linux-kernel - Re: [PATCH 02/11] persistent_ram: Fix buffer size clamping during writes

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Message-ID: <20120513203835.GA30082@lizard>
Date:	Sun, 13 May 2012 13:38:36 -0700
From:	Anton Vorontsov <anton.vorontsov@...aro.org>
To:	Dan Carpenter <dan.carpenter@...cle.com>
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Kees Cook <keescook@...omium.org>,
	Colin Cross <ccross@...roid.com>, devel@...verdev.osuosl.org,
	linaro-kernel@...ts.linaro.org, Arnd Bergmann <arnd@...db.de>,
	patches@...aro.org, Marco Stornelli <marco.stornelli@...il.com>,
	Stephen Boyd <sboyd@...eaurora.org>,
	linux-kernel@...r.kernel.org, arve@...roid.com,
	Jesper Juhl <jj@...osbits.net>,
	John Stultz <john.stultz@...aro.org>,
	Rebecca Schultz Zavin <rebecca@...roid.com>,
	WANG Cong <xiyou.wangcong@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	kernel-team@...roid.com, Thomas Meyer <thomas@...3r.de>
Subject: Re: [PATCH 02/11] persistent_ram: Fix buffer size clamping during
 writes

On Sun, May 13, 2012 at 07:56:01PM +0300, Dan Carpenter wrote:
> On Fri, May 11, 2012 at 05:17:17PM -0700, Anton Vorontsov wrote:
> > This is a longstanding bug, almost unnoticeable when calling
> > persistent_ram_write() for small buffers.
> > 
> > But when called for large data buffers, the write routine behaves
> > incorrectly, as the size may never update: instead of clamping
> > the size to the maximum buffer size, buffer_size_add_clamp() returns
> > an error (which is never checked by the write routine, btw).
> > 
> > To fix this, we now use buffer_size_add() that actually clamps the
> > size to the max value.
> > 
> > Also remove buffer_size_add_clamp(), it is no longer needed.
> > 
> 
> Say if you did notice it, what would that look like?  It's just that
> something gets lost instead of written to the screen right?

Yep. Suppose the ring buffer size is 4096 bytes, when somebody tries to
write a data in a 2000 bytes chunk, the first write will succeed (buffer
size will be 2000), but the second now 3000-bytes write will left the
size equal to 2000, instead of clamping it to 4096.

When we had a large buffer but a small writes (e.g. ram_console usage
scenario), this is almost unnoticeable. But when we started using large
writes the bug showed up.

Thanks,

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