From: Florian Weimer <fweimer@redhat.com>
To: "H.J. Lu" <hjl.tools@gmail.com>
Cc: x86-64-abi <x86-64-abi@googlegroups.com>,
Binutils <binutils@sourceware.org>,
GNU C Library <libc-alpha@sourceware.org>
Subject: Re: PT_NOTE alignment, NT_GNU_PROPERTY_TYPE_0, glibc and gold
Date: Thu, 16 Aug 2018 15:31:07 +0200 [thread overview]
Message-ID: <480f513b-cfee-311a-0793-55eec81cd0fa@redhat.com> (raw)
In-Reply-To: <CAMe9rOpZ7WuEmUxtUCtEmdg5Obv8ZutFnLr7Bv9q5V6NZZ4PHw@mail.gmail.com>
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.
>> 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.
>> 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.
> 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;
Thanks,
Florian
next prev parent reply other threads:[~2018-08-16 13:31 UTC|newest]
Thread overview: 29+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <CAMe9rOrrayKnc_cPm4SmnDnUGLbBUmOYMBTMOM8KLAHVmb=rUQ@mail.gmail.com>
2018-08-16 13:00 ` PT_NOTE alignment, NT_GNU_PROPERTY_TYPE_0, glibc and gold (was: Re: [PATCH] Document GNU_PROPERTY_X86_ISA_1_[USED|NEEDED]) Florian Weimer
2018-08-16 13:19 ` H.J. Lu
2018-08-16 13:29 ` H.J. Lu
2018-08-16 13:31 ` Florian Weimer [this message]
2018-08-16 13:39 ` PT_NOTE alignment, NT_GNU_PROPERTY_TYPE_0, glibc and gold H.J. Lu
2018-08-16 14:01 ` Florian Weimer
2018-08-16 14:43 ` H.J. Lu
2018-08-16 14:21 ` Florian Weimer
2018-08-16 17:46 ` H.J. Lu
2018-08-16 19:16 ` Mark Wielaard
2018-08-16 19:36 ` H.J. Lu
2018-08-17 6:04 ` Mark Wielaard
2018-08-17 6:20 ` Florian Weimer
2018-08-17 6:41 ` Mark Wielaard
2018-08-17 15:10 ` H.J. Lu
2018-08-17 21:05 ` Mark Wielaard
2018-08-21 23:02 ` Cary Coutant
2018-08-22 9:39 ` Florian Weimer
2018-08-22 10:08 ` Mark Wielaard
2018-08-22 23:36 ` Cary Coutant
2018-08-24 18:39 ` Florian Weimer
2018-08-23 14:41 ` Michael Matz
2018-08-23 14:43 ` Florian Weimer
2018-08-17 15:13 ` H.J. Lu
2018-09-19 19:22 ` Florian Weimer
2018-09-21 12:55 ` Michael Matz
2018-09-21 13:04 ` Florian Weimer
2018-09-26 17:39 ` Cary Coutant
2018-09-26 18:36 ` H.J. Lu
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