[PATCH 2/9] perf symbols: Add support for x86 .plt.sec

[Adrian Hunter <adrian.hunter@intel.com>]

The section .plt.sec was originally added for MPX and was first called
.plt.bnd. While MPX has been deprecated, .plt.sec is now also used for IBT.
On x86_64, IBT seems to be enabled by default, but can be switched off
using gcc option -fcf-protection=none. On 32-bit, option -z ibt will
enable IBT.

With .plt.sec, calls are made into .plt.sec instead of .plt, so it
makes more sense to put the symbols there instead of .plt. A notable
difference is that .plt.sec does not have a header entry.

For x86, when synthesizing symbols for plt, use offset and entry size of
.plt.sec instead of .plt when there is a .plt.sec section.

Example on Ubuntu 22.04 gcc 11.3:

  Before:

    $ cat tstpltlib.c
    void fn1(void) {}
    void fn2(void) {}
    void fn3(void) {}
    void fn4(void) {}
    $ cat tstplt.c
    void fn1(void);
    void fn2(void);
    void fn3(void);
    void fn4(void);

    int main()
    {
            fn4();
            fn1();
            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 -o tstplt tstplt.c -L . -ltstpltlib -Wl,-rpath=$(pwd)
    $ readelf -SW tstplt | grep 'plt\|Name'
      [Nr] Name              Type            Address          Off    Size   ES Flg Lk Inf Al
      [11] .rela.plt         RELA            0000000000000698 000698 000060 18  AI  6  24  8
      [13] .plt              PROGBITS        0000000000001020 001020 000050 10  AX  0   0 16
      [14] .plt.got          PROGBITS        0000000000001070 001070 000010 10  AX  0   0 16
      [15] .plt.sec          PROGBITS        0000000000001080 001080 000040 10  AX  0   0 16
    $ perf record -e intel_pt//u --filter 'filter main @ ./tstplt' ./tstplt
    [ perf record: Woken up 1 times to write data ]
    [ perf record: Captured and wrote 0.015 MB perf.data ]
    $ perf script --itrace=be --ns -F+flags,-event,+addr,-period,-comm,-tid,-cpu,-dso
    38970.522546686:   tr strt                               0 [unknown] =>     55fc222a81a9 main+0x0
    38970.522546686:   tr end  call               55fc222a81b1 main+0x8 =>     55fc222a80a0 [unknown]
    38970.522546687:   tr strt                               0 [unknown] =>     55fc222a81b6 main+0xd
    38970.522546687:   tr end  call               55fc222a81b6 main+0xd =>     55fc222a8080 [unknown]
    38970.522546688:   tr strt                               0 [unknown] =>     55fc222a81bb main+0x12
    38970.522546688:   tr end  call               55fc222a81bb main+0x12 =>     55fc222a80b0 [unknown]
    38970.522546688:   tr strt                               0 [unknown] =>     55fc222a81c0 main+0x17
    38970.522546688:   tr end  call               55fc222a81c0 main+0x17 =>     55fc222a8090 [unknown]
    38970.522546689:   tr strt                               0 [unknown] =>     55fc222a81c5 main+0x1c
    38970.522546894:   tr end  return             55fc222a81cb main+0x22 =>     7f3a4dc29d90 __libc_start_call_main+0x80

  After:

    $ perf script --itrace=be --ns -F+flags,-event,+addr,-period,-comm,-tid,-cpu,-dso
    38970.522546686:   tr strt                               0 [unknown] =>     55fc222a81a9 main+0x0
    38970.522546686:   tr end  call               55fc222a81b1 main+0x8 =>     55fc222a80a0 fn4@plt+0x0
    38970.522546687:   tr strt                               0 [unknown] =>     55fc222a81b6 main+0xd
    38970.522546687:   tr end  call               55fc222a81b6 main+0xd =>     55fc222a8080 fn1@plt+0x0
    38970.522546688:   tr strt                               0 [unknown] =>     55fc222a81bb main+0x12
    38970.522546688:   tr end  call               55fc222a81bb main+0x12 =>     55fc222a80b0 fn2@plt+0x0
    38970.522546688:   tr strt                               0 [unknown] =>     55fc222a81c0 main+0x17
    38970.522546688:   tr end  call               55fc222a81c0 main+0x17 =>     55fc222a8090 fn3@plt+0x0
    38970.522546689:   tr strt                               0 [unknown] =>     55fc222a81c5 main+0x1c
    38970.522546894:   tr end  return             55fc222a81cb main+0x22 =>     7f3a4dc29d90 __libc_start_call_main+0x80

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
---
 tools/perf/util/symbol-elf.c | 30 +++++++++++++++++++++++-------
 1 file changed, 23 insertions(+), 7 deletions(-)

diff --git a/tools/perf/util/symbol-elf.c b/tools/perf/util/symbol-elf.c
index 9328c162d68f..bb1b5cb3ff12 100644
--- a/tools/perf/util/symbol-elf.c
+++ b/tools/perf/util/symbol-elf.c
@@ -379,6 +379,11 @@ static bool get_plt_sizes(struct dso *dso, GElf_Ehdr *ehdr, GElf_Shdr *shdr_plt,
 	return false;
 }
 
+static bool machine_is_x86(GElf_Half e_machine)
+{
+	return e_machine == EM_386 || e_machine == EM_X86_64;
+}
+
 /*
  * We need to check if we have a .dynsym, so that we can handle the
  * .plt, synthesizing its symbols, that aren't on the symtabs (be it
@@ -391,8 +396,8 @@ int dso__synthesize_plt_symbols(struct dso *dso, struct symsrc *ss)
 	uint32_t nr_rel_entries, idx;
 	GElf_Sym sym;
 	u64 plt_offset, plt_header_size, plt_entry_size;
-	GElf_Shdr shdr_plt;
-	struct symbol *f;
+	GElf_Shdr shdr_plt, plt_sec_shdr;
+	struct symbol *f, *plt_sym;
 	GElf_Shdr shdr_rel_plt, shdr_dynsym;
 	Elf_Data *syms, *symstrs;
 	Elf_Scn *scn_plt_rel, *scn_symstrs, *scn_dynsym;
@@ -422,10 +427,23 @@ int dso__synthesize_plt_symbols(struct dso *dso, struct symsrc *ss)
 		return 0;
 
 	/* Add a symbol for .plt header */
-	f = symbol__new(shdr_plt.sh_offset, plt_header_size, STB_GLOBAL, STT_FUNC, ".plt");
-	if (!f)
+	plt_sym = symbol__new(shdr_plt.sh_offset, plt_header_size, STB_GLOBAL, STT_FUNC, ".plt");
+	if (!plt_sym)
 		goto out_elf_end;
-	symbols__insert(&dso->symbols, f);
+	symbols__insert(&dso->symbols, plt_sym);
+
+	/* Only x86 has .plt.sec */
+	if (machine_is_x86(ehdr.e_machine) &&
+	    elf_section_by_name(elf, &ehdr, &plt_sec_shdr, ".plt.sec", NULL)) {
+		if (!get_plt_sizes(dso, &ehdr, &plt_sec_shdr, &plt_header_size, &plt_entry_size))
+			return 0;
+		/* Extend .plt symbol to entire .plt */
+		plt_sym->end = plt_sym->start + shdr_plt.sh_size;
+		/* Use .plt.sec offset */
+		plt_offset = plt_sec_shdr.sh_offset;
+	} else {
+		plt_offset = shdr_plt.sh_offset + plt_header_size;
+	}
 
 	scn_dynsym = ss->dynsym;
 	shdr_dynsym = ss->dynshdr;
@@ -474,8 +492,6 @@ int dso__synthesize_plt_symbols(struct dso *dso, struct symsrc *ss)
 		goto out_elf_end;
 
 	nr_rel_entries = shdr_rel_plt.sh_size / shdr_rel_plt.sh_entsize;
-	plt_offset = shdr_plt.sh_offset;
-	plt_offset += plt_header_size;
 
 	ri.is_rela = shdr_rel_plt.sh_type == SHT_RELA;
 
-- 
2.34.1