linux-kernel - Re: [PATCH 2/4] vfs: explicitly cast s_maxbytes in fiemap_check

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Date:	Sat, 8 Aug 2009 07:20:31 -0400
From:	Jeff Layton <jlayton@...hat.com>
To:	Johannes Weiner <hannes@...xchg.org>
Cc:	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	akpm@...ux-foundation.org, hch@...radead.org, rlove@...gle.com,
	msb@...gle.com, viro@...iv.linux.org.uk
Subject: Re: [PATCH 2/4] vfs: explicitly cast s_maxbytes in
 fiemap_check_ranges

On Sat, 8 Aug 2009 00:12:52 +0200
Johannes Weiner <hannes@...xchg.org> wrote:

> On Fri, Aug 07, 2009 at 02:57:39PM -0400, Jeff Layton wrote:
> > If fiemap_check_ranges is passed a large enough value, then it's
> > possible that the value would be cast to a signed value for comparison
> > against s_maxbytes when we change it to loff_t. Make sure that doesn't
> > happen by explicitly casting s_maxbytes to an unsigned value for the
> > purposes of comparison.
> 
> I think this is unneeded, C garuantees that in this case the signed
> value will get promoted to an unsigned value, not the other way round.
> 

After looking at this again, I think you're correct. do_sendfile was
actually casting s_maxbytes to a signed value which is why it was
broken there.

I can drop this patch if the consensus is to do so. I still think
however that it doesn't hurt to do explict casts when comparing signed
and unsigned values to remove any potential for ambiguity.

-- 
Jeff Layton <jlayton@...hat.com>
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