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

From: Stephane Eranian
Date: Mon Nov 23 2015 - 12:36:49 EST

On Fri, Nov 20, 2015 at 1:18 PM, Brian Robbins <brianrob@xxxxxxxxxxxxx> wrote:
> 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.
The debug info section is optional and contains the line table info. This
can be used should you want to display source code along with assembly.

>> 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?
If you have a new binary format, I think you'd have to write the parser 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.
I will post V8 this week.
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