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Re: PT_NOTE alignment, NT_GNU_PROPERTY_TYPE_0, glibc and gold
On Thu, Aug 16, 2018 at 6:31 AM, Florian Weimer <firstname.lastname@example.org> wrote:
> On 08/16/2018 03:19 PM, H.J. Lu wrote:
>> On Thu, Aug 16, 2018 at 6:00 AM, Florian Weimer <email@example.com>
>>> 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:
>>>> 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
>> 2.1.7 Alignment of Note Sections
>> All entries in a PT_NOTE segment have the same alignment which equals to
>> 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
>> 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.
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)
>>> the glibc dynamic loader when it comes to alignment of PT_NOTE segments.
>>> glibc will disregard property notes in ELFCLASS64 objects which have
>>> 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
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 <firstname.lastname@example.org>
>> 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))
This code is correct. NT_GNU_PROPERTY_TYPE_0 follows gABI.