 
	Steven Rostedt <rostedt@...dmis.org>, Ingo Molnar <mingo@...nel.org>, 
	Arnaldo Carvalho de Melo <acme@...nel.org>, linux-kernel@...r.kernel.org, 
	Indu Bhagat <indu.bhagat@...cle.com>, Mark Rutland <mark.rutland@....com>, 
	Alexander Shishkin <alexander.shishkin@...ux.intel.com>, Jiri Olsa <jolsa@...nel.org>, 
	Namhyung Kim <namhyung@...nel.org>, Ian Rogers <irogers@...gle.com>, 
	Adrian Hunter <adrian.hunter@...el.com>, linux-perf-users@...r.kernel.org, 
	Mark Brown <broonie@...nel.org>, linux-toolchains@...r.kernel.org, 
	Jordan Rome <jordalgo@...a.com>, Sam James <sam@...too.org>, linux-trace-kernel@...r.kerne.org, 
	Jens Remus <jremus@...ux.ibm.com>, Mathieu Desnoyers <mathieu.desnoyers@...icios.com>, 
	Florian Weimer <fweimer@...hat.com>, Andy Lutomirski <luto@...nel.org>, bpf <bpf@...r.kernel.org>
Subject: Re: [PATCH v3 09/19] unwind: Introduce sframe user space unwinding

On Fri, Nov 1, 2024 at 12:29 PM Josh Poimboeuf <jpoimboe@...nel.org> wrote:
>
> On Fri, Nov 01, 2024 at 11:34:48AM -0700, Andrii Nakryiko wrote:
> > 00200000-170ad000 r--p 00000000 07:01 5
> > 172ac000-498e7000 r-xp 16eac000 07:01 5
> > 49ae7000-49b8b000 r--p 494e7000 07:01 5
> > 49d8b000-4a228000 rw-p 4958b000 07:01 5
> > 4a228000-4c677000 rw-p 00000000 00:00 0
> > 4c800000-4ca00000 r-xp 49c00000 07:01 5
> > 4ca00000-4f600000 r-xp 49e00000 07:01 5
> > 4f600000-5b270000 r-xp 4ca00000 07:01 5
> >
> > Sorry, I'm probably dense and missing something. But from the example
> > process above, isn't this check violated already? Or it's two
> > different things? Not sure, honestly.
>
> It's hard to tell exactly what's going on, did you strip the file names?

Yes, I did, of course. But as I said, they all belong to the same main
binary of the process.

>
> The sframe limitation is per file, not per address space.  I assume
> these are one file:
>
> > 172ac000-498e7000 r-xp 16eac000 07:01 5
>
> and these are another:
>
> > 4c800000-4ca00000 r-xp 49c00000 07:01 5
> > 4ca00000-4f600000 r-xp 49e00000 07:01 5
> > 4f600000-5b270000 r-xp 4ca00000 07:01 5
>
> Multiple mappings for a single file is fine, as long as they're
> contiguous.

No all of what I posted above belongs to the same file (except
"4a228000-4c677000 rw-p 00000000 00:00 0" which doesn't have
associated file, but I suspect it originally was part of this file, we
do some tricks with re-mmap()'ing stuff due to huge pages usage).

>
> > > Actually I just double checked and even the kernel's ELF loader assumes
> > > that each executable has only a single text start+end address pair.
> >
> > See above, very confused by such assumptions, but I'm hoping we are
> > talking about two different things here.
>
> The "contiguous text" thing seems enforced by the kernel for
> executables.  However it doesn't manage shared libraries, those are
> mapped by the loader, e.g. /lib64/ld-linux-x86-64.so.2.
>
> At a quick glance I can't tell if /lib64/ld-linux-x86-64.so.2 enforces
> that.
>
> > > There's no point in adding complexity to support some hypothetical.  I
> > > can remove the printk though.
> >
> > We are talking about fundamental things like format for supporting
> > frame pointer-less stack trace capture. It will take years to adopt
> > SFrame everywhere, so I think it's prudent to think a bit ahead beyond
> > just saying "no real application should need more than 4GB text", IMO.
>
> I don't think anybody is saying that...
>
> --
> Josh