linux-kernel - Re: [PATCH v2 4/4] perf-probe: Warn if the target function is GNU Indirect function

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Date:   Fri, 10 Jul 2020 14:25:29 -0300
From:   Arnaldo Carvalho de Melo <acme@...nel.org>
To:     Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
Cc:     Masami Hiramatsu <mhiramat@...nel.org>,
        Arnaldo Carvalho de Melo <acme@...hat.com>,
        Oleg Nesterov <oleg@...hat.com>, linux-kernel@...r.kernel.org,
        Andi Kleen <andi@...stfloor.org>,
        Andi Kleen <ak@...ux.intel.com>
Subject: Re: [PATCH v2 4/4] perf-probe: Warn if the target function is GNU
 Indirect function

Em Fri, Jul 10, 2020 at 07:27:12PM +0530, Srikar Dronamraju escreveu:
> * Masami Hiramatsu <mhiramat@...nel.org> [2020-07-10 22:11:33]:
> 
> > Warn if the probe target function is GNU indirect function (GNU_IFUNC)
> > because it may not what the user want to probe.
> > 
> > The GNU indirect function ( https://sourceware.org/glibc/wiki/GNU_IFUNC )
> > is the dynamic solved symbol at runtime. IFUNC function is a selector
> > which is invoked from the elf loader, but the symbol address of the
> > function which will be modified by the IFUNC is same as the IFUNC in
> > the symbol table. This can confuse users who is trying to probe on
> > such functions.
> > 
> > For example, the memcpy is one of IFUNC.
> > 
> > # perf probe -x /lib64/libc-2.30.so -a memcpy
> > # perf probe -l
> >   probe_libc:memcpy    (on __new_memcpy_ifunc@..._64/multiarch/memcpy.c in /usr/lib64/libc-2.30.so)
> > 
> > the probe is put on a IFUNC.
> > 
> > # perf record -e probe_libc:memcpy --call-graph dwarf -aR ./perf
> > 
> > Thus, I decided to warn user when the perf probe detects the probe point
> > is on the GNU IFUNC symbol. Someone who wants to probe an IFUNC symbol to
> > debug the IFUNC function, they can ignore this warning.
> > 
> > Reported-by: Andi Kleen <andi@...stfloor.org>
> > Signed-off-by: Masami Hiramatsu <mhiramat@...nel.org>
> 
> Looks good to me.
> 
> Reviewed-by: Srikar Dronamraju <srikar@...ux.vnet.ibm.com>

Thanks, applied.

- Arnaldo