Warn if the probe target function is a GNU indirect function (GNU_IFUNC)
because it may not be what the user wants to probe.

The GNU indirect function ( https://sourceware.org/glibc/wiki/GNU_IFUNC )
is the dynamic symbol solved at runtime. An 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 the same as the IFUNC in
the symbol table. This can confuse users trying to probe such functions.

For example, memcpy is an IFUNC.

  probe_libc:memcpy    (on __new_memcpy_ifunc@x86_64/multiarch/memcpy.c in /usr/lib64/libc-2.30.so)

the probe is put on an IFUNC.

  perf  1742 [000] 26201.715632: probe_libc:memcpy: (7fdaa53824c0)
              7fdaa53824c0 __new_memcpy_ifunc+0x0 (inlined)
              7fdaa5d4a980 elf_machine_rela+0x6c0 (inlined)
              7fdaa5d4a980 elf_dynamic_do_Rela+0x6c0 (inlined)
              7fdaa5d4a980 _dl_relocate_object+0x6c0 (/usr/lib64/ld-2.30.so)
              7fdaa5d42155 dl_main+0x1cc5 (/usr/lib64/ld-2.30.so)
              7fdaa5d5831a _dl_sysdep_start+0x54a (/usr/lib64/ld-2.30.so)
              7fdaa5d3ffeb _dl_start_final+0x25b (inlined)
              7fdaa5d3ffeb _dl_start+0x25b (/usr/lib64/ld-2.30.so)
              7fdaa5d3f117 .annobin_rtld.c+0x7 (inlined)

And the event is invoked from the ELF loader instead of the target
program's main code.

Moreover, at this moment, we can not probe on the function which will
be selected by the IFUNC, because it is determined at runtime. But
uprobe will be prepared before running the target binary.

Thus, I decided to warn user when 'perf probe' detects that the probe
point is on an GNU IFUNC symbol. Someone who wants to probe an IFUNC
symbol to debug the IFUNC function can ignore this warning.

Committer notes:

I.e., this warning will be emitted if the probe point is an IFUNC:

  "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"

Complete set of steps:

  # readelf -sW /lib64/libc-2.29.so  | grep IFUNC | tail
   22196: 0000000000109a80   183 IFUNC   GLOBAL DEFAULT   14 __memcpy_chk
   22214: 00000000000b7d90   191 IFUNC   GLOBAL DEFAULT   14 __gettimeofday
   22336: 000000000008b690    60 IFUNC   GLOBAL DEFAULT   14 memchr
   22350: 000000000008b9b0    89 IFUNC   GLOBAL DEFAULT   14 __stpcpy
   22420: 000000000008bb10    76 IFUNC   GLOBAL DEFAULT   14 __strcasecmp_l
   22582: 000000000008a970    60 IFUNC   GLOBAL DEFAULT   14 strlen
   22585: 00000000000a54d0    92 IFUNC   WEAK   DEFAULT   14 wmemset
   22600: 000000000010b030    92 IFUNC   GLOBAL DEFAULT   14 __wmemset_chk
   22618: 000000000008b8a0   183 IFUNC   GLOBAL DEFAULT   14 __mempcpy
   22675: 000000000008ba70    76 IFUNC   WEAK   DEFAULT   14 strcasecmp
  #
  # perf probe -x /lib64/libc-2.29.so strlen
  Warning: The probe function (strlen) is a GNU indirect function.
  Consider identifying the final function used at run time and set the probe directly on that.
  Added new event:
    probe_libc:strlen    (on strlen in /usr/lib64/libc-2.29.so)

  You can now use it in all perf tools, such as:

  	perf record -e probe_libc:strlen -aR sleep 1

  #
Reported-by: Andi Kleen <andi@firstfloor.org>
Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
Link: http://lore.kernel.org/lkml/159438669349.62703.5978345670436126948.stgit@devnote2Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>