linux-kernel - Re: [PATCH] perf tools: don't adjust symbols in vDSO

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Date:	Fri, 26 Jun 2015 13:23:08 +0100
From:	Will Deacon <will.deacon@....com>
To:	Adrian Hunter <adrian.hunter@...el.com>
Cc:	"acme@...hat.com" <acme@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Kristina Martsenko <Kristina.Martsenko@....com>,
	Vladimir Nikulichev <nvs@...icks.com>,
	Namhyung Kim <namhyung@...nel.org>
Subject: Re: [PATCH] perf tools: don't adjust symbols in vDSO

On Thu, Jun 25, 2015 at 02:32:13PM +0100, Adrian Hunter wrote:
> On 24/06/15 19:17, Will Deacon wrote:
> > Commit 922d0e4d9f04 ("perf tools: Adjust symbols in VDSO") changed the
> > ELF symbol parsing so that the vDSO is treated the same as ET_EXEC and
> > ET_REL binaries despite being an ET_DYN.
> > 
> > This causes objdump, which expects relative addresses, not to produce
> > any output in conjunction with perf annotate, which cheerfully passes
> > absolute addresses when trying to disassemble vDSO functions.
> > 
> > This patch avoids marking the vDSO as requiring adjustment of symbol
> > addresses, allowing the relative program counter to be used instead.
> > 
> > Cc: Vladimir Nikulichev <nvs@...icks.com>
> > Cc: Adrian Hunter <adrian.hunter@...el.com>
> > Cc: Namhyung Kim <namhyung@...nel.org>
> > Reported-by: Kristina Martsenko <kristina.martsenko@....com>
> > Signed-off-by: Will Deacon <will.deacon@....com>
> > ---
> > 
> > Not sure why I've just started seeing this, but it appears to affect
> > both x86 and arm64. Also, if I revert the patch above then the issue
> > it supposedly fixed doesn't resurface. Maybe it was just masking another
> > bug that has since been addressed?
> 
> No the problem still appears on older kernels.

Can you be more specific, please? I tried with a 3.16 kernel (that I happen
to be running on my box) but perf doesn't even detect the vdso there,
regardless of this patch.

> Probably could look at the vdso section/program headers to decide if it
> needs adjustment or not.

Did the x86 kernel change in this regard? Why isn't the vDSO always ET_DYN?

Will
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