[PATCH] D18055: ELF: Implement --build-id.

[Rui Ueyama via llvm-commits]

I sent out http://reviews.llvm.org/D18091 for review. It implements FNV1
hash.

On Fri, Mar 11, 2016 at 9:37 AM, Ed Maste <emaste at freebsd.org> wrote:

> On 11 March 2016 at 11:56, Rui Ueyama <ruiu at google.com> wrote:
> > Non-secure hash function such as FNV1 can be written in 10 lines of code
> or
> > so, so we don't necessarily have to use ADT/Hashing.h.
> >
> > The question is whether we need to use a secure hash or not. If we can
> > accept a collision of build ID (although it's a very low probability), we
> > should use a non-secure hash. If not, we need to use a secure hash. It
> seems
> > to me that everybody's basically okay with a very low probability of
> > collision for the sake of performance? So here's my plan.
> >
> >  - Use a fast non-secure hash function as default for --build-id
> >  - Implement --build-id=md5 (GNU ld and gold both have this option) so
> that
> > users can choose the secure hash function
> >
> > Any objections/concerns?
>
> This sounds broadly fine to me.
>
> Note that ld.bfd and gold support md5, sha1, and a 128-bit random
> number ("uuid"). sha1 seems to be the default for both ld.bfd and
> gold, and if we implement only one "secure" hash I think that should
> be it.
>
> Gold also has a mode that parallelizes by hashing chunks of the output
> file, and then hashing those hashes. It's unfortunate to expose so
> many options to the user though. I'm not sure what makes the most
> sense for us with respect to providing useful knobs, and bfd/gold
> compatibility. In my ideal world we'd have only one option in lld
> that's both fast and secure.
>
> One other item to consider, some tools make expectations about the
> size of the build-id data. For example, "file" uses the size to report
> the putative algorithm used. It reports "sha1" for a 20-byte build-id
> and "md5/uuid" for a 16-byte build-id.
>
> % cc -fuse-ld=gold -Wl,--build-id hello.c
> % file a.out
> a.out: ELF 64-bit LSB executable, x86-64, version 1 (FreeBSD),
> dynamically linked, interpreter /libexec/ld-elf.so.1, for FreeBSD 10.1
> (1001507), BuildID[sha1]=74c58ef86747a092d357a42cc106eb4688e15333, not
> stripped
> %
>

The build-id note section has the size field for the build-id so that any
tools that reads an ID is able to know the size of it. In our case, because
it is 64-bit FNV1, it's set to 8 (byte.)


> In any case I'd be happy with what you describe above, but with a
> small preference for sha1 over md5.
>
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