PT_NOTE alignment, NT_GNU_PROPERTY_TYPE_0, glibc and gold

[Florian Weimer]

On 08/23/2018 01:36 AM, Cary Coutant wrote:
>>> Why should the loader have to go parsing everything in a generic PT_NOTE
>>> segment anyway?
>>
>> With 8-byte alignment, in practice, it is currently quite fast to discover
>> the new notes because the other notes have 4-byte alignment, so the PT_NOTE
>> segment with the property notes should be pleasantly short.
> 
> I understand the benefit of having a PT_NOTE segment containing
> nothing but the one note, and this is certainly expedient, but it's
> quite ugly and, I think, fragile. This really underscores the value of
> defining a bespoke segment type for this one purpose.

Yes, the existing note processing in linkers makes this fragile.

> How difficult would it be for glibc if the linker would consume the
> existing .note.gnu.property sections, then generate an output section
> with the same format (maybe minus the SHT_NOTE overhead), but using a
> new section type in its own new segment type? You could continue to
> look for 8-byte aligned legacy PT_NOTE segments, but could also look
> for the new PT_GNU_PROPERTY segment.

It does not sound difficult to implement.  This is based on the 
assumption that the current dynamic loader ignores unknown segment types 
(which I think is true).

But I don't view this a win because it doesn't take the 8-byte notes out 
of circulation.  We'd still have to support both indefinitely, and would 
have update GCC as well.  We can just keep using the existing code.

Thanks,
Florian