Re: PT_NOTE alignment, NT_GNU_PROPERTY_TYPE_0, glibc and gold

["H.J. Lu" <hjl.tools@gmail.com>]

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.

>>> Is the link editor supposed to maintain separate segments for notes with
>>> different alignments?  Or is it possible to merge the notes into a single
>>> segment, potentially after adjusting alignment?
>>>
>>
>> It is possible.  We just need to place 4-byte aligned notes after 8-byte
>> aligned notes.
>
>
> Based on section 2.1.7, this would not be valid by itself because the
> section needs to have 8-byte alignment (to satisfy the property notes
> requirement).  All notes in the segment need to have the same alignment
> (because p_align is supposed to be used for parsing).  So reordering alone
> will not produce a valid segment.
>
> Part of the problem is that the note header is 12 bytes (not a multiple of
> 8), and that the name and descriptor lengths do not include the padding
> (which makes sense), so you really need a correct source of alignment.
>
> If we want to generate a single segment (and I think we should), we need to
> realign the notes to a common alignment, either 4 or 8 bytes.  That's what
> gold seems todo right now, with 4-byte alignment.

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

>>> Is the link editor *required* to produce 8-byte alignment for notes in
>>> ELFCLASS64 objects?
>>
>>
>> It is decided by the alignment of NOTE section, not by linker.
>>
>>> Currently, we do not have agreement between binutils (particularly gold)
>>> and
>>> the glibc dynamic loader when it comes to alignment of PT_NOTE segments.
>>> glibc will disregard property notes in ELFCLASS64 objects which have
>>> 4-byte
>>> alignment, but gold produces such notes.  This needs to be fixed.
>>
>>
>> I don't believe this is true.  See above.
>
>
> Which part?  I see the 4-byte segment alignment with gold from
> binutils-2.31.1-11.fc29.x86_64.
>

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.

>> After this commit:
>>
>> commit 8d81ce0c6d6ca923571e8b2bac132929f9a02973
>> Author: H.J. Lu <hjl.tools@gmail.com>
>> Date:   Tue Nov 28 09:56:47 2017 -0800
>>
>>      Properly compute offsets of note descriptor and next note [BZ #22370]
>
> …
>>
>> glibc can handle both 4 byte and 8 byte NOTE alignments.
>
>
> There's still this code in glibc, in sysdeps/x86/dl-prop.h:
>
>   /* The NT_GNU_PROPERTY_TYPE_0 note must be aliged to 4 bytes in
>      32-bit objects and to 8 bytes in 64-bit objects.  Skip notes
>      with incorrect alignment.  */
>   if (align != (__ELF_NATIVE_CLASS / 8))
>     return;
>

This code is correct.  NT_GNU_PROPERTY_TYPE_0 follows gABI.


-- 
H.J.