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

[Srikar Dronamraju <srikar@linux.vnet.ibm.com>]

* Masami Hiramatsu <mhiramat@kernel.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@x86_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@firstfloor.org>
> Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>

Looks good to me.

Reviewed-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>

> ---
>   Changes in v2:
>    - Check GNU_IFUNC only for uprobe
>    - Show function name instead of the address.
>    - Update the warning message according to Andi's comment.
> ---
>  tools/perf/util/probe-event.c |    8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
> 
> diff --git a/tools/perf/util/probe-event.c b/tools/perf/util/probe-event.c
> index 8cd1224e5f4c..679447f13c20 100644
> --- a/tools/perf/util/probe-event.c
> +++ b/tools/perf/util/probe-event.c
> @@ -375,9 +375,13 @@ static int find_alternative_probe_point(struct debuginfo *dinfo,
> 
>  	/* Find the address of given function */
>  	map__for_each_symbol_by_name(map, pp->function, sym) {
> -		if (uprobes)
> +		if (uprobes) {
>  			address = sym->start;
> -		else
> +			if (sym->type == STT_GNU_IFUNC)
> +				pr_warning("Warning: The probe function (%s) is a GNU indirect function.\n"
> +					   "Consider identifying the final function used at run time and set the probe directly on that.\n",
> +					   pp->function);
> +		} else
>  			address = map->unmap_ip(map, sym->start) - map->reloc;
>  		break;
>  	}
> 

-- 
Thanks and Regards
Srikar Dronamraju