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

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Message-ID: <20250124151340.2c173332@gandalf.local.home>
Date: Fri, 24 Jan 2025 15:13:40 -0500
From: Steven Rostedt <rostedt@...dmis.org>
To: Josh Poimboeuf <jpoimboe@...nel.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, 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
something.

-- Steve