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

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Message-Id: <1844C797-F3C7-4EA7-BBE8-520A7609C743@cam.ac.uk>
Date:	Mon, 22 Sep 2014 16:46:01 +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 16:33, Linus Torvalds <torvalds@...ux-foundation.org> wrote:
> On Mon, Sep 22, 2014 at 8:29 AM, Anton Altaparmakov <aia21@....ac.uk> wrote:
>> 
>> You could do "block & ~(sector_t)(size - 1)" instead of "(sector_t)index << sizebits" if you prefer but not sure that is an improvement!
> 
> No, it would be even worse. Something like
> 
>  block & ~(sector_t)((size >> 9) - 1)
> 
> because block is the sector number (ie 512-byte) and size is in bytes.

Oops, sorry.  But I think you got it wrong, too as you are ignoring the PAGE_SIZE - as was I but it is what we need to align to in addition to the problem of "size" being in bytes.  So I think the correct mask is actually based on sizebits which reflects the number of blocks per page thus:

	block & ~(sector_t)((1 << sizebits) - 1)

In any case the shift is the lesser evil I think as it is at least obviously correct whilst getting the right mask has taken us a few iterations of correcting each other! (-:

PS. Thank you for taking my patch and correcting the misleading description!

Best regards,

	Anton

>           Linus

-- 
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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