linux-kernel - Re: [PATCH v4 17/39] unwind_user/sframe: Add support for reading .sframe headers

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Message-ID: <20250124223909.sw7nfdjx4da46vyp@jpoimboe>
Date: Fri, 24 Jan 2025 14:39:09 -0800
From: Josh Poimboeuf <jpoimboe@...nel.org>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: Andrii Nakryiko <andrii.nakryiko@...il.com>,
	Indu Bhagat <indu.bhagat@...cle.com>, x86@...nel.org,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	linux-kernel@...r.kernel.org, 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.kernel.org,
	Jens Remus <jremus@...ux.ibm.com>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Florian Weimer <fweimer@...hat.com>,
	Andy Lutomirski <luto@...nel.org>,
	Masami Hiramatsu <mhiramat@...nel.org>,
	Weinan Liu <wnliu@...gle.com>
Subject: Re: [PATCH v4 17/39] unwind_user/sframe: Add support for reading
 .sframe headers

On Fri, Jan 24, 2025 at 03:13:40PM -0500, Steven Rostedt wrote:
> On Fri, 24 Jan 2025 11:21:59 -0800
> Josh Poimboeuf <jpoimboe@...nel.org> wrote:
> 
> > > given SFRAME_F_FRAME_POINTER in the header, is it really that
> > > nonsensical and illegal to have zero FDEs/FREs? Maybe we should allow
> > > that?  
> > 
> > It would seem a bit silly to create an empty .sframe section just to set
> > that SFRAME_F_FRAME_POINTER bit.  Regardless, there's nothing the kernel
> > can do with that.
> > 
> > > > +               dbg("no fde/fre entries\n");
> > > > +               return -EINVAL;
> > > > +       }
> > > > +
> > > > +       header_end = sec->sframe_start + SFRAME_HEADER_SIZE(shdr);
> > > > +       if (header_end >= sec->sframe_end) {  
> > > 
> > > if we allow zero FDEs/FREs, header_end == sec->sframe_end is legal, right?  
> > 
> > I suppose so, but again I'm not seeing any reason to support that.
> 
> Hmm, could that be useful for implementing a way to dynamically grow or
> shrink an sframe because of jits? I'm just thinking about placeholders or
> sohething.

Maybe?

I was thinking the kernel would have sframe_section placeholders for JIT
code sections, so when sframe_find() retrieves the struct for a given
IP, it sees the JIT flag is set along with a pointer to the in-memory
shared "sframe section", then goes to read that to get the corresponding
sframe entry (insert erratic hand waving).

It's still early days but it's quite possible the in-memory "sframe
section" formats might end up looking pretty different from the .sframe
file section spec.

-- 
Josh