Re: PT_NOTE alignment, NT_GNU_PROPERTY_TYPE_0, glibc and gold (was: Re: [PATCH] Document GNU_PROPERTY_X86_ISA_1_[USED|NEEDED])

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

On Thu, Aug 16, 2018 at 6:19 AM, H.J. Lu <hjl.tools@gmail.com> 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.
>
>
>> 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.

Wait.  We can't since the p_align field is used to compute the next note.
We need at least 2 NOTE segments where there are both 4-byte and
8-byte aligned notes.

>> 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.
>
> 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]
>
>     A note header has 3 4-bytes fields, followed by note name and note
>     descriptor.  According to gABI, in a note entry, the note name field,
>     not note name size, is padded for the note descriptor.  And the note
>     descriptor field, not note descriptor size, is padded for the next
>     note entry.  Notes are aligned to 4 bytes in 32-bit objects and 8 bytes
>     in 64-bit objects.
>
>     For all GNU notes, the name is "GNU" which is 4 bytes.  They have the
>     same format in the first 16 bytes in both 32-bit and 64-bit objects.
>     They differ by note descriptor size and note type.  So far, .note.ABI-tag
>     and .note.gnu.build-id notes are always aligned to 4 bytes.  The exsting
>     codes compute the note size by aligning the note name size and note
>     descriptor size to 4 bytes.  It happens to produce the same value as
>     the actual note size by luck since the name size is 4 and offset of the
>     note descriptor is 16.  But it will produce the wrong size when note
>     alignment is 8 bytes in 64-bit objects.
>
>     This patch defines ELF_NOTE_DESC_OFFSET and ELF_NOTE_NEXT_OFFSET to
>     properly compute offsets of note descriptor and next note.  It uses
>     alignment of PT_NOTE segment to support both 4-byte and 8-byte note
>     alignments in 64-bit objects.  To handle PT_NOTE segments with
>     incorrect alignment, which may lead to an infinite loop, if segment
>     alignment is less than 4, we treate alignment as 4 bytes since some
>     note segments have 0 or 1 byte alignment.
>
> glibc can handle both 4 byte and 8 byte NOTE alignments.
>
>
> --
> H.J.



-- 
H.J.