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
