PT_NOTE alignment, NT_GNU_PROPERTY_TYPE_0, glibc and gold

[Florian Weimer]

On 08/16/2018 09:16 PM, Mark Wielaard wrote:
> On Thu, Aug 16, 2018 at 04:21:05PM +0200, Florian Weimer wrote:
>> On 08/16/2018 03:39 PM, H.J. Lu wrote:
>>> 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.
>>
>> I filed: https://sourceware.org/bugzilla/show_bug.cgi?id=23535
> 
> I don't think this is a bug in gold, but one in ld:
> https://sourceware.org/bugzilla/show_bug.cgi?id=22749
> 
> In the GNU abi all ELF Notes are arrays of 32bit words
> (and so 4-byte aligned). This is the same for most other
> ELF systems. Making the ELF Notes fields 64bit words (and
> so 8-byte aligned) in ELFCLASS64 would indeed be what gabi
> literally says, but not what GNU systems, and others, follow.

The binaries are already out there and will be around for a while, even 
if we reduce the alignment now.  So generic ELF parsers need to cope 
with these binaries anyway.

> Having a mix of 4-byte words and 8-byte words ELF Notes
> in the same ELF file seems unnecessarily confusing and
> introduces extra segments and sections.

There are no 8-byte word ELF notes on GNU systems, that's an HP-UX 
feature.  The new 8-byte-aligned notes still have a four-byte header.

But like you, I don't yet see the value of the 8-byte alignment.  We 
could decide that the current gold behavior is valid, fix glibc, and 
move on.

Thanks,
Florian