Re: [PATCH v2] libebl: recognize FDO Packaging Metadata ELF note

[Mark Wielaard <mark@klomp.org>]

Hi Luca,

On Sat, 2022-03-26 at 18:19 +0000, Luca Boccassi wrote:
> > Already working on the updated script, the native type is exactly
> > the
> > approach I was taking, this works fine on a Debian machine on s390x
> > (and also on x86_64), eg:
> > 
> > -        BYTE(0x04) BYTE(0x00) BYTE(0x00) BYTE(0x00) /* Length of
> > Owner including NUL */
> > -        BYTE(0x47) BYTE(0x00) BYTE(0x00) BYTE(0x00) /* Length of
> > Value including NUL */
> > -        BYTE(0x7e) BYTE(0x1a) BYTE(0xfe) BYTE(0xca) /* Note ID */
> > +        LONG(0x04)                                  /* Length of
> > Owner including NUL */
> > +        LONG(0x0047)                                /* Length of
> > Value including NUL */
> > +        LONG(0xcafe1a7e)                            /* Note ID */
> > 
> > The rest of the fields are C strings so no change needed, I
> > believe.
> > 
> > Does this look right to you as well?
> 
> Here's the fix:
> 
> https://src.fedoraproject.org/rpms/package-notes/pull-request/3#
> 
> Now it's up to the Fedora folks what to do with it. I tested the
> updated script on Debian x86_64 and s390x, and it all works as
> intended. Sorry again for the breakage!

Yes, that looks correct. Note that the example on 
https://systemd.io/COREDUMP_PACKAGE_METADATA/ also uses BYTEs for
everything, instead of LONGs for the namesz, descsz and type words.

This also seems to make sure everything is aligned (at 4 bytes). An ELF
note is defined as an array of (4 byte) words. Where the first 3
(n_namesz, n_descsz, n_type) have a special interpretation. Your name
string is also exactly 4 bytes "FDO\0", so you don't need any extra
padding to make the start of the descriptor be aligned. And since you
don't add any other notes to the section you don't need to explicitly
pad the description. The linker should take care of that in case it
merges note sections/segments.

Still I would really recommend trying to add native support to linkers
for package notes, just like they support build-ids by default. That
also makes it easier for the linker to simply merge the notes. Trying
to do this with inserting a linker script really feels very fragile.

Cheers,

Mark