* Re: Alignment and sizes of note sections in 64-bit ELF objects
[not found] ` <CAMe9rOp4tDf619hCJUMMjd=MGFobSEtNgddrczWR9trb6JishQ@mail.gmail.com>
@ 2017-01-01 0:00 ` Suprateeka R Hegde
2017-01-01 0:00 ` H.J. Lu
0 siblings, 1 reply; 2+ messages in thread
From: Suprateeka R Hegde @ 2017-01-01 0:00 UTC (permalink / raw)
To: generic-abi, H.J. Lu; +Cc: gnu-gabi
(I took a while to recollect the discussion)
On 16-Nov-2017 10:11 PM, H.J. Lu wrote:
> On Mon, Apr 3, 2017 at 11:48 AM, Suprateeka R Hegde
> <hegdesmailbox@gmail.com> wrote:
>> On 03-Apr-2017 08:49 PM, Ian Lance Taylor wrote:
>>> On Mon, Apr 3, 2017 at 8:16 AM, H.J. Lu <hjl.tools@gmail.com> wrote:
>>>> According to gABI, in 64-bit objects, each note entry is an array of 8-bye
>>>> words in the format of the target processor. But I got
>>>>
>>>> [ 2] .note.ABI-tag NOTE 0000000000000254 000254
>>>> 000020 00 A 0 0 4
>>>> [ 3] .note.gnu.build-id NOTE 0000000000000274 000274
>>>> 000024 00 A 0 0 4
>>>>
>>>> on Linux/x86-64. .note.ABI-tag size is 32, but it isn't aligned at 8 bytes.
>>>> .note.gnu.build-id size is 36, which isn't multiple of 8 bytes. Should
>>>> note sections in 64-bit ELF objects be multiple of 8 bytes as well as
>>>> aligned to 8 bytes?
>>
>> On HP-UX, it is 8 byte alignment. And the size becomes multiple of 8
>> bytes automatically, as we ensure 8 byte alignment for the next note
>> entry. This is as per the gABI.
>>
>>> This is the comment I wrote in gold when I looked into this:
>>>
>>> // Authorities all agree that the values in a .note field should
>>> // be aligned on 4-byte boundaries for 32-bit binaries. However,
>>> // they differ on what the alignment is for 64-bit binaries.
>>> // The GABI says unambiguously they take 8-byte alignment:
>>> // http://sco.com/developers/gabi/latest/ch5.pheader.html#note_section
>>> // Other documentation says alignment should always be 4 bytes:
>>> // http://www.netbsd.org/docs/kernel/elf-notes.html#note-format
>>
>> Thats interesting. But why should it differ from GABI?
>>
>> May be because: In both ILP32 and LP64 model, integer is 4 bytes. And
>> the documentation says "integer" in parenthesis for the fields.
>>
>> For example:
>> Name Size
>> 4 bytes (integer)
>> Desc Size
>> 4 bytes (integer)
>>
>> In contrast to GABI, NetBSD seems to have attributed these fields with
>> "integer".
>>
>
> Linux has been using 4 byte alignment for .note.ABI-tag note and
> .note.gnu.build-id note in 64-bit objects. We can't change their
> alignment to 8 bytes. Can we add a footnote in gABI to address it?
It should be more clear than that. See below.
> In reality, this isn't a real issue since all notes in one PT_LOAD segment
> must have the same alignment which equals to p_align.
p_align of PT_LOAD or PT_NOTE?
> Note parser
> can use p_align of PT_LOAD segment for note alignment,
Why not p_align of PT_NOTE? p_align of PT_LOAD seems to be (on my
Ubuntu) set to the 2MiB pagesize value.
> instead of
> assuming alignment based on ELF file class.
The gABI description of ELF class based alignment may be because gABI
does not talk anything about PT_NOTE actually being part of PT_LOAD.
PT_NOTE could be a separate segment on its own outside PT_LOAD, though
almost all implementations make it part of PT_LOAD.
> BTW, should we document that all notes in one PT_LOAD segment
> must have the same alignment which equals to p_align?
Why not p_align of PT_NOTE? And p_align of PT_NOTE may be set to the
maximum of all sh_addralign of all SHT_NOTE that make up the PT_NOTE.
--
Supra
^ permalink raw reply [flat|nested] 2+ messages in thread
* Re: Alignment and sizes of note sections in 64-bit ELF objects
2017-01-01 0:00 ` Alignment and sizes of note sections in 64-bit ELF objects Suprateeka R Hegde
@ 2017-01-01 0:00 ` H.J. Lu
0 siblings, 0 replies; 2+ messages in thread
From: H.J. Lu @ 2017-01-01 0:00 UTC (permalink / raw)
To: Suprateeka R Hegde
Cc: Generic System V Application Binary Interface, gnu-gabi
On Mon, Nov 20, 2017 at 7:27 AM, Suprateeka R Hegde
<hegdesmailbox@gmail.com> wrote:
> (I took a while to recollect the discussion)
>
> On 16-Nov-2017 10:11 PM, H.J. Lu wrote:
>> On Mon, Apr 3, 2017 at 11:48 AM, Suprateeka R Hegde
>> <hegdesmailbox@gmail.com> wrote:
>>> On 03-Apr-2017 08:49 PM, Ian Lance Taylor wrote:
>>>> On Mon, Apr 3, 2017 at 8:16 AM, H.J. Lu <hjl.tools@gmail.com> wrote:
>>>>> According to gABI, in 64-bit objects, each note entry is an array of 8-bye
>>>>> words in the format of the target processor. But I got
>>>>>
>>>>> [ 2] .note.ABI-tag NOTE 0000000000000254 000254
>>>>> 000020 00 A 0 0 4
>>>>> [ 3] .note.gnu.build-id NOTE 0000000000000274 000274
>>>>> 000024 00 A 0 0 4
>>>>>
>>>>> on Linux/x86-64. .note.ABI-tag size is 32, but it isn't aligned at 8 bytes.
>>>>> .note.gnu.build-id size is 36, which isn't multiple of 8 bytes. Should
>>>>> note sections in 64-bit ELF objects be multiple of 8 bytes as well as
>>>>> aligned to 8 bytes?
>>>
>>> On HP-UX, it is 8 byte alignment. And the size becomes multiple of 8
>>> bytes automatically, as we ensure 8 byte alignment for the next note
>>> entry. This is as per the gABI.
>>>
>>>> This is the comment I wrote in gold when I looked into this:
>>>>
>>>> // Authorities all agree that the values in a .note field should
>>>> // be aligned on 4-byte boundaries for 32-bit binaries. However,
>>>> // they differ on what the alignment is for 64-bit binaries.
>>>> // The GABI says unambiguously they take 8-byte alignment:
>>>> // http://sco.com/developers/gabi/latest/ch5.pheader.html#note_section
>>>> // Other documentation says alignment should always be 4 bytes:
>>>> // http://www.netbsd.org/docs/kernel/elf-notes.html#note-format
>>>
>>> Thats interesting. But why should it differ from GABI?
>>>
>>> May be because: In both ILP32 and LP64 model, integer is 4 bytes. And
>>> the documentation says "integer" in parenthesis for the fields.
>>>
>>> For example:
>>> Name Size
>>> 4 bytes (integer)
>>> Desc Size
>>> 4 bytes (integer)
>>>
>>> In contrast to GABI, NetBSD seems to have attributed these fields with
>>> "integer".
>>>
>>
>> Linux has been using 4 byte alignment for .note.ABI-tag note and
>> .note.gnu.build-id note in 64-bit objects. We can't change their
>> alignment to 8 bytes. Can we add a footnote in gABI to address it?
>
> It should be more clear than that. See below.
>
>> In reality, this isn't a real issue since all notes in one PT_LOAD segment
>> must have the same alignment which equals to p_align.
>
> p_align of PT_LOAD or PT_NOTE?
>
>> Note parser
>> can use p_align of PT_LOAD segment for note alignment,
>
> Why not p_align of PT_NOTE? p_align of PT_LOAD seems to be (on my
> Ubuntu) set to the 2MiB pagesize value.
>
>> instead of
>> assuming alignment based on ELF file class.
>
> The gABI description of ELF class based alignment may be because gABI
> does not talk anything about PT_NOTE actually being part of PT_LOAD.
> PT_NOTE could be a separate segment on its own outside PT_LOAD, though
> almost all implementations make it part of PT_LOAD.
>
>> BTW, should we document that all notes in one PT_LOAD segment
>> must have the same alignment which equals to p_align?
>
> Why not p_align of PT_NOTE? And p_align of PT_NOTE may be set to the
> maximum of all sh_addralign of all SHT_NOTE that make up the PT_NOTE.
Typo. It should be PT_NOTE, instead of PT_LOAD.
--
H.J.
^ permalink raw reply [flat|nested] 2+ messages in thread
end of thread, other threads:[~2017-11-20 15:51 UTC | newest]
Thread overview: 2+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
[not found] <CAMe9rOoWPk84b9HoeiJdF4B_X090jfQQXCyNUm1_R22uNSHOkw@mail.gmail.com>
[not found] ` <CAOganjUx7N4qTEEM0DDdn0izhO_UPv9miB7w4O0abU=p4GmT0w@mail.gmail.com>
[not found] ` <6de4d8f7-a08e-7aac-64d0-3e41795c9143@gmail.com>
[not found] ` <CAMe9rOp4tDf619hCJUMMjd=MGFobSEtNgddrczWR9trb6JishQ@mail.gmail.com>
2017-01-01 0:00 ` Alignment and sizes of note sections in 64-bit ELF objects Suprateeka R Hegde
2017-01-01 0:00 ` H.J. Lu
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).