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


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.

> 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.


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