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

Adrian Hunter
2023-01-31T10:18:12Z

On 31/01/23 01:26, Namhyung Kim wrote:
> On Fri, Jan 27, 2023 at 07:02:22PM +0200, Adrian Hunter wrote:
>> 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);
>
> What about EM_386? Now I'm seeing segfaults on 32 bit test programs
> with .plt.got section.

Thanks for looking at this.

Looks like a late change in get_plt_got_name() did not
get tested for 32-bit. It should be checking di->sorted
first. EM_386 is not supported by this logic.

I will send a V2

>
> Thanks,
> Namhyung
>
>
>> +
>> 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
>>
>>