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Re: PT_NOTE alignment, NT_GNU_PROPERTY_TYPE_0, glibc and gold


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


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