perf does not resolve plt symbols from libstdc++ right (.plt.sec problem)

[Jiri Slaby]

Hi,

this e-mails is a follow-up of my report at:
https://bugzilla.suse.com/show_bug.cgi?id=1180681

There is a problem with *@plt symbols in some libraries, they are 
unresolved by perf (memcmp@plt in this case):
>     0.26%  main2    /usr/lib64/libstdc++.so.6.0.28            0xa51a0 
           l [.] 0x00000000000a51a0

On the other hand, plt symbols in other libraries are fine (memset@plt 
in this case):
>     0.17%  main2    /usr/lib64/libantlr4-runtime.so.4.8       0x4ed10 
           l [.] memset@plt

I dumped memcmp's .plt.rela entries in perf:
/usr/lib64/libantlr4-runtime.so.4.8: 154th addr=4e9d0 plt_off=4e020 
hdr=10 entry=10
/usr/lib64/libstdc++.so.6.0.28: 772th addr=a1070 plt_off=9e020 hdr=10 
entry=10

The difference (offset) of stdc++'s memcmp is 0xa51a0 (correct) - 
0xa1070 (perf's computed) = 0x4130.

The problem is perf assumes nth entry of .plt.rela to correspond to nth 
function in .plt, but memcmp is in .plt.sec in libstdc++.so:

> Relocation section '.rela.plt' at offset 0x97900 contains 1018 entries:
>     Offset             Info             Type               Symbol's 
Value  Symbol's Name + Addend
> ...
> 00000000001dc838  0000007800000007 R_X86_64_JUMP_SLOT 
0000000000000000 memcmp@GLIBC_2.2.5 + 0

Perf does this with the rela entries:
https://github.com/torvalds/linux/blob/f5e6c330254ae691f6d7befe61c786eb5056007e/tools/perf/util/symbol-elf.c#L385

It takes a symbol index from sym.r_info. Then it resolves its name from 
.dynsym, appending "@plt" to it. Then this name is added to perf's 
symbol table along with address which is computed as .rela.plt index 
multiplied by entry size (shdr_plt.sh_entsize) plus plt header 
(shdr_plt.sh_entsize on x86_64 too).

And from this comes (almost) the offset above:
> $ objdump -h /usr/lib64/libstdc++.so.6|grep -E ' .plt(\.sec)? '
>  12 .plt          00003fb0  000000000009e020  000000000009e020 
0009e020  2**4
>  14 .plt.sec      00003fa0  00000000000a2160  00000000000a2160 
000a2160  2**4

0xa2160-0x9e020 = 0x4140. I assume the 0x10 difference is that perf adds 
shdr_plt.sh_entsize (0x10) to the offset to skip the first .plt entry 
(header).

Richard writes:
======
.plt.sec is IIRC the "second" (sec) PLT entry - the one that will be 
used on the second call (and on).  This is used / emitted for ELF object 
instrumented for Intel CET.  The details escape me for the moment but I 
hope the x86 ABI documents this (and the constraints) in detail.
======

How should perf find out whether to consider .plt or .plt.sec? Or 
generally, how to properly find an address of *@plt symbols like 
memcmp@plt above?

thanks,
--
js
suse labs