[PATCH 7/9] perf symbols: Allow for static executables with .plt

[Adrian Hunter]

A statically linked executable can have a .plt due to IFUNCs, in which
case .symtab is used not .dynsym. Check the section header link to see
if that is the case, and then use symtab instead.

Example:

  Before:

    $ cat tstifunc.c
    #include <stdio.h>

    void thing1(void)
    {
            printf("thing1\n");
    }

    void thing2(void)
    {
            printf("thing2\n");
    }

    typedef void (*thing_fn_t)(void);

    thing_fn_t thing_ifunc(void)
    {
            int x;

            if (x & 1)
                    return thing2;
            return thing1;
    }

    void thing(void) __attribute__ ((ifunc ("thing_ifunc")));

    int main()
    {
            thing();
            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 -static -Wall -Wextra -Wno-uninitialized -o tstifuncstatic tstifunc.c
    $ readelf -SW tstifuncstatic | grep 'Name\|plt\|dyn'
      [Nr] Name              Type            Address          Off    Size   ES Flg Lk Inf Al
      [ 4] .rela.plt         RELA            00000000004002e8 0002e8 000258 18  AI 29  20  8
      [ 6] .plt              PROGBITS        0000000000401020 001020 000190 00  AX  0   0 16
      [20] .got.plt          PROGBITS        00000000004c5000 0c4000 0000e0 08  WA  0   0  8
    $ perf record -e intel_pt//u --filter 'filter main @ ./tstifuncstatic' ./tstifuncstatic
    thing1
    [ perf record: Woken up 1 times to write data ]
    [ perf record: Captured and wrote 0.008 MB perf.data ]
    $ perf script --itrace=be --ns -F+flags,-event,+addr,-period,-comm,-tid,-cpu,-dso
    15786.690189535:   tr strt                               0 [unknown] =>           4017cd main+0x0
    15786.690189535:   tr end  call                     4017d5 main+0x8 =>           401170 [unknown]
    15786.690197660:   tr strt                               0 [unknown] =>           4017da main+0xd
    15786.690197660:   tr end  return                   4017e0 main+0x13 =>           401c1a __libc_start_call_main+0x6a

  After:

    $ perf script --itrace=be --ns -F+flags,-event,+addr,-period,-comm,-tid,-cpu,-dso
    15786.690189535:   tr strt                               0 [unknown] =>           4017cd main+0x0
    15786.690189535:   tr end  call                     4017d5 main+0x8 =>           401170 thing_ifunc@plt+0x0
    15786.690197660:   tr strt                               0 [unknown] =>           4017da main+0xd
    15786.690197660:   tr end  return                   4017e0 main+0x13 =>           401c1a __libc_start_call_main+0x6a

Signed-off-by: Adrian Hunter <adrian.hunter@xxxxxxxxx>
---
 tools/perf/util/symbol-elf.c | 30 ++++++++++++++++++++----------
 tools/perf/util/symsrc.h     |  1 +
 2 files changed, 21 insertions(+), 10 deletions(-)

diff --git a/tools/perf/util/symbol-elf.c b/tools/perf/util/symbol-elf.c
index 8f7802097c72..9e265a726418 100644
--- a/tools/perf/util/symbol-elf.c
+++ b/tools/perf/util/symbol-elf.c
@@ -483,7 +483,6 @@ int dso__synthesize_plt_symbols(struct dso *dso, struct symsrc *ss)
 	GElf_Shdr shdr_rel_plt, shdr_dynsym;
 	Elf_Data *syms, *symstrs;
 	Elf_Scn *scn_plt_rel, *scn_symstrs, *scn_dynsym;
-	size_t dynsym_idx;
 	GElf_Ehdr ehdr;
 	char sympltname[1024];
 	Elf *elf;
@@ -530,13 +529,6 @@ int dso__synthesize_plt_symbols(struct dso *dso, struct symsrc *ss)
 		lazy_plt = true;
 	}
 
-	scn_dynsym = ss->dynsym;
-	shdr_dynsym = ss->dynshdr;
-	dynsym_idx = ss->dynsym_idx;
-
-	if (scn_dynsym == NULL)
-		return 0;
-
 	scn_plt_rel = elf_section_by_name(elf, &ehdr, &shdr_rel_plt,
 					  ".rela.plt", NULL);
 	if (scn_plt_rel == NULL) {
@@ -550,8 +542,25 @@ int dso__synthesize_plt_symbols(struct dso *dso, struct symsrc *ss)
 	    shdr_rel_plt.sh_type != SHT_REL)
 		return 0;
 
-	if (shdr_rel_plt.sh_link != dynsym_idx)
+	if (!shdr_rel_plt.sh_link)
+		return 0;
+
+	if (shdr_rel_plt.sh_link == ss->dynsym_idx) {
+		scn_dynsym = ss->dynsym;
+		shdr_dynsym = ss->dynshdr;
+	} else if (shdr_rel_plt.sh_link == ss->symtab_idx) {
+		/*
+		 * A static executable can have a .plt due to IFUNCs, in which
+		 * case .symtab is used not .dynsym.
+		 */
+		scn_dynsym = ss->symtab;
+		shdr_dynsym = ss->symshdr;
+	} else {
 		goto out_elf_end;
+	}
+
+	if (!scn_dynsym)
+		return 0;
 
 	/*
 	 * Fetch the relocation section to find the idxes to the GOT
@@ -1077,8 +1086,9 @@ int symsrc__init(struct symsrc *ss, struct dso *dso, const char *name,
 
 	ss->is_64_bit = (gelf_getclass(elf) == ELFCLASS64);
 
+	ss->symtab_idx = 0;
 	ss->symtab = elf_section_by_name(elf, &ehdr, &ss->symshdr, ".symtab",
-			NULL);
+			&ss->symtab_idx);
 	if (ss->symshdr.sh_type != SHT_SYMTAB)
 		ss->symtab = NULL;
 
diff --git a/tools/perf/util/symsrc.h b/tools/perf/util/symsrc.h
index 2665b4bde751..edf82028c9e6 100644
--- a/tools/perf/util/symsrc.h
+++ b/tools/perf/util/symsrc.h
@@ -26,6 +26,7 @@ struct symsrc {
 	GElf_Shdr	     opdshdr;
 
 	Elf_Scn		     *symtab;
+	size_t		     symtab_idx;
 	GElf_Shdr	     symshdr;
 
 	Elf_Scn		     *dynsym;
-- 
2.34.1