Commit ce4c8e79 authored by Adrian Hunter's avatar Adrian Hunter Committed by Arnaldo Carvalho de Melo
Browse files

perf symbols: Get symbols for .plt.got for x86-64 



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)

Reviewed-by: default avatarNamhyung Kim <namhyung@kernel.org>
Signed-off-by: default avatarAdrian Hunter <adrian.hunter@intel.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Link: https://lore.kernel.org/r/20230131131625.6964-10-adrian.hunter@intel.com


Signed-off-by: default avatarArnaldo Carvalho de Melo <acme@redhat.com>
parent 51a188ad

tools/perf/util/symbol-elf.c

+157 −4
Original line number Diff line number Diff line
@@ -466,28 +466,181 @@ 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)

{

	struct rela_dyn vi, *vr;

	const char *sym_name;

	char *demangled;

	GElf_Sym sym;

	u32 disp;



	if (!di->sorted)

		return false;



	disp = get_x86_64_plt_disp(di->plt_got_data->d_buf + i);

	if (!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) {

		if (!get_plt_got_name(&shdr, i, &di, buf, buf_sz))

			snprintf(buf, buf_sz, "offset_%#" PRIx64 "@plt", (u64)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;

}



/*