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

Namhyung Kim
2023-01-30T23:26:09Z

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,
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
>
>