linux-kernel - Re: [PATCH v3 17/21] vmcore: check NT_VMCORE_PAD as a mark indicating the end of ELF note buffer

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Message-ID: <87r4j98l1q.fsf@xmission.com>
Date:	Wed, 20 Mar 2013 20:54:25 -0700
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	HATAYAMA Daisuke <d.hatayama@...fujitsu.com>
Cc:	vgoyal@...hat.com, cpw@....com, kumagai-atsushi@....nes.nec.co.jp,
	lisa.mitchell@...com, heiko.carstens@...ibm.com,
	akpm@...ux-foundation.org, kexec@...ts.infradead.org,
	linux-kernel@...r.kernel.org, zhangyanfei@...fujitsu.com
Subject: Re: [PATCH v3 17/21] vmcore: check NT_VMCORE_PAD as a mark indicating the end of ELF note buffer

HATAYAMA Daisuke <d.hatayama@...fujitsu.com> writes:

> From: "Eric W. Biederman" <ebiederm@...ssion.com>
> Subject: Re: [PATCH v3 17/21] vmcore: check NT_VMCORE_PAD as a mark indicating the end of ELF note buffer
> Date: Tue, 19 Mar 2013 14:11:51 -0700
>
>> HATAYAMA Daisuke <d.hatayama@...fujitsu.com> writes:
>> 
>>> Modern kernel marks the end of ELF note buffer with NT_VMCORE_PAD type
>>> note in order to make the buffer satisfy mmap()'s page-size boundary
>>> requirement. This patch makes finishing reading each buffer if the
>>> note type now being read is NT_VMCORE_PAD type.
>> 
>> Ick.  Even with a pad header you can mark the end with an empty header,
>> and my memory may be deceiving me but I believe an empty header is
>> specified by the ELF ABI docs.
>> 
>> Beyond which I don't quite see the point of any of this as all of these
>> headers need to be combined into a single note section before being
>> presented to user space.
>
> Though this patch might get unecessary later, I cannot find part
> explaining necessity of marking end of ELF segmetns with an empty
> header in ELF spec. For example:

You are right.  It appears to be our own invention and just part of
the ABI of taking a crash dump.  Something we should sit down and
document someday.

> Also, it's possible to get size of a whole part of ELF note segments
> from p_memsz or p_filesz, and gdb and binutils are reading the note
> segments until reaching the size.

Agreed.  Except in our weird case where we generate the notes on the
fly, and generate the NOTE segment header much earlier.

Eric
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