[PATCH V8 1/4] Add dso__is_pie call to identify ELF PIE

[Leo Yan]

On 9/9/2024 10:30 PM, Steve Clevenger wrote:
> 
> Add dso__is_pie global to read the .dynamic section DT_FLAGS_1 entry for
> the DF_1_PIE flag. This identifies position executable code.
> 
> Signed-off-by: Steve Clevenger <scclevenger at os.amperecomputing.com>
> Reviewed-by: Leo Yan <leo.yan at arm.com>
> ---
>  tools/perf/util/symbol-elf.c | 61 ++++++++++++++++++++++++++++++++++++
>  tools/perf/util/symbol.h     |  1 +
>  2 files changed, 62 insertions(+)
> 
> diff --git a/tools/perf/util/symbol-elf.c b/tools/perf/util/symbol-elf.c
> index e398abfd13a0..babe47976922 100644
> --- a/tools/perf/util/symbol-elf.c
> +++ b/tools/perf/util/symbol-elf.c
> @@ -662,6 +662,67 @@ static int dso__synthesize_plt_got_symbols(struct dso *dso, Elf *elf,
>         return err;
>  }
> 
> +/*
> + * Check dynamic section DT_FLAGS_1 for a Position Independent
> + * Executable (PIE).
> + */
> +bool dso__is_pie(struct dso *dso)
> +{
> +       Elf *elf = NULL;
> +       Elf_Scn *scn = NULL;
> +       GElf_Ehdr ehdr;
> +       GElf_Shdr shdr;
> +       bool is_pie = false;
> +       char dso_path[PATH_MAX];
> +       int fd = -1;
> +
> +       if (!dso || (elf_version(EV_CURRENT) == EV_NONE))
> +               goto exit;      // false
> +
> +       dso__build_id_filename(dso, dso_path, sizeof(dso_path), false);
> +
> +       fd = open(dso_path, O_RDONLY);
> +
> +       if (fd < 0) {
> +               pr_debug("%s: cannot read cached %s.\n", __func__, dso_path);
> +               goto exit;      // false
> +       }
> +
> +       elf = elf_begin(fd, ELF_C_READ, NULL);
> +       gelf_getehdr(elf, &ehdr);
> +
> +       if (ehdr.e_type == ET_DYN) {
> +               Elf_Data *data;
> +               GElf_Dyn *entry;
> +               int n_entries = shdr.sh_size / sizeof(GElf_Dyn);

I took time to play this series on Arm64 machine and found issue in above
sentence. As the 'shdr' strucutre is read out from elf_section_by_name()
below, but it calculates the entries before reading out the section header.
Therefore, I observed that program will not be detected as PIE executable due
to 'n_entries' is 0.

With fixing this bug, then I observed the regression caused by patch 02.

Below are steps:

- Build test program with below command:

  # gcc -pie -Wl,-z,relro,-z,now -o test test.c
  # readelf readelf -a test | grep FLAGS
  0x000000000000001e (FLAGS)              BIND_NOW
  0x000000006ffffffb (FLAGS_1)            Flags: NOW PIE

  The test program is a simple infinite loop. I run this program in a docker
  container with Fedora 40.

- Record trace data:

  # perf record -e cycles -p 149531
  # perf script

            test  149531 483168.078485:       4986 cycles:      aaaacde401b4
[unknown] (/home/test)
            test  149531 483168.078564:     134207 cycles:      aaaacde401b4
[unknown] (/home/test)
            test  149531 483168.079305:    1257097 cycles:      aaaacde401b4
[unknown] (/home/test)

  You can see it fails to parse symbol and print out [unknown].

  After I reverted patch 02 in this series, then:

  # perf script
            test  149531 483168.078485:       4986 cycles:      aaaacde401b4
main+0xc (/home/test)
            test  149531 483168.078564:     134207 cycles:      aaaacde401b4
main+0xc (/home/test)
            test  149531 483168.079305:    1257097 cycles:      aaaacde401b4
main+0xc (/home/test)

Not sure if I miss anything for PIE executable, seems to me, we should drop
the first two patches and just pass pg_off to python script?

Thanks,
Leo

> +
> +               scn = elf_section_by_name(elf, &ehdr, &shdr, ".dynamic", NULL);
> +               if (!scn)
> +                       goto exit_close;        // false
> +
> +               data = (Elf_Data *) elf_getdata(scn, NULL);
> +               if (!data || !data->d_buf)
> +                       goto exit_close;        // false
> +
> +               // check DT_FLAGS_1
> +               for (int i = 0; i < n_entries; i++) {
> +                       entry = ((GElf_Dyn *) data->d_buf) + i;
> +                       if (entry->d_tag == DT_FLAGS_1) {
> +                               if ((entry->d_un.d_val & DF_1_PIE) != 0) {
> +                                       is_pie = true;
> +                                       break;
> +                               }
> +                       }
> +               } // end for
> +       }
> +
> +exit_close:
> +       elf_end(elf);
> +       close(fd);
> +exit:
> +       return is_pie;
> +}
> +
>  /*
>   * 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
> diff --git a/tools/perf/util/symbol.h b/tools/perf/util/symbol.h
> index 3fb5d146d9b1..33ea2596ce31 100644
> --- a/tools/perf/util/symbol.h
> +++ b/tools/perf/util/symbol.h
> @@ -127,6 +127,7 @@ void dso__insert_symbol(struct dso *dso,
>                         struct symbol *sym);
>  void dso__delete_symbol(struct dso *dso,
>                         struct symbol *sym);
> +bool dso__is_pie(struct dso *dso);
> 
>  struct symbol *dso__find_symbol(struct dso *dso, u64 addr);
>  struct symbol *dso__find_symbol_nocache(struct dso *dso, u64 addr);
> --
> 2.44.0
>