[PATCH 3/9] perf symbols: Sort plt relocations for x86

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

For x86, with the addition of IFUNCs, relocation information becomes
disordered with respect to plt. Correct that by sorting the relocations by
offset.

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 0.029 MB perf.data ]
$ perf script --itrace=be --ns -F+flags,-event,+addr,-period,-comm,-tid,-cpu,-dso
20417.302513948: tr strt 0 [unknown] => 5629a74892be main+0x0
20417.302513948: tr end call 5629a74892c6 main+0x8 => 5629a7489110 fn2@plt+0x0
20417.302513949: tr strt 0 [unknown] => 5629a74892cb main+0xd
20417.302513949: tr end call 5629a74892cb main+0xd => 5629a74890f0 fn3@plt+0x0
20417.302513950: tr strt 0 [unknown] => 5629a74892d0 main+0x12
20417.302513950: tr end call 5629a74892d0 main+0x12 => 5629a74890d0 __stack_chk_fail@plt+0x0
20417.302528114: tr strt 0 [unknown] => 5629a74892d5 main+0x17
20417.302528114: tr end call 5629a74892d5 main+0x17 => 5629a7489120 getrandom@plt+0x0
20417.302528115: tr strt 0 [unknown] => 5629a74892da main+0x1c
20417.302528115: tr end call 5629a74892da main+0x1c => 5629a7489100 fn4@plt+0x0
20417.302528115: tr strt 0 [unknown] => 5629a74892df main+0x21
20417.302528115: tr end return 5629a74892e5 main+0x27 => 7ff14da29d90 __libc_start_call_main+0x80

After:

$ perf script --itrace=be --ns -F+flags,-event,+addr,-period,-comm,-tid,-cpu,-dso
20417.302513948: tr strt 0 [unknown] => 5629a74892be main+0x0
20417.302513948: tr end call 5629a74892c6 main+0x8 => 5629a7489110 fn4@plt+0x0
20417.302513949: tr strt 0 [unknown] => 5629a74892cb main+0xd
20417.302513949: tr end call 5629a74892cb main+0xd => 5629a74890f0 fn1@plt+0x0
20417.302513950: tr strt 0 [unknown] => 5629a74892d0 main+0x12
20417.302513950: tr end call 5629a74892d0 main+0x12 => 5629a74890d0 offset_0x10d0@plt+0x0
20417.302528114: tr strt 0 [unknown] => 5629a74892d5 main+0x17
20417.302528114: tr end call 5629a74892d5 main+0x17 => 5629a7489120 fn2@plt+0x0
20417.302528115: tr strt 0 [unknown] => 5629a74892da main+0x1c
20417.302528115: tr end call 5629a74892da main+0x1c => 5629a7489100 fn3@plt+0x0
20417.302528115: tr strt 0 [unknown] => 5629a74892df main+0x21
20417.302528115: tr end return 5629a74892e5 main+0x27 => 7ff14da29d90 __libc_start_call_main+0x80

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
---
tools/perf/util/symbol-elf.c | 60 ++++++++++++++++++++++++++++++++++--
1 file changed, 57 insertions(+), 3 deletions(-)

diff --git a/tools/perf/util/symbol-elf.c b/tools/perf/util/symbol-elf.c
index bb1b5cb3ff12..07cfcf8f40e3 100644
--- a/tools/perf/util/symbol-elf.c
+++ b/tools/perf/util/symbol-elf.c
@@ -324,6 +324,8 @@ static char *demangle_sym(struct dso *dso, int kmodule, const char *elf_name)
}

struct rel_info {
+ u32 nr_entries;
+ u32 *sorted;
bool is_rela;
Elf_Data *reldata;
GElf_Rela rela;
@@ -332,6 +334,7 @@ struct rel_info {

static u32 get_rel_symidx(struct rel_info *ri, u32 idx)
{
+ idx = ri->sorted ? ri->sorted[idx] : idx;
if (ri->is_rela) {
gelf_getrela(ri->reldata, idx, &ri->rela);
return GELF_R_SYM(ri->rela.r_info);
@@ -340,6 +343,49 @@ static u32 get_rel_symidx(struct rel_info *ri, u32 idx)
return GELF_R_SYM(ri->rel.r_info);
}

+static u64 get_rel_offset(struct rel_info *ri, u32 x)
+{
+ if (ri->is_rela) {
+ GElf_Rela rela;
+
+ gelf_getrela(ri->reldata, x, &rela);
+ return rela.r_offset;
+ } else {
+ GElf_Rel rel;
+
+ gelf_getrel(ri->reldata, x, &rel);
+ return rel.r_offset;
+ }
+}
+
+static int rel_cmp(const void *a, const void *b, void *r)
+{
+ struct rel_info *ri = r;
+ u64 a_offset = get_rel_offset(ri, *(const u32 *)a);
+ u64 b_offset = get_rel_offset(ri, *(const u32 *)b);
+
+ return a_offset < b_offset ? -1 : (a_offset > b_offset ? 1 : 0);
+}
+
+static int sort_rel(struct rel_info *ri)
+{
+ size_t sz = sizeof(ri->sorted[0]);
+ u32 i;
+
+ ri->sorted = calloc(ri->nr_entries, sz);
+ if (!ri->sorted)
+ return -1;
+ for (i = 0; i < ri->nr_entries; i++)
+ ri->sorted[i] = i;
+ qsort_r(ri->sorted, ri->nr_entries, sz, rel_cmp, ri);
+ return 0;
+}
+
+static void exit_rel(struct rel_info *ri)
+{
+ free(ri->sorted);
+}
+
static bool get_plt_sizes(struct dso *dso, GElf_Ehdr *ehdr, GElf_Shdr *shdr_plt,
u64 *plt_header_size, u64 *plt_entry_size)
{
@@ -393,7 +439,7 @@ static bool machine_is_x86(GElf_Half e_machine)
*/
int dso__synthesize_plt_symbols(struct dso *dso, struct symsrc *ss)
{
- uint32_t nr_rel_entries, idx;
+ uint32_t idx;
GElf_Sym sym;
u64 plt_offset, plt_header_size, plt_entry_size;
GElf_Shdr shdr_plt, plt_sec_shdr;
@@ -491,11 +537,18 @@ int dso__synthesize_plt_symbols(struct dso *dso, struct symsrc *ss)
if (symstrs->d_size == 0)
goto out_elf_end;

- nr_rel_entries = shdr_rel_plt.sh_size / shdr_rel_plt.sh_entsize;
+ ri.nr_entries = shdr_rel_plt.sh_size / shdr_rel_plt.sh_entsize;

ri.is_rela = shdr_rel_plt.sh_type == SHT_RELA;

- for (idx = 0; idx < nr_rel_entries; idx++) {
+ /*
+ * x86 doesn't insert IFUNC relocations in .plt order, so sort to get
+ * back in order.
+ */
+ if (machine_is_x86(ehdr.e_machine) && sort_rel(&ri))
+ goto out_elf_end;
+
+ for (idx = 0; idx < ri.nr_entries; idx++) {
const char *elf_name = NULL;
char *demangled = NULL;

@@ -523,6 +576,7 @@ int dso__synthesize_plt_symbols(struct dso *dso, struct symsrc *ss)

err = 0;
out_elf_end:
+ exit_rel(&ri);
if (err == 0)
return nr;
pr_debug("%s: problems reading %s PLT info.\n",
--
2.34.1