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

[Will Deacon <will.deacon@arm.com>]

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@arm.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.

Will