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

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Message-ID: <20150627090354.GA29944@arm.com>
Date:	Sat, 27 Jun 2015 10:03:54 +0100
From:	Will Deacon <will.deacon@....com>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Adrian Hunter <adrian.hunter@...el.com>,
	"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 Fri, Jun 26, 2015 at 04:44:14PM +0100, Andy Lutomirski wrote:
> On Fri, Jun 26, 2015 at 8:29 AM, Will Deacon <will.deacon@....com> wrote:
> > On Fri, Jun 26, 2015 at 04:18:59PM +0100, Andy Lutomirski wrote:
> >> On Fri, Jun 26, 2015 at 7:58 AM, Will Deacon <will.deacon@....com> wrote:
> >> > (CC'ing Andy, since the removal of VDSO_PRELINK is user-visible here)
> >>
> >> What arch is this?  I removed VDSO_PRELINK entirely from x86 a while
> >> back, and now x86's vdso has a base address of 0 before relocations,
> >> and everything works just fine.
> >
> > I think this is only x86, since it's the removal of VDSO_PRELINK that
> > has changed things.
> >
> >> (Except one ancient glibc, which fails if the vdso is relocated at
> >> all.  We no longer support that version of glibc unless you turn off
> >> the vdso entirely.)
> >
> > The problem is that perf expects to objdump portions of the vdso using
> > --start-address=foo and --stop-address=bar, but these addresses have changed
> > from being offset by VDSO_PRELINK to 0x0.
> >
> > Thankfully, it looks like perf tool was always broken in this regard, but
> > I figured you might like to be aware of the issue. I guess perf just needs
> > to add on the load address of the vdso .text section to its relative
> > addresses before passing them to objdump.
> >
> 
> Given that we've randomized the vdso load address on x86 for years, I
> don't see how perf ever worked here.  There was a brief period during
> which we actually loaded the vdso at the address VDSO_PRELINK, but
> that's long gone.

The symbol resolution works by applying the vdso symbol offset to the
[vdso] vma base, but perf annotate had indeed been broken the whole time.

> Is there a patch I should be looking at?

I've cooked something, so I'll send it out now.

Will
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