linux-kernel - Re: [PATCH RESEND 1/3] printk: convert byte-buffer to variable-length record buffer

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Date:	Sat, 26 May 2012 04:11:43 -0700
From:	Anton Vorontsov <anton.vorontsov@...aro.org>
To:	Kay Sievers <kay@...y.org>
Cc:	Greg Kroah-Hartmann <greg@...ah.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org,
	Kees Cook <keescook@...omium.org>,
	Colin Cross <ccross@...roid.com>,
	Tony Luck <tony.luck@...el.com>,
	John Stultz <john.stultz@...aro.org>
Subject: Re: [PATCH RESEND 1/3] printk: convert byte-buffer to
 variable-length record buffer

Hello Kay,

On Thu, May 03, 2012 at 02:29:13AM +0200, Kay Sievers wrote:
> From: Kay Sievers <kay@...y.org>
> Subject: printk: convert byte-buffer to variable-length record buffer

Bad news, this patch breaks all kmsg_dump users.

Per current printk.c:

 * The 'struct log' buffer header must never be directly exported to
 * userspace, it is a kernel-private implementation detail that might
 * need to be changed in the future, when the requirements change.

But kmsg_dump users get the raw log, not text. Some users may even
use strlen() on the buffers, e.g. ramoops module was using it just
recently, before persistent_ram rework.

So, starting from this patch every kmsg_dump user is getting the
binary stuff, which is barely usable, and breaks a longstanding
user-visible ABIs (e.g. mtdoops, oops dumping into nvram on PowerPC,
and ramoops of course).

I'm not sure how we can fix this without allocating a huge buffer
(and thus wasting memory), or reworking the whole kmsg_dump concept
and its users.

Ideas?

p.s. And, btw, during HW bring-up, when there's even no serial
console usable, I somewhat liked to directly print log_buf memory
from a JTAG debugger, and thus see the kernel logs. Now that'll
look ugly. Heh. :-)

-- 
Anton Vorontsov
Email: cbouatmailru@...il.com
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