[PATCH 9/9] perf symbols: Get symbols for .plt.got for x86-64

Adrian Hunter
2023-01-27T17:04:24Z

For x86_64, determine a symbol for .plt.got entries. That requires
computing the target offset and finding that in .rela.dyn, which in
turn means .rela.dyn needs to be sorted by offset.

Example:

In this example, the GNU C Library is using .plt.got for malloc and
free.

Before:

$ 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.
$ perf record -e intel_pt//u uname
Linux
[ perf record: Woken up 1 times to write data ]
[ perf record: Captured and wrote 0.027 MB perf.data ]
$ perf script --itrace=be --ns -F-event,+addr,-period,-comm,-tid,-cpu > /tmp/cmp1.txt

After:

$ perf script --itrace=be --ns -F-event,+addr,-period,-comm,-tid,-cpu > /tmp/cmp2.txt
$ diff /tmp/cmp1.txt /tmp/cmp2.txt | head -12
15509,15510c15509,15510
< 27046.755390907: 7f0b2943e3ab _nl_normalize_codeset+0x5b (/usr/lib/x86_64-linux-gnu/libc.so.6) => 7f0b29428380 offset_0x28380@plt+0x0 (/usr/lib/x86_64-linux-gnu/libc.so.6)
< 27046.755390907: 7f0b29428384 offset_0x28380@plt+0x4 (/usr/lib/x86_64-linux-gnu/libc.so.6) => 7f0b294a5120 malloc+0x0 (/usr/lib/x86_64-linux-gnu/libc.so.6)
---
> 27046.755390907: 7f0b2943e3ab _nl_normalize_codeset+0x5b (/usr/lib/x86_64-linux-gnu/libc.so.6) => 7f0b29428380 malloc@plt+0x0 (/usr/lib/x86_64-linux-gnu/libc.so.6)
> 27046.755390907: 7f0b29428384 malloc@plt+0x4 (/usr/lib/x86_64-linux-gnu/libc.so.6) => 7f0b294a5120 malloc+0x0 (/usr/lib/x86_64-linux-gnu/libc.so.6)
15821,15822c15821,15822
< 27046.755394865: 7f0b2943850c _nl_load_locale_from_archive+0x5bc (/usr/lib/x86_64-linux-gnu/libc.so.6) => 7f0b29428370 offset_0x28370@plt+0x0 (/usr/lib/x86_64-linux-gnu/libc.so.6)
< 27046.755394865: 7f0b29428374 offset_0x28370@plt+0x4 (/usr/lib/x86_64-linux-gnu/libc.so.6) => 7f0b294a5460 cfree@GLIBC_2.2.5+0x0 (/usr/lib/x86_64-linux-gnu/libc.so.6)
---
> 27046.755394865: 7f0b2943850c _nl_load_locale_from_archive+0x5bc (/usr/lib/x86_64-linux-gnu/libc.so.6) => 7f0b29428370 free@plt+0x0 (/usr/lib/x86_64-linux-gnu/libc.so.6)
> 27046.755394865: 7f0b29428374 free@plt+0x4 (/usr/lib/x86_64-linux-gnu/libc.so.6) => 7f0b294a5460 cfree@GLIBC_2.2.5+0x0 (/usr/lib/x86_64-linux-gnu/libc.so.6)

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
---
tools/perf/util/symbol-elf.c | 158 ++++++++++++++++++++++++++++++++++-
1 file changed, 154 insertions(+), 4 deletions(-)

diff --git a/tools/perf/util/symbol-elf.c b/tools/perf/util/symbol-elf.c
index 254116d40e59..4fc8e7fc10f4 100644
--- a/tools/perf/util/symbol-elf.c
+++ b/tools/perf/util/symbol-elf.c
@@ -466,28 +466,178 @@ static bool machine_is_x86(GElf_Half e_machine)
return e_machine == EM_386 || e_machine == EM_X86_64;
}

+struct rela_dyn {
+ GElf_Addr offset;
+ u32 sym_idx;
+};
+
+struct rela_dyn_info {
+ struct dso *dso;
+ Elf_Data *plt_got_data;
+ u32 nr_entries;
+ struct rela_dyn *sorted;
+ Elf_Data *dynsym_data;
+ Elf_Data *dynstr_data;
+ Elf_Data *rela_dyn_data;
+};
+
+static void exit_rela_dyn(struct rela_dyn_info *di)
+{
+ free(di->sorted);
+}
+
+static int cmp_offset(const void *a, const void *b)
+{
+ const struct rela_dyn *va = a;
+ const struct rela_dyn *vb = b;
+
+ return va->offset < vb->offset ? -1 : (va->offset > vb->offset ? 1 : 0);
+}
+
+static int sort_rela_dyn(struct rela_dyn_info *di)
+{
+ u32 i, n;
+
+ di->sorted = calloc(di->nr_entries, sizeof(di->sorted[0]));
+ if (!di->sorted)
+ return -1;
+
+ /* Get data for sorting: the offset and symbol index */
+ for (i = 0, n = 0; i < di->nr_entries; i++) {
+ GElf_Rela rela;
+ u32 sym_idx;
+
+ gelf_getrela(di->rela_dyn_data, i, &rela);
+ sym_idx = GELF_R_SYM(rela.r_info);
+ if (sym_idx) {
+ di->sorted[n].sym_idx = sym_idx;
+ di->sorted[n].offset = rela.r_offset;
+ n += 1;
+ }
+ }
+
+ /* Sort by offset */
+ di->nr_entries = n;
+ qsort(di->sorted, n, sizeof(di->sorted[0]), cmp_offset);
+
+ return 0;
+}
+
+static void get_rela_dyn_info(Elf *elf, GElf_Ehdr *ehdr, struct rela_dyn_info *di, Elf_Scn *scn)
+{
+ GElf_Shdr rela_dyn_shdr;
+ GElf_Shdr shdr;
+
+ di->plt_got_data = elf_getdata(scn, NULL);
+
+ scn = elf_section_by_name(elf, ehdr, &rela_dyn_shdr, ".rela.dyn", NULL);
+ if (!scn || !rela_dyn_shdr.sh_link || !rela_dyn_shdr.sh_entsize)
+ return;
+
+ di->nr_entries = rela_dyn_shdr.sh_size / rela_dyn_shdr.sh_entsize;
+ di->rela_dyn_data = elf_getdata(scn, NULL);
+
+ scn = elf_getscn(elf, rela_dyn_shdr.sh_link);
+ if (!scn || !gelf_getshdr(scn, &shdr) || !shdr.sh_link)
+ return;
+
+ di->dynsym_data = elf_getdata(scn, NULL);
+ di->dynstr_data = elf_getdata(elf_getscn(elf, shdr.sh_link), NULL);
+
+ if (!di->plt_got_data || !di->dynstr_data || !di->dynsym_data || !di->rela_dyn_data)
+ return;
+
+ /* Sort into offset order */
+ sort_rela_dyn(di);
+}
+
+/* Get instruction displacement from a plt entry for x86_64 */
+static u32 get_x86_64_plt_disp(const u8 *p)
+{
+ u8 endbr64[] = {0xf3, 0x0f, 0x1e, 0xfa};
+ int n = 0;
+
+ /* Skip endbr64 */
+ if (!memcmp(p, endbr64, sizeof(endbr64)))
+ n += sizeof(endbr64);
+ /* Skip bnd prefix */
+ if (p[n] == 0xf2)
+ n += 1;
+ /* jmp with 4-byte displacement */
+ if (p[n] == 0xff && p[n + 1] == 0x25) {
+ n += 2;
+ /* Also add offset from start of entry to end of instruction */
+ return n + 4 + le32toh(*(const u32 *)(p + n));
+ }
+ return 0;
+}
+
+static bool get_plt_got_name(GElf_Shdr *shdr, size_t i,
+ struct rela_dyn_info *di,
+ char *buf, size_t buf_sz)
+{
+ void *p = di->plt_got_data->d_buf + i;
+ u32 disp = get_x86_64_plt_disp(p);
+ struct rela_dyn vi, *vr;
+ const char *sym_name;
+ char *demangled;
+ GElf_Sym sym;
+
+ if (!di->sorted || !disp)
+ return false;
+
+ /* Compute target offset of the .plt.got entry */
+ vi.offset = shdr->sh_offset + di->plt_got_data->d_off + i + disp;
+
+ /* Find that offset in .rela.dyn (sorted by offset) */
+ vr = bsearch(&vi, di->sorted, di->nr_entries, sizeof(di->sorted[0]), cmp_offset);
+ if (!vr)
+ return false;
+
+ /* Get the associated symbol */
+ gelf_getsym(di->dynsym_data, vr->sym_idx, &sym);
+ sym_name = elf_sym__name(&sym, di->dynstr_data);
+ demangled = demangle_sym(di->dso, 0, sym_name);
+ if (demangled != NULL)
+ sym_name = demangled;
+
+ snprintf(buf, buf_sz, "%s@plt", sym_name);
+
+ free(demangled);
+
+ return *sym_name;
+}
+
static int dso__synthesize_plt_got_symbols(struct dso *dso, Elf *elf,
GElf_Ehdr *ehdr,
char *buf, size_t buf_sz)
{
+ struct rela_dyn_info di = { .dso = dso };
struct symbol *sym;
GElf_Shdr shdr;
Elf_Scn *scn;
+ int err = -1;
size_t i;

scn = elf_section_by_name(elf, ehdr, &shdr, ".plt.got", NULL);
if (!scn || !shdr.sh_entsize)
return 0;

+ if (ehdr->e_machine == EM_X86_64)
+ get_rela_dyn_info(elf, ehdr, &di, scn);
+
for (i = 0; i < shdr.sh_size; i += shdr.sh_entsize) {
- snprintf(buf, buf_sz, "offset_%#zx@plt", shdr.sh_offset + i);
+ if (!get_plt_got_name(&shdr, i, &di, buf, buf_sz))
+ 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;
+ goto out;
symbols__insert(&dso->symbols, sym);
}
-
- return 0;
+ err = 0;
+out:
+ exit_rela_dyn(&di);
+ return err;
}

/*
--
2.34.1