linux-kernel - Re: [PATCH] Fix nasty 32-bit overflow bug in buffer i/o code.

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Message-Id: <55E81CB2-AB4B-41E2-88D5-B33F7D8B1DB8@cam.ac.uk>
Date:	Mon, 22 Sep 2014 16:29:56 +0100
From:	Anton Altaparmakov <aia21@....ac.uk>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	Hugh Dickins <hughd@...gle.com>,
	stable <stable@...r.kernel.org>
Subject: Re: [PATCH] Fix nasty 32-bit overflow bug in buffer i/o code.

Hi Linus,

On 22 Sep 2014, at 16:18, Linus Torvalds <torvalds@...ux-foundation.org> wrote:
> On Sun, Sep 21, 2014 at 5:53 PM, Anton Altaparmakov <aia21@....ac.uk> wrote:
>> 
>> This patch fixes this issue by type casting "index" to sector_t before
>> doing the left shift.
> 
> Ugh. Does the simpler patch to just pass in "block" work as well?

That doesn't work because nothing rounds down "block" to the first block in the page and init_page_buffers() requires "block" to be the first block in the page.

The shift right followed by shift left achieves the "rounding down" required.

You could do "block & ~(sector_t)(size - 1)" instead of "(sector_t)index << sizebits" if you prefer but not sure that is an improvement!

Best regards,

	Anton

>                    Linus
> <patch.diff>


-- 
Anton Altaparmakov <aia21 at cam.ac.uk> (replace at with @)
University of Cambridge Information Services, Roger Needham Building
7 JJ Thomson Avenue, Cambridge, CB3 0RB, UK

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