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: <20130322.093042.393566604.d.hatayama@jp.fujitsu.com>
Date:	Fri, 22 Mar 2013 09:30:42 +0900 (JST)
From:	HATAYAMA Daisuke <d.hatayama@...fujitsu.com>
To:	vgoyal@...hat.com
Cc:	ebiederm@...ssion.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

From: Vivek Goyal <vgoyal@...hat.com>
Subject: Re: [PATCH v3 17/21] vmcore: check NT_VMCORE_PAD as a mark indicating the end of ELF note buffer
Date: Thu, 21 Mar 2013 10:36:56 -0400

> On Wed, Mar 20, 2013 at 08:54:25PM -0700, Eric W. Biederman wrote:
> 
> [..]
>> > 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.
> 
> And in our case we don't know the size of ELF note. Kernel is not
> exporting the size. So kexec-tools is putting an upper limit of 1024
> and putting that value in p_memsz and p_filesz fields.
> 
> Given the fact that we are reserving elf notes at boot. That means
> we know the size of ELF notes. It should make sense to export it
> to user space and let kexec-tools put right values.
> 
> In fact looks like /sys/kernel/vmcoreinfo is exporting two values. Address
> and size. (This is kind of violation of sysfs poilcy of one value per
> file). But for per cpu notes, we are exporting only address and not
> size.

IIRC, Greg Norman pointed out this violation of vmcoreinfo file when
he found some monthes ago.

> 
> /sys/devices/system/cpu/cpu<n>/crash_notes
> 
> May be we should export another file
> 
> /sys/devices/system/cpu/cpu<n>/crash_notes_size
> 
> and let kexec-tools parse it.

Anyway, I think of this issue as beyond the scope of what I'm working
here...

Thanks.
HATAYAMA, Daisuke

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