On Mon, Mar 29, 2021 at 2:38 AM Richard Biener <rguent...@suse.de> wrote: > > On Mon, 29 Mar 2021, Jiri Slaby wrote: > > > Any ideas on this? > > > > On 11. 01. 21, 7:31, Jiri Slaby wrote: > > > 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. > > I just checked and the x86_64 psABI doesn't say anything about .plt.sec
The second PLT is documented in section 13.2 Dynamic Linking in x86-64 psABI. Please see elf_x86_64_get_synthetic_symtab in binutils for PLT symbol processing. -- H.J.
