linux-kernel - Re: [RFC PATCH 2/8] perf probe powerpc: Fix symbol fixup issues due to ELF type

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Date:	Wed, 10 Dec 2014 20:50:50 +1100
From:	Michael Ellerman <mpe@...erman.id.au>
To:	"Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com>
Cc:	linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org,
	acme@...nel.org, ananth@...ibm.com
Subject: Re: [RFC PATCH 2/8] perf probe powerpc: Fix symbol fixup issues due
 to ELF type

On Tue, 2014-12-09 at 23:04 +0530, Naveen N. Rao wrote:
> If using the symbol table, symbol addresses are not being fixed up
> properly, resulting in probes being placed at wrong addresses:
> 
>   # perf probe do_fork
>   Added new event:
>     probe:do_fork        (on do_fork)
> 
>   You can now use it in all perf tools, such as:
> 
> 	  perf record -e probe:do_fork -aR sleep 1
> 
>   # cat /sys/kernel/debug/tracing/kprobe_events
>   p:probe/do_fork _text+635952
>   # printf "%x" 635952
>   9b430
>   # grep do_fork /boot/System.map
>   c0000000000ab430 T .do_fork

OK, but why is that happening? And why does checking for ET_DYN fix it?

> Fix by checking for ELF type ET_DYN used by ppc64 kernels.

We sometimes produce ET_DYN kernels, but only if CONFIG_RELOCATABLE=y.

cheers


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