[PATCH 1/9] perf symbols: Correct plt entry sizes for x86

Adrian Hunter
2023-01-27T17:03:10Z

In 32-bit executables the .plt entry size can be set to 4 when it is really
16. In fact the only sizes used for x86 (32 or 64 bit) are 8 or 16, so
check for those and, if not, use the alignment to choose which it is.

Example on Ubuntu 22.04 gcc 11.3:

Before:

$ cat tstpltlib.c
void fn1(void) {}
void fn2(void) {}
void fn3(void) {}
void fn4(void) {}
$ cat tstplt.c
void fn1(void);
void fn2(void);
void fn3(void);
void fn4(void);

int main()
{
fn4();
fn1();
fn2();
fn3();
return 0;
}
$ gcc --version
gcc (Ubuntu 11.3.0-1ubuntu1~22.04) 11.3.0
Copyright (C) 2021 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
$ gcc -m32 -Wall -Wextra -shared -o libtstpltlib32.so tstpltlib.c
$ gcc -m32 -Wall -Wextra -o tstplt32 tstplt.c -L . -ltstpltlib32 -Wl,-rpath=$(pwd)
$ perf record -e intel_pt//u --filter 'filter main @ ./tstplt32' ./tstplt32
[ perf record: Woken up 1 times to write data ]
[ perf record: Captured and wrote 0.011 MB perf.data ]
$ readelf -SW tstplt32 | grep 'plt\|Name'
[Nr] Name Type Addr Off Size ES Flg Lk Inf Al
[10] .rel.plt REL 0000041c 00041c 000028 08 AI 5 22 4
[12] .plt PROGBITS 00001030 001030 000060 04 AX 0 0 16 <- ES is 0x04, should be 0x10
[13] .plt.got PROGBITS 00001090 001090 000008 08 AX 0 0 8
$ perf script --itrace=be --ns -F+flags,-event,+addr,-period,-comm,-tid,-cpu,-dso
17894.383903029: tr strt 0 [unknown] => 565b81cd main+0x0
17894.383903029: tr end call 565b81d4 main+0x7 => 565b80d0 __x86.get_pc_thunk.bx+0x0
17894.383903031: tr strt 0 [unknown] => 565b81d9 main+0xc
17894.383903031: tr end call 565b81df main+0x12 => 565b8070 [unknown]
17894.383903032: tr strt 0 [unknown] => 565b81e4 main+0x17
17894.383903032: tr end call 565b81e4 main+0x17 => 565b8050 [unknown]
17894.383903033: tr strt 0 [unknown] => 565b81e9 main+0x1c
17894.383903033: tr end call 565b81e9 main+0x1c => 565b8080 [unknown]
17894.383903033: tr strt 0 [unknown] => 565b81ee main+0x21
17894.383903033: tr end call 565b81ee main+0x21 => 565b8060 [unknown]
17894.383903237: tr strt 0 [unknown] => 565b81f3 main+0x26
17894.383903237: tr end return 565b81fc main+0x2f => f7c21519 [unknown]

After:

$ perf script --itrace=be --ns -F+flags,-event,+addr,-period,-comm,-tid,-cpu,-dso
17894.383903029: tr strt 0 [unknown] => 565b81cd main+0x0
17894.383903029: tr end call 565b81d4 main+0x7 => 565b80d0 __x86.get_pc_thunk.bx+0x0
17894.383903031: tr strt 0 [unknown] => 565b81d9 main+0xc
17894.383903031: tr end call 565b81df main+0x12 => 565b8070 fn4@plt+0x0
17894.383903032: tr strt 0 [unknown] => 565b81e4 main+0x17
17894.383903032: tr end call 565b81e4 main+0x17 => 565b8050 fn1@plt+0x0
17894.383903033: tr strt 0 [unknown] => 565b81e9 main+0x1c
17894.383903033: tr end call 565b81e9 main+0x1c => 565b8080 fn2@plt+0x0
17894.383903033: tr strt 0 [unknown] => 565b81ee main+0x21
17894.383903033: tr end call 565b81ee main+0x21 => 565b8060 fn3@plt+0x0
17894.383903237: tr strt 0 [unknown] => 565b81f3 main+0x26
17894.383903237: tr end return 565b81fc main+0x2f => f7c21519 [unknown]

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
---
tools/perf/util/symbol-elf.c | 17 +++++++++++++----
1 file changed, 13 insertions(+), 4 deletions(-)

diff --git a/tools/perf/util/symbol-elf.c b/tools/perf/util/symbol-elf.c
index aa62735aea7b..9328c162d68f 100644
--- a/tools/perf/util/symbol-elf.c
+++ b/tools/perf/util/symbol-elf.c
@@ -360,14 +360,23 @@ static bool get_plt_sizes(struct dso *dso, GElf_Ehdr *ehdr, GElf_Shdr *shdr_plt,
*plt_header_size = 128;
*plt_entry_size = 32;
return true;
+ case EM_386:
+ case EM_X86_64:
+ *plt_entry_size = shdr_plt->sh_entsize;
+ /* Size is 8 or 16, if not, assume alignment indicates size */
+ if (*plt_entry_size != 8 && *plt_entry_size != 16)
+ *plt_entry_size = shdr_plt->sh_addralign == 8 ? 8 : 16;
+ *plt_header_size = *plt_entry_size;
+ break;
default: /* FIXME: s390/alpha/mips/parisc/poperpc/sh/xtensa need to be checked */
*plt_header_size = shdr_plt->sh_entsize;
*plt_entry_size = shdr_plt->sh_entsize;
- if (*plt_entry_size)
- return true;
- pr_debug("Missing PLT entry size for %s\n", dso->long_name);
- return false;
+ break;
}
+ if (*plt_entry_size)
+ return true;
+ pr_debug("Missing PLT entry size for %s\n", dso->long_name);
+ return false;
}

/*
--
2.34.1