[PATCH 2/9] perf symbols: Add support for x86 .plt.sec

Adrian Hunter
2023-01-27T17:03:12Z

The section .plt.sec was originally added for MPX and was first called
.plt.bnd. While MPX has been deprecated, .plt.sec is now also used for IBT.
On x86_64, IBT seems to be enabled by default, but can be switched off
using gcc option -fcf-protection=none. On 32-bit, option -z ibt will
enable IBT.

With .plt.sec, calls are made into .plt.sec instead of .plt, so it
makes more sense to put the symbols there instead of .plt. A notable
difference is that .plt.sec does not have a header entry.

For x86, when synthesizing symbols for plt, use offset and entry size of
.plt.sec instead of .plt when there is a .plt.sec section.

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 -Wall -Wextra -shared -o libtstpltlib.so tstpltlib.c
$ gcc -Wall -Wextra -o tstplt tstplt.c -L . -ltstpltlib -Wl,-rpath=$(pwd)
$ readelf -SW tstplt | grep 'plt\|Name'
[Nr] Name Type Address Off Size ES Flg Lk Inf Al
[11] .rela.plt RELA 0000000000000698 000698 000060 18 AI 6 24 8
[13] .plt PROGBITS 0000000000001020 001020 000050 10 AX 0 0 16
[14] .plt.got PROGBITS 0000000000001070 001070 000010 10 AX 0 0 16
[15] .plt.sec PROGBITS 0000000000001080 001080 000040 10 AX 0 0 16
$ perf record -e intel_pt//u --filter 'filter main @ ./tstplt' ./tstplt
[ perf record: Woken up 1 times to write data ]
[ perf record: Captured and wrote 0.015 MB perf.data ]
$ perf script --itrace=be --ns -F+flags,-event,+addr,-period,-comm,-tid,-cpu,-dso
38970.522546686: tr strt 0 [unknown] => 55fc222a81a9 main+0x0
38970.522546686: tr end call 55fc222a81b1 main+0x8 => 55fc222a80a0 [unknown]
38970.522546687: tr strt 0 [unknown] => 55fc222a81b6 main+0xd
38970.522546687: tr end call 55fc222a81b6 main+0xd => 55fc222a8080 [unknown]
38970.522546688: tr strt 0 [unknown] => 55fc222a81bb main+0x12
38970.522546688: tr end call 55fc222a81bb main+0x12 => 55fc222a80b0 [unknown]
38970.522546688: tr strt 0 [unknown] => 55fc222a81c0 main+0x17
38970.522546688: tr end call 55fc222a81c0 main+0x17 => 55fc222a8090 [unknown]
38970.522546689: tr strt 0 [unknown] => 55fc222a81c5 main+0x1c
38970.522546894: tr end return 55fc222a81cb main+0x22 => 7f3a4dc29d90 __libc_start_call_main+0x80

After:

$ perf script --itrace=be --ns -F+flags,-event,+addr,-period,-comm,-tid,-cpu,-dso
38970.522546686: tr strt 0 [unknown] => 55fc222a81a9 main+0x0
38970.522546686: tr end call 55fc222a81b1 main+0x8 => 55fc222a80a0 fn4@plt+0x0
38970.522546687: tr strt 0 [unknown] => 55fc222a81b6 main+0xd
38970.522546687: tr end call 55fc222a81b6 main+0xd => 55fc222a8080 fn1@plt+0x0
38970.522546688: tr strt 0 [unknown] => 55fc222a81bb main+0x12
38970.522546688: tr end call 55fc222a81bb main+0x12 => 55fc222a80b0 fn2@plt+0x0
38970.522546688: tr strt 0 [unknown] => 55fc222a81c0 main+0x17
38970.522546688: tr end call 55fc222a81c0 main+0x17 => 55fc222a8090 fn3@plt+0x0
38970.522546689: tr strt 0 [unknown] => 55fc222a81c5 main+0x1c
38970.522546894: tr end return 55fc222a81cb main+0x22 => 7f3a4dc29d90 __libc_start_call_main+0x80

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
---
tools/perf/util/symbol-elf.c | 30 +++++++++++++++++++++++-------
1 file changed, 23 insertions(+), 7 deletions(-)

diff --git a/tools/perf/util/symbol-elf.c b/tools/perf/util/symbol-elf.c
index 9328c162d68f..bb1b5cb3ff12 100644
--- a/tools/perf/util/symbol-elf.c
+++ b/tools/perf/util/symbol-elf.c
@@ -379,6 +379,11 @@ static bool get_plt_sizes(struct dso *dso, GElf_Ehdr *ehdr, GElf_Shdr *shdr_plt,
return false;
}

+static bool machine_is_x86(GElf_Half e_machine)
+{
+ return e_machine == EM_386 || e_machine == EM_X86_64;
+}
+
/*
* 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
@@ -391,8 +396,8 @@ int dso__synthesize_plt_symbols(struct dso *dso, struct symsrc *ss)
uint32_t nr_rel_entries, idx;
GElf_Sym sym;
u64 plt_offset, plt_header_size, plt_entry_size;
- GElf_Shdr shdr_plt;
- struct symbol *f;
+ GElf_Shdr shdr_plt, plt_sec_shdr;
+ struct symbol *f, *plt_sym;
GElf_Shdr shdr_rel_plt, shdr_dynsym;
Elf_Data *syms, *symstrs;
Elf_Scn *scn_plt_rel, *scn_symstrs, *scn_dynsym;
@@ -422,10 +427,23 @@ int dso__synthesize_plt_symbols(struct dso *dso, struct symsrc *ss)
return 0;

/* Add a symbol for .plt header */
- f = symbol__new(shdr_plt.sh_offset, plt_header_size, STB_GLOBAL, STT_FUNC, ".plt");
- if (!f)
+ plt_sym = symbol__new(shdr_plt.sh_offset, plt_header_size, STB_GLOBAL, STT_FUNC, ".plt");
+ if (!plt_sym)
goto out_elf_end;
- symbols__insert(&dso->symbols, f);
+ symbols__insert(&dso->symbols, plt_sym);
+
+ /* Only x86 has .plt.sec */
+ if (machine_is_x86(ehdr.e_machine) &&
+ elf_section_by_name(elf, &ehdr, &plt_sec_shdr, ".plt.sec", NULL)) {
+ if (!get_plt_sizes(dso, &ehdr, &plt_sec_shdr, &plt_header_size, &plt_entry_size))
+ return 0;
+ /* Extend .plt symbol to entire .plt */
+ plt_sym->end = plt_sym->start + shdr_plt.sh_size;
+ /* Use .plt.sec offset */
+ plt_offset = plt_sec_shdr.sh_offset;
+ } else {
+ plt_offset = shdr_plt.sh_offset + plt_header_size;
+ }

scn_dynsym = ss->dynsym;
shdr_dynsym = ss->dynshdr;
@@ -474,8 +492,6 @@ int dso__synthesize_plt_symbols(struct dso *dso, struct symsrc *ss)
goto out_elf_end;

nr_rel_entries = shdr_rel_plt.sh_size / shdr_rel_plt.sh_entsize;
- plt_offset = shdr_plt.sh_offset;
- plt_offset += plt_header_size;

ri.is_rela = shdr_rel_plt.sh_type == SHT_RELA;

--
2.34.1