[PATCH V8 1/4] Add dso__is_pie call to identify ELF PIE
Leo Yan
leo.yan at arm.com
Wed Oct 9 14:13:42 PDT 2024
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
>
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