From: Alex Elder <aelder@sgi.com>
To: Dave Chinner <david@fromorbit.com>
Cc: xfs@oss.sgi.com
Subject: Re: [PATCH] xfs: prevent 32bit overflow in space reservation
Date: Fri, 03 Sep 2010 08:48:03 -0500 [thread overview]
Message-ID: <1283521683.3094.10.camel@doink> (raw)
In-Reply-To: <20100903000146.GA705@dastard>
On Fri, 2010-09-03 at 10:01 +1000, Dave Chinner wrote:
> On Thu, Sep 02, 2010 at 10:51:19AM -0500, Alex Elder wrote:
> > On Thu, 2010-09-02 at 15:17 +1000, Dave Chinner wrote:
> > > From: Dave Chinner <dchinner@redhat.com>
> > >
> > > If we attempt to preallocate more than 2^32 blocks of space in a
. . .
> > > + resblks = min_t(xfs_fileoff_t, (e - s), (MAXEXTLEN * nimaps));
> >
> > I guess it's clear that MAXEXTLEN fits in 32 bits because of
> > sizeof (xfs_extlen_t).
>
> True, but if sizeof(xfs_extlen_t) was the limiting factor,
> then the mulitply could still cause 32bit overflows.
>
> The real reason is that MAXEXTLEN defines the maximum extent length
> supported by the on disk bmap btree record format. The record format
> defines the extent length in FSBs to be:
>
> #define MAXEXTLEN ((xfs_extlen_t)0x001fffff) /* 21 bits */
>
> and as such fits easily into the 32 bit limit.
Yes, I recognized that but didn't mention it. However...
> > And inspection shows that nimaps is
> > just 1, so this does the 32-bit limiting. But that just
> > seems indirect.
>
> nimaps can be up to:
>
> #define XFS_BMAP_MAX_NMAP 4
...I had not noticed that nimap could have been changed from
its value 1 by the xfs_bmapi() call, so the point you make is
important.
> So if we change the loop to do more allocations per loop, then
> the code will already handle it correctly. :)
Yes. And like I said, just adjusting the comment explains
why it is safe.
-Alex
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prev parent reply other threads:[~2010-09-03 13:48 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2010-09-02 5:17 [PATCH] xfs: prevent 32bit overflow in space reservation Dave Chinner
2010-09-02 12:16 ` Christoph Hellwig
2010-09-02 15:51 ` Alex Elder
2010-09-03 0:01 ` Dave Chinner
2010-09-03 13:48 ` Alex Elder [this message]
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