[PATCH 5/9] perf symbols: Add support for IFUNC symbols for x86_64

Adrian Hunter
2023-01-27T17:03:37Z

For x86_64, the GNU linker is putting IFUNC information in the relocation
addend, so use it to try to find a symbol for plt entries that refer to
IFUNCs.

Example:

Before:

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

void fn1(void);
void fn2(void);
void fn3(void);
void fn4(void);

int main()
{
fn4();
fn1();
thing();
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 -Wno-uninitialized -o tstpltifunc tstpltifunc.c -L . -ltstpltlib -Wl,-rpath="$(pwd)"
$ readelf -rW tstpltifunc | grep -A99 plt
Relocation section '.rela.plt' at offset 0x738 contains 8 entries:
Offset Info Type Symbol's Value Symbol's Name + Addend
0000000000003f98 0000000300000007 R_X86_64_JUMP_SLOT 0000000000000000 puts@GLIBC_2.2.5 + 0
0000000000003fa8 0000000400000007 R_X86_64_JUMP_SLOT 0000000000000000 __stack_chk_fail@GLIBC_2.4 + 0
0000000000003fb0 0000000500000007 R_X86_64_JUMP_SLOT 0000000000000000 fn1 + 0
0000000000003fb8 0000000600000007 R_X86_64_JUMP_SLOT 0000000000000000 fn3 + 0
0000000000003fc0 0000000800000007 R_X86_64_JUMP_SLOT 0000000000000000 fn4 + 0
0000000000003fc8 0000000900000007 R_X86_64_JUMP_SLOT 0000000000000000 fn2 + 0
0000000000003fd0 0000000b00000007 R_X86_64_JUMP_SLOT 0000000000000000 getrandom@GLIBC_2.25 + 0
0000000000003fa0 0000000000000025 R_X86_64_IRELATIVE 125d
$ perf record -e intel_pt//u --filter 'filter main @ ./tstpltifunc' ./tstpltifunc
thing2
[ perf record: Woken up 1 times to write data ]
[ perf record: Captured and wrote 0.016 MB perf.data ]
$ perf script --itrace=be --ns -F+flags,-event,+addr,-period,-comm,-tid,-cpu,-dso
21860.073683659: tr strt 0 [unknown] => 561e212c42be main+0x0
21860.073683659: tr end call 561e212c42c6 main+0x8 => 561e212c4110 fn4@plt+0x0
21860.073683661: tr strt 0 [unknown] => 561e212c42cb main+0xd
21860.073683661: tr end call 561e212c42cb main+0xd => 561e212c40f0 fn1@plt+0x0
21860.073683661: tr strt 0 [unknown] => 561e212c42d0 main+0x12
21860.073683661: tr end call 561e212c42d0 main+0x12 => 561e212c40d0 offset_0x10d0@plt+0x0
21860.073698451: tr strt 0 [unknown] => 561e212c42d5 main+0x17
21860.073698451: tr end call 561e212c42d5 main+0x17 => 561e212c4120 fn2@plt+0x0
21860.073698451: tr strt 0 [unknown] => 561e212c42da main+0x1c
21860.073698451: tr end call 561e212c42da main+0x1c => 561e212c4100 fn3@plt+0x0
21860.073698452: tr strt 0 [unknown] => 561e212c42df main+0x21
21860.073698452: tr end return 561e212c42e5 main+0x27 => 7fb51cc29d90 __libc_start_call_main+0x80

After:

$ perf script --itrace=be --ns -F+flags,-event,+addr,-period,-comm,-tid,-cpu,-dso
21860.073683659: tr strt 0 [unknown] => 561e212c42be main+0x0
21860.073683659: tr end call 561e212c42c6 main+0x8 => 561e212c4110 fn4@plt+0x0
21860.073683661: tr strt 0 [unknown] => 561e212c42cb main+0xd
21860.073683661: tr end call 561e212c42cb main+0xd => 561e212c40f0 fn1@plt+0x0
21860.073683661: tr strt 0 [unknown] => 561e212c42d0 main+0x12
21860.073683661: tr end call 561e212c42d0 main+0x12 => 561e212c40d0 thing_ifunc@plt+0x0
21860.073698451: tr strt 0 [unknown] => 561e212c42d5 main+0x17
21860.073698451: tr end call 561e212c42d5 main+0x17 => 561e212c4120 fn2@plt+0x0
21860.073698451: tr strt 0 [unknown] => 561e212c42da main+0x1c
21860.073698451: tr end call 561e212c42da main+0x1c => 561e212c4100 fn3@plt+0x0
21860.073698452: tr strt 0 [unknown] => 561e212c42df main+0x21
21860.073698452: tr end return 561e212c42e5 main+0x27 => 7fb51cc29d90 __libc_start_call_main+0x80

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
---
tools/perf/util/symbol-elf.c | 38 +++++++++++++++++++++++++++++++++++-
1 file changed, 37 insertions(+), 1 deletion(-)

diff --git a/tools/perf/util/symbol-elf.c b/tools/perf/util/symbol-elf.c
index 07cfcf8f40e3..a002fc0bea03 100644
--- a/tools/perf/util/symbol-elf.c
+++ b/tools/perf/util/symbol-elf.c
@@ -381,6 +381,42 @@ static int sort_rel(struct rel_info *ri)
return 0;
}

+/*
+ * For x86_64, the GNU linker is putting IFUNC information in the relocation
+ * addend.
+ */
+static bool addend_may_be_ifunc(GElf_Ehdr *ehdr, struct rel_info *ri)
+{
+ return ehdr->e_machine == EM_X86_64 && ri->is_rela &&
+ GELF_R_TYPE(ri->rela.r_info) == R_X86_64_IRELATIVE;
+}
+
+static bool get_ifunc_name(Elf *elf, struct dso *dso, GElf_Ehdr *ehdr,
+ struct rel_info *ri, char *buf, size_t buf_sz)
+{
+ u64 addr = ri->rela.r_addend;
+ struct symbol *sym;
+ GElf_Phdr phdr;
+
+ if (!addend_may_be_ifunc(ehdr, ri))
+ return false;
+
+ if (elf_read_program_header(elf, addr, &phdr))
+ return false;
+
+ addr -= phdr.p_vaddr - phdr.p_offset;
+
+ sym = dso__find_symbol_nocache(dso, addr);
+
+ /* Expecting the address to be an IFUNC or IFUNC alias */
+ if (!sym || sym->start != addr || (sym->type != STT_GNU_IFUNC && !sym->ifunc_alias))
+ return false;
+
+ snprintf(buf, buf_sz, "%s@plt", sym->name);
+
+ return true;
+}
+
static void exit_rel(struct rel_info *ri)
{
free(ri->sorted);
@@ -560,7 +596,7 @@ int dso__synthesize_plt_symbols(struct dso *dso, struct symsrc *ss)
elf_name = demangled;
if (*elf_name)
snprintf(sympltname, sizeof(sympltname), "%s@plt", elf_name);
- else
+ else if (!get_ifunc_name(elf, dso, &ehdr, &ri, sympltname, sizeof(sympltname)))
snprintf(sympltname, sizeof(sympltname),
"offset_%#" PRIx64 "@plt", plt_offset);
free(demangled);
--
2.34.1