linux-kernel - Re: [PATCH] perf: Fallback to JIT support for mmap'd non-ELF binaries.

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Date:	Fri, 20 Nov 2015 13:18:55 -0800 (PST)
From:	Brian Robbins <brianrob@...rosoft.com>
To:	Peter Zijlstra <peterz@...radead.org>
cc:	Brian Robbins <brianrob@...rosoft.com>,
	Ingo Molnar <mingo@...hat.com>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Stephane Eranian <eranian@...gle.com>
Subject: Re: [PATCH] perf: Fallback to JIT support for mmap'd non-ELF
 binaries.



On Fri, 20 Nov 2015, Peter Zijlstra wrote:

> On Thu, Nov 19, 2015 at 11:45:45PM +0000, Brian Robbins wrote:
> 
> > Thank you for the feedback.  The file format is similar to PE, but is
> > not identical.  So, we would be implementing something very scoped,
> > which doesn't feel right to me.
> 
> *groan* you just had to go and invent yet another executable format,
> right? :-)
Nah, it just has some extra stuff in it that makes this less desirable - 
it's technically PE, but anyway.
> 
> > I am interested in the new JIT support, however my understanding from
> > the information that I've read is that it requires kernel support in
> > 4.x, though I can't seem to find where I read that.  I want to make
> > sure that this works on older kernels (3.x) as well.
> 
> As I think Stephane explained, this is only required if you need to
> match up kernel and userspace timestamps, which is important for dynamic
> code generation, less so for static code in a weird format.

Yes, agreed.

> 
> So what the new JIT stuff does is online write 'fake' ELF files with
> symbol sections and (optionally?) dwarf debug info for line numbers.
> 
> Since you don't dynamically generate code, you can offline generate
> these ELF files and redirect the symbol parser bits to that (we already
> look for debug ELF files in various locations), or...
> 
> > The reason I went with this approach is because it is simple for
> > runtimes to implement and has no requirement that perf understand the
> > file format.  I am open to feedback if there is a preferred solution
> > that would still work for older kernels as well.
> 
> Since, someone somewhere needs to go parse this funny new file format
> anyhow to either generate /tmp files or fake ELF files or whatever, you
> might as well put that decoder in perf?
> 
> Or just ship these fake ELF files in /usr/lib/debug/ or whatever the
> 'right' location for the distro at hand is.
>

This seems like a reasonable approach.

Stephane, are your changes available for public consumption?  Last I 
recall, the patches were still in review.

Thanks.
-Brian 
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