[PATCH 8/9] perf symbols: Start adding support for .plt.got for x86

Adrian Hunter
2023-01-27T17:04:11Z

For x86, .plt.got is used, for example, when the address is taken of a
dynamically linked function. Start adding support by synthesizing a
symbol for each entry. A subsequent patch will attempt to get a better
name for the symbol.

Example:

Before:

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

void callfn(void (*fn)(void))
{
fn();
}

int main()
{
fn4();
fn1();
callfn(fn3);
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 -Wall -Wextra -shared -o libtstpltlib.so tstpltlib.c
$ gcc -Wall -Wextra -o tstpltgot tstpltgot.c -L . -ltstpltlib -Wl,-rpath="$(pwd)"
$ readelf -SW tstpltgot | grep 'Name\|plt\|dyn'
[Nr] Name Type Address Off Size ES Flg Lk Inf Al
[ 6] .dynsym DYNSYM 00000000000003d8 0003d8 0000f0 18 A 7 1 8
[ 7] .dynstr STRTAB 00000000000004c8 0004c8 0000c6 00 A 0 0 1
[10] .rela.dyn RELA 00000000000005d8 0005d8 0000d8 18 A 6 0 8
[11] .rela.plt RELA 00000000000006b0 0006b0 000048 18 AI 6 24 8
[13] .plt PROGBITS 0000000000001020 001020 000040 10 AX 0 0 16
[14] .plt.got PROGBITS 0000000000001060 001060 000020 10 AX 0 0 16
[15] .plt.sec PROGBITS 0000000000001080 001080 000030 10 AX 0 0 16
[23] .dynamic DYNAMIC 0000000000003d90 002d90 000210 10 WA 7 0 8
$ perf record -e intel_pt//u --filter 'filter main @ ./tstpltgot , filter callfn @ ./tstpltgot' ./tstpltgot
[ perf record: Woken up 1 times to write data ]
[ perf record: Captured and wrote 0.011 MB perf.data ]
$ perf script --itrace=be --ns -F+flags,-event,+addr,-period,-comm,-tid,-cpu,-dso
28393.810326915: tr strt 0 [unknown] => 562350baa1b2 main+0x0
28393.810326915: tr end call 562350baa1ba main+0x8 => 562350baa090 fn4@plt+0x0
28393.810326917: tr strt 0 [unknown] => 562350baa1bf main+0xd
28393.810326917: tr end call 562350baa1bf main+0xd => 562350baa080 fn1@plt+0x0
28393.810326917: tr strt 0 [unknown] => 562350baa1c4 main+0x12
28393.810326917: call 562350baa1ce main+0x1c => 562350baa199 callfn+0x0
28393.810326917: tr end call 562350baa1ad callfn+0x14 => 7f607d36110f fn3+0x0
28393.810326922: tr strt 0 [unknown] => 562350baa1af callfn+0x16
28393.810326922: return 562350baa1b1 callfn+0x18 => 562350baa1d3 main+0x21
28393.810326922: tr end call 562350baa1d3 main+0x21 => 562350baa0a0 fn2@plt+0x0
28393.810326924: tr strt 0 [unknown] => 562350baa1d8 main+0x26
28393.810326924: tr end call 562350baa1d8 main+0x26 => 562350baa060 [unknown] <- call to fn3 via .plt.got
28393.810326925: tr strt 0 [unknown] => 562350baa1dd main+0x2b
28393.810326925: tr end return 562350baa1e3 main+0x31 => 7f607d029d90 __libc_start_call_main+0x80

After:

$ perf script --itrace=be --ns -F+flags,-event,+addr,-period,-comm,-tid,-cpu,-dso
28393.810326915: tr strt 0 [unknown] => 562350baa1b2 main+0x0
28393.810326915: tr end call 562350baa1ba main+0x8 => 562350baa090 fn4@plt+0x0
28393.810326917: tr strt 0 [unknown] => 562350baa1bf main+0xd
28393.810326917: tr end call 562350baa1bf main+0xd => 562350baa080 fn1@plt+0x0
28393.810326917: tr strt 0 [unknown] => 562350baa1c4 main+0x12
28393.810326917: call 562350baa1ce main+0x1c => 562350baa199 callfn+0x0
28393.810326917: tr end call 562350baa1ad callfn+0x14 => 7f607d36110f fn3+0x0
28393.810326922: tr strt 0 [unknown] => 562350baa1af callfn+0x16
28393.810326922: return 562350baa1b1 callfn+0x18 => 562350baa1d3 main+0x21
28393.810326922: tr end call 562350baa1d3 main+0x21 => 562350baa0a0 fn2@plt+0x0
28393.810326924: tr strt 0 [unknown] => 562350baa1d8 main+0x26
28393.810326924: tr end call 562350baa1d8 main+0x26 => 562350baa060 offset_0x1060@plt+0x0
28393.810326925: tr strt 0 [unknown] => 562350baa1dd main+0x2b
28393.810326925: tr end return 562350baa1e3 main+0x31 => 7f607d029d90 __libc_start_call_main+0x80

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
---
tools/perf/util/symbol-elf.c | 29 +++++++++++++++++++++++++++++
1 file changed, 29 insertions(+)

diff --git a/tools/perf/util/symbol-elf.c b/tools/perf/util/symbol-elf.c
index 9e265a726418..254116d40e59 100644
--- a/tools/perf/util/symbol-elf.c
+++ b/tools/perf/util/symbol-elf.c
@@ -466,6 +466,30 @@ static bool machine_is_x86(GElf_Half e_machine)
return e_machine == EM_386 || e_machine == EM_X86_64;
}

+static int dso__synthesize_plt_got_symbols(struct dso *dso, Elf *elf,
+ GElf_Ehdr *ehdr,
+ char *buf, size_t buf_sz)
+{
+ struct symbol *sym;
+ GElf_Shdr shdr;
+ Elf_Scn *scn;
+ size_t i;
+
+ scn = elf_section_by_name(elf, ehdr, &shdr, ".plt.got", NULL);
+ if (!scn || !shdr.sh_entsize)
+ return 0;
+
+ for (i = 0; i < shdr.sh_size; i += shdr.sh_entsize) {
+ snprintf(buf, buf_sz, "offset_%#zx@plt", shdr.sh_offset + i);
+ sym = symbol__new(shdr.sh_offset + i, shdr.sh_entsize, STB_GLOBAL, STT_FUNC, buf);
+ if (!sym)
+ return -1;
+ symbols__insert(&dso->symbols, sym);
+ }
+
+ return 0;
+}
+
/*
* We need to check if we have a .dynsym, so that we can handle the
* .plt, synthesizing its symbols, that aren't on the symtabs (be it
@@ -514,6 +538,11 @@ int dso__synthesize_plt_symbols(struct dso *dso, struct symsrc *ss)
goto out_elf_end;
symbols__insert(&dso->symbols, plt_sym);

+ /* Only x86 has .plt.got */
+ if (machine_is_x86(ehdr.e_machine) &&
+ dso__synthesize_plt_got_symbols(dso, elf, &ehdr, sympltname, sizeof(sympltname)))
+ goto out_elf_end;
+
/* Only x86 has .plt.sec */
if (machine_is_x86(ehdr.e_machine) &&
elf_section_by_name(elf, &ehdr, &plt_sec_shdr, ".plt.sec", NULL)) {
--
2.34.1