Re: PT_NOTE alignment, NT_GNU_PROPERTY_TYPE_0, glibc and gold

[Florian Weimer <fweimer at redhat dot com>]

On 08/16/2018 03:39 PM, H.J. Lu wrote:
> On Thu, Aug 16, 2018 at 6:31 AM, Florian Weimer <fweimer@redhat.com> wrote:
> > On 08/16/2018 03:19 PM, H.J. Lu wrote:
> > > On Thu, Aug 16, 2018 at 6:00 AM, Florian Weimer <fweimer@redhat.com>
> > > wrote:
> > > > On 08/07/2018 10:41 PM, H.J. Lu wrote:
> > > > > The .note.gnu.property section with NT_GNU_PROPERTY_TYPE_0 has been
> > > > > added to Linux Extensions to gABI:
> > > > >
> > > > > https://github.com/hjl-tools/linux-abi
> > > > >
> > > > > GNU_PROPERTY_X86_ISA_1_USED and GNU_PROPERTY_X86_ISA_1_NEEDED are
> > > > > processor-specific program property types for i386 and x86-64.
> > > >
> > > >
> > > >
> > > > The specification is incomplete as far as alignment matters are
> > > > concerned.
> > >
> > >
> > > https://github.com/hjl-tools/linux-abi/wiki/linux-abi-draft.pdf
> > >
> > > has
> > >
> > > 2.1.7 Alignment of Note Sections
> > >
> > > All entries in a PT_NOTE segment have the same alignment which equals to
> > > the
> > > p_align field in program header.
> > > According to gABI, each note entry should be aligned to 4 bytes in 32-bit
> > > objects or 8 bytes in 64-bit objects. But .note.ABI-tag section (see
> > > Section 2.1.6) and .note.gnu.build-id section (see Section 2.1.4) are
> > > aligned
> > > to 4 bytes in both 32-bit and 64-bit objects. Note parser should use
> > > p_align for
> > > note alignment, instead of assuming alignment based on ELF file class.
> >
> >
> > This is still ambiguous, particularly based on your comments below.
>
> https://github.com/hjl-tools/linux-abi/wiki/linux-abi-draft.pdf
>
> conforms to gABI unless stated otherwise.

I still think there is ambiguity.

> I was wrong.  We need 2 NOTE segments one fore 8-byte alignment and
> one for 4-byte alignment.

If one needs to generate two segments, the ABI documents should say so. 
Otherwise, linker implementors will assume that there are other ways to 
implement this.

> glibc only discards 4-byte aligned NT_GNU_PROPERTY_TYPE_0 note
> since NT_GNU_PROPERTY_TYPE_0 note follows gABI.  If gold
> generates 4 byte alignment, it is a gold bug.

See above.  I don't have a strong opinion how we make gold and glibc 
interoperate, but if gold is wrong, the ABI documents should be made 
more precise.

Thanks,
Florian