linux-kernel - Re: file offset corruption on 32-bit machines?

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Message-ID: <20080414194550.GN7385@csclub.uwaterloo.ca>
Date:	Mon, 14 Apr 2008 15:45:50 -0400
From:	lsorense@...lub.uwaterloo.ca (Lennart Sorensen)
To:	Jan Kara <jack@...e.cz>
Cc:	Bodo Eggert <7eggert@....de>, Diego Calleja <diegocg@...il.com>,
	Jiri Kosina <jkosina@...e.cz>, Michal Hocko <mhocko@...e.cz>,
	Meelis Roos <mroos@...ux.ee>,
	Linux Kernel list <linux-kernel@...r.kernel.org>,
	linux-fsdevel@...r.kernel.org
Subject: Re: file offset corruption on 32-bit machines?

On Mon, Apr 14, 2008 at 09:42:46PM +0200, Jan Kara wrote:
> > Why would you want to?  You can already set the filepointer explicitly
> > to any value you want if you have the filehandle.
> > 
> > If you had a file with some security checks for whether the user could
> > read from it implemented based on locations then you would check it when
> > you read/write not when you seek, since after all you could just keep
> > reading until you get to the desired position.
>   Yes and no - for example if you manage to corrupt f_pos so that it
> becomes negative, you have won because it is checked only in seek, pread,
> pwrite, but not in read or write which rely on the check in seek...

The only file that could possibly implement any such silly security
based on position would be in /proc or /sys or similar, in which case
whatever driver implements it can check the position during any
read/write operation, and it would have to if it wants to implement such
a silly security system.

Any sane system would put the secured data in a seperate file from the
unsecured data obviously.

Trying to read from a negative position on a normal file should clearly
fail, and if it doesn't then that is a seperate issue to fix and has
nothing to do with the file position being set atomicly.

-- 
Len Sorensen
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