Re: [PATCH] perf genelf: Switch from SHA-1 to BLAKE2s for build ID generation

[Eric Biggers <ebiggers@kernel.org>]

On Fri, Dec 05, 2025 at 10:53:38AM -0800, Ian Rogers wrote:
> >  static struct buildid_note {
> >         Elf_Note desc;          /* descsz: size of build-id, must be multiple of 4 */
> >         char     name[4];       /* GNU\0 */
> > -       u8       build_id[SHA1_DIGEST_SIZE];
> > +       u8       build_id[20];
> 
> nit: Could we add a comment on where the value of 20 is coming from?
> We could use something like: sizeof(((struct
> perf_record_header_build_id*)0)->data). But a comment is enough imo.
> 
> Tested-by: Ian Rogers <irogers@google.com>
> 
> Thanks,
> Ian

It was already a literal 20 before "perf genelf: Remove libcrypto
dependency and use built-in sha1()".  So this change just restores the
previous literal 20.

This entire struct, not just the build ID field, appears to be an
external data format.  If you'd like to add a comment that describes
where this external data format is defined, presumably either the ELF
specification or binutils, that would be helpful.  But it seems outside
the scope of this change.

(I did put the literal 20 in the BLAKE2s test as well.  I'll put a
comment there that mentions that it matches the length used in the
non-test code.)

- Eric