[PATCH 7/9] perf symbols: Allow for static executables with .plt

Adrian Hunter
2023-01-27T17:04:00Z

A statically linked executable can have a .plt due to IFUNCs, in which
case .symtab is used not .dynsym. Check the section header link to see
if that is the case, and then use symtab instead.

Example:

Before:

$ cat tstifunc.c
#include <stdio.h>

void thing1(void)
{
printf("thing1\n");
}

void thing2(void)
{
printf("thing2\n");
}

typedef void (*thing_fn_t)(void);

thing_fn_t thing_ifunc(void)
{
int x;

if (x & 1)
return thing2;
return thing1;
}

void thing(void) __attribute__ ((ifunc ("thing_ifunc")));

int main()
{
thing();
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 -static -Wall -Wextra -Wno-uninitialized -o tstifuncstatic tstifunc.c
$ readelf -SW tstifuncstatic | grep 'Name\|plt\|dyn'
[Nr] Name Type Address Off Size ES Flg Lk Inf Al
[ 4] .rela.plt RELA 00000000004002e8 0002e8 000258 18 AI 29 20 8
[ 6] .plt PROGBITS 0000000000401020 001020 000190 00 AX 0 0 16
[20] .got.plt PROGBITS 00000000004c5000 0c4000 0000e0 08 WA 0 0 8
$ perf record -e intel_pt//u --filter 'filter main @ ./tstifuncstatic' ./tstifuncstatic
thing1
[ perf record: Woken up 1 times to write data ]
[ perf record: Captured and wrote 0.008 MB perf.data ]
$ perf script --itrace=be --ns -F+flags,-event,+addr,-period,-comm,-tid,-cpu,-dso
15786.690189535: tr strt 0 [unknown] => 4017cd main+0x0
15786.690189535: tr end call 4017d5 main+0x8 => 401170 [unknown]
15786.690197660: tr strt 0 [unknown] => 4017da main+0xd
15786.690197660: tr end return 4017e0 main+0x13 => 401c1a __libc_start_call_main+0x6a

After:

$ perf script --itrace=be --ns -F+flags,-event,+addr,-period,-comm,-tid,-cpu,-dso
15786.690189535: tr strt 0 [unknown] => 4017cd main+0x0
15786.690189535: tr end call 4017d5 main+0x8 => 401170 thing_ifunc@plt+0x0
15786.690197660: tr strt 0 [unknown] => 4017da main+0xd
15786.690197660: tr end return 4017e0 main+0x13 => 401c1a __libc_start_call_main+0x6a

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
---
tools/perf/util/symbol-elf.c | 30 ++++++++++++++++++++----------
tools/perf/util/symsrc.h | 1 +
2 files changed, 21 insertions(+), 10 deletions(-)

diff --git a/tools/perf/util/symbol-elf.c b/tools/perf/util/symbol-elf.c
index 8f7802097c72..9e265a726418 100644
--- a/tools/perf/util/symbol-elf.c
+++ b/tools/perf/util/symbol-elf.c
@@ -483,7 +483,6 @@ int dso__synthesize_plt_symbols(struct dso *dso, struct symsrc *ss)
GElf_Shdr shdr_rel_plt, shdr_dynsym;
Elf_Data *syms, *symstrs;
Elf_Scn *scn_plt_rel, *scn_symstrs, *scn_dynsym;
- size_t dynsym_idx;
GElf_Ehdr ehdr;
char sympltname[1024];
Elf *elf;
@@ -530,13 +529,6 @@ int dso__synthesize_plt_symbols(struct dso *dso, struct symsrc *ss)
lazy_plt = true;
}

- scn_dynsym = ss->dynsym;
- shdr_dynsym = ss->dynshdr;
- dynsym_idx = ss->dynsym_idx;
-
- if (scn_dynsym == NULL)
- return 0;
-
scn_plt_rel = elf_section_by_name(elf, &ehdr, &shdr_rel_plt,
".rela.plt", NULL);
if (scn_plt_rel == NULL) {
@@ -550,8 +542,25 @@ int dso__synthesize_plt_symbols(struct dso *dso, struct symsrc *ss)
shdr_rel_plt.sh_type != SHT_REL)
return 0;

- if (shdr_rel_plt.sh_link != dynsym_idx)
+ if (!shdr_rel_plt.sh_link)
+ return 0;
+
+ if (shdr_rel_plt.sh_link == ss->dynsym_idx) {
+ scn_dynsym = ss->dynsym;
+ shdr_dynsym = ss->dynshdr;
+ } else if (shdr_rel_plt.sh_link == ss->symtab_idx) {
+ /*
+ * A static executable can have a .plt due to IFUNCs, in which
+ * case .symtab is used not .dynsym.
+ */
+ scn_dynsym = ss->symtab;
+ shdr_dynsym = ss->symshdr;
+ } else {
goto out_elf_end;
+ }
+
+ if (!scn_dynsym)
+ return 0;

/*
* Fetch the relocation section to find the idxes to the GOT
@@ -1077,8 +1086,9 @@ int symsrc__init(struct symsrc *ss, struct dso *dso, const char *name,

ss->is_64_bit = (gelf_getclass(elf) == ELFCLASS64);

+ ss->symtab_idx = 0;
ss->symtab = elf_section_by_name(elf, &ehdr, &ss->symshdr, ".symtab",
- NULL);
+ &ss->symtab_idx);
if (ss->symshdr.sh_type != SHT_SYMTAB)
ss->symtab = NULL;

diff --git a/tools/perf/util/symsrc.h b/tools/perf/util/symsrc.h
index 2665b4bde751..edf82028c9e6 100644
--- a/tools/perf/util/symsrc.h
+++ b/tools/perf/util/symsrc.h
@@ -26,6 +26,7 @@ struct symsrc {
GElf_Shdr opdshdr;

Elf_Scn *symtab;
+ size_t symtab_idx;
GElf_Shdr symshdr;

Elf_Scn *dynsym;
--
2.34.1