* Masami Hiramatsu <mhira...@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 <a...@firstfloor.org>
> Signed-off-by: Masami Hiramatsu <mhira...@kernel.org>
Looks good to me.
Reviewed-by: Srikar Dronamraju <sri...@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
