Re: [PATCHv3 bpf-next 5/9] selftests/bpf: Add read_buildid function

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

On Thu, Mar 16, 2023 at 10:03 AM Jiri Olsa <jolsa@kernel.org> wrote:
>
> Adding read_build_id function that parses out build id from
> specified binary.
>
> It will replace extract_build_id and also be used in following
> changes.
>
> Signed-off-by: Jiri Olsa <jolsa@kernel.org>
> ---
>  tools/testing/selftests/bpf/trace_helpers.c | 86 +++++++++++++++++++++
>  tools/testing/selftests/bpf/trace_helpers.h |  5 ++
>  2 files changed, 91 insertions(+)
>
> diff --git a/tools/testing/selftests/bpf/trace_helpers.c b/tools/testing/selftests/bpf/trace_helpers.c
> index 934bf28fc888..72b38a41f574 100644
> --- a/tools/testing/selftests/bpf/trace_helpers.c
> +++ b/tools/testing/selftests/bpf/trace_helpers.c
> @@ -11,6 +11,9 @@
>  #include <linux/perf_event.h>
>  #include <sys/mman.h>
>  #include "trace_helpers.h"
> +#include <linux/limits.h>
> +#include <libelf.h>
> +#include <gelf.h>
>
>  #define TRACEFS_PIPE   "/sys/kernel/tracing/trace_pipe"
>  #define DEBUGFS_PIPE   "/sys/kernel/debug/tracing/trace_pipe"
> @@ -234,3 +237,86 @@ ssize_t get_rel_offset(uintptr_t addr)
>         fclose(f);
>         return -EINVAL;
>  }
> +
> +static int
> +parse_build_id_buf(const void *note_start, Elf32_Word note_size,
> +                  char *build_id)

nit: single line

should we pass buffer size instead of assuming at least BPF_BUILD_ID_SIZE below?

> +{
> +       Elf32_Word note_offs = 0, new_offs;
> +
> +       while (note_offs + sizeof(Elf32_Nhdr) < note_size) {
> +               Elf32_Nhdr *nhdr = (Elf32_Nhdr *)(note_start + note_offs);
> +
> +               if (nhdr->n_type == 3 && nhdr->n_namesz == sizeof("GNU") &&
> +                   !strcmp((char *)(nhdr + 1), "GNU") && nhdr->n_descsz > 0 &&
> +                   nhdr->n_descsz <= BPF_BUILD_ID_SIZE) {
> +                       memcpy(build_id, note_start + note_offs +
> +                              ALIGN(sizeof("GNU"), 4) + sizeof(Elf32_Nhdr), nhdr->n_descsz);
> +                       memset(build_id + nhdr->n_descsz, 0, BPF_BUILD_ID_SIZE - nhdr->n_descsz);
> +                       return (int) nhdr->n_descsz;
> +               }
> +
> +               new_offs = note_offs + sizeof(Elf32_Nhdr) +
> +                          ALIGN(nhdr->n_namesz, 4) + ALIGN(nhdr->n_descsz, 4);
> +               if (new_offs >= note_size)
> +                       break;

while condition() above would handle this, so this check appears not necessary?

so just assign note_offs directly?


> +               note_offs = new_offs;
> +       }
> +
> +       return -EINVAL;

nit: -ENOENT or -ESRCH?

> +}
> +
> +/* Reads binary from *path* file and returns it in the *build_id*
> + * which is expected to be at least BPF_BUILD_ID_SIZE bytes.
> + * Returns size of build id on success. On error the error value
> + * is returned.
> + */
> +int read_build_id(const char *path, char *build_id)
> +{
> +       int fd, err = -EINVAL;
> +       Elf *elf = NULL;
> +       GElf_Ehdr ehdr;
> +       size_t max, i;
> +
> +       fd = open(path, O_RDONLY | O_CLOEXEC);
> +       if (fd < 0)
> +               return -errno;
> +
> +       (void)elf_version(EV_CURRENT);
> +
> +       elf = elf_begin(fd, ELF_C_READ, NULL);

ELF_C_READ_MMAP ?

> +       if (!elf)
> +               goto out;
> +       if (elf_kind(elf) != ELF_K_ELF)
> +               goto out;
> +       if (gelf_getehdr(elf, &ehdr) == NULL)

nit: !gelf_getehdr()

> +               goto out;
> +       if (ehdr.e_ident[EI_CLASS] != ELFCLASS64)
> +               goto out;

does this have to be 64-bit specific?... you are using gelf stuff, you
can be bitness-agnostic here

> +
> +       for (i = 0; i < ehdr.e_phnum; i++) {
> +               GElf_Phdr mem, *phdr;
> +               char *data;
> +
> +               phdr = gelf_getphdr(elf, i, &mem);
> +               if (!phdr)
> +                       goto out;
> +               if (phdr->p_type != PT_NOTE)
> +                       continue;

I don't know where ELF + build ID spec is (if at all), but it seems to
always be in the ".note.gnu.build-id" section, so should we check the
name here?


> +               data = elf_rawfile(elf, &max);
> +               if (!data)
> +                       goto out;
> +               if (phdr->p_offset >= max || (phdr->p_offset + phdr->p_memsz >= max))

`phdr->p_offset + phdr->p_memsz == max` would be fine, no?

> +                       goto out;
> +               err = parse_build_id_buf(data + phdr->p_offset, phdr->p_memsz, build_id);
> +               if (err > 0)
> +                       goto out;
> +               err = -EINVAL;
> +       }
> +
> +out:
> +       if (elf)
> +               elf_end(elf);
> +       close(fd);
> +       return err;
> +}
> diff --git a/tools/testing/selftests/bpf/trace_helpers.h b/tools/testing/selftests/bpf/trace_helpers.h
> index 53efde0e2998..bc3b92057033 100644
> --- a/tools/testing/selftests/bpf/trace_helpers.h
> +++ b/tools/testing/selftests/bpf/trace_helpers.h
> @@ -4,6 +4,9 @@
>
>  #include <bpf/libbpf.h>
>
> +#define __ALIGN_MASK(x, mask)  (((x)+(mask))&~(mask))
> +#define ALIGN(x, a)            __ALIGN_MASK(x, (typeof(x))(a)-1)
> +
>  struct ksym {
>         long addr;
>         char *name;
> @@ -23,4 +26,6 @@ void read_trace_pipe(void);
>  ssize_t get_uprobe_offset(const void *addr);
>  ssize_t get_rel_offset(uintptr_t addr);
>
> +int read_build_id(const char *path, char *build_id);
> +
>  #endif
> --
> 2.39.2
>