linux-kernel - Re: [Patch 1/4][kernel][slimdump] Add new elf-note of type NT

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Date:	Tue, 4 Oct 2011 10:30:12 -0400
From:	Vivek Goyal <vgoyal@...hat.com>
To:	"K.Prasad" <prasad@...ux.vnet.ibm.com>
Cc:	linux-kernel@...r.kernel.org, crash-utility@...hat.com,
	kexec@...ts.infradead.org, Andi Kleen <andi@...stfloor.org>,
	"Luck, Tony" <tony.luck@...el.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>, anderson@...hat.com,
	tachibana@....nes.nec.co.jp, oomichi@....nes.nec.co.jp
Subject: Re: [Patch 1/4][kernel][slimdump] Add new elf-note of type
 NT_NOCOREDUMP to capture slimdump

On Mon, Oct 03, 2011 at 01:02:03PM +0530, K.Prasad wrote:
> There are certain types of crashes induced by faulty hardware in which
> capturing crashing kernel's memory (through kdump) makes no sense (or sometimes
> dangerous).
> 
> A case in point, is unrecoverable memory errors (resulting in fatal machine
> check exceptions) in which reading from the faulty memory location from the
> kexec'ed kernel will cause double fault and system reset (leaving no
> information for the user).
> 
> This patch introduces a framework called 'slimdump' enabled through a new
> elf-note NT_NOCOREDUMP. Any error whose cause cannot be attributed to a
> software error and cannot be detected by analysing the kernel memory may
> decide to add this elf-note to the vmcore and indicate the futility of
> such an exercise. Tools such as 'kexec', 'makedumpfile' and 'crash' are
> also modified in tandem to recognise this new elf-note and capture
> 'slimdump'.
> 
> The physical address and size of the NT_NOCOREDUMP are made available to the
> user-space through a "/sys/kernel/nt_nocoredump" sysfs file (just like other
> kexec related files).

Even if kernel has to signal to user space the reason for crash, why not
add this info to existing vmcoreinfo note. Something like another filed.
PANIC_MCE=1.

Secondly, the note name NT_NOCOREDUMP itself sounds binding. Kernel can
export the reason of panic and then it is up to user space what do they
want to do with it.

So to me,  

> 
> Signed-off-by: K.Prasad <prasad@...ux.vnet.ibm.com>
> ---
>  arch/x86/kernel/cpu/mcheck/mce.c |   28 ++++++++++++++++++++++++++++
>  include/linux/elf.h              |   18 ++++++++++++++++++
>  include/linux/kexec.h            |    1 +
>  kernel/kexec.c                   |   11 +++++++++++
>  kernel/ksysfs.c                  |   10 ++++++++++
>  5 files changed, 68 insertions(+), 0 deletions(-)
> 
> diff --git a/arch/x86/kernel/cpu/mcheck/mce.c b/arch/x86/kernel/cpu/mcheck/mce.c
> index 08363b0..483b2fc 100644
> --- a/arch/x86/kernel/cpu/mcheck/mce.c
> +++ b/arch/x86/kernel/cpu/mcheck/mce.c
> @@ -238,6 +238,34 @@ static atomic_t mce_paniced;
>  static int fake_panic;
>  static atomic_t mce_fake_paniced;
>  
> +void arch_add_nocoredump_note(u32 *buf)
> +{
> +	struct elf_note note;
> +	const char note_name[] = "PANIC_MCE";
> +	const char desc_msg[] = "Crash induced due to a fatal machine "
> +				"check error";
> +

Again, note_name and desc_msg seem to be only two exports. Frankly desc
string seems pretty obivious and we should be able to ignore it. So just
exporting PANIC_MCE=true or something like that in case of MCE. 

Thanks
Vivek
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