perf symbols: Allow for static executables with .plt

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

Reviewed-by: Namhyung Kim <namhyung@kernel.org>
Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Link: https://lore.kernel.org/r/20230131131625.6964-8-adrian.hunter@intel.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

diff --git a/tools/perf/util/symbol-elf.c b/tools/perf/util/symbol-elf.c
index 8f7802097c72390ae67bf6b4b31b6e5c4a49df9a..9e265a726418d6fac4c08f86c860258e674f8095 100644 (file)
--- 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 2665b4bde7513cd8289a76f1cc345fa0ca9e973c..edf82028c9e62bfba6f7a5ac2a37b14ab7d21c11 100644 (file)
--- 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;