From: "H.J. Lu" <hjl.tools@gmail.com>
To: Florian Weimer <fweimer@redhat.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 (was: Re: [PATCH] Document GNU_PROPERTY_X86_ISA_1_[USED|NEEDED])
Date: Thu, 16 Aug 2018 06:29:34 -0700 [thread overview]
Message-ID: <CAMe9rOoK83K8-9zZxRvSqYzWgB4ZCKPXb9abvsjDLLEKFohZXA@mail.gmail.com> (raw)
In-Reply-To: <CAMe9rOpZ7WuEmUxtUCtEmdg5Obv8ZutFnLr7Bv9q5V6NZZ4PHw@mail.gmail.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.
next prev parent reply other threads:[~2018-08-16 13:29 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 [this message]
2018-08-16 13:31 ` PT_NOTE alignment, NT_GNU_PROPERTY_TYPE_0, glibc and gold Florian Weimer
2018-08-16 13:39 ` 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
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
List information: https://www.gnu.org/software/libc/involved.html
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=CAMe9rOoK83K8-9zZxRvSqYzWgB4ZCKPXb9abvsjDLLEKFohZXA@mail.gmail.com \
--to=hjl.tools@gmail.com \
--cc=binutils@sourceware.org \
--cc=fweimer@redhat.com \
--cc=libc-alpha@sourceware.org \
--cc=x86-64-abi@googlegroups.com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for read-only IMAP folder(s) and NNTP newsgroup(s).