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

[Jiri Olsa <olsajiri@gmail.com>]

On Thu, Mar 16, 2023 at 03:23:03PM -0700, Andrii Nakryiko wrote:
> 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>

I'll send this separatelly as bpf/selftests fix so doesn't get lost

> > ---
> >  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

ok

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

ok

> 
> > +{
> > +       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?

good idea, it will simplify that

> 
> 
> > +               note_offs = new_offs;
> > +       }
> > +
> > +       return -EINVAL;
> 
> nit: -ENOENT or -ESRCH?

I kept the same error as is in kernel, but ENOENT makes more sense

> 
> > +}
> > +
> > +/* 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 ?

ok

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

ok

> 
> > +               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

right, I don't think it's needed, will check

> 
> > +
> > +       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?

this section name is not manadatory as stated in
  https://fedoraproject.org/wiki/RolandMcGrath/BuildID

  The new section is canonically called .note.gnu.build-id, but the name is not normative,
  and the section can be merged with other SHT_NOTE sections. The ELF note headers give
  name "GNU" and type 3 (NT_GNU_BUILD_ID) for a build ID note.

> 
> 
> > +               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?

right, will change

thanks,
jirka

> 
> > +                       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
> >