From owner-freebsd-bugs  Thu Dec 23 11:41:11 1999
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Date: Thu, 23 Dec 1999 11:40:40 -0800
From: Tim Kientzle <kientzle@acm.org>
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To: gnats-admin@FreeBSD.org, freebsd-bugs@FreeBSD.org
Subject: Re: kern/15639: msdosfs can't read large (3.8gig) files
References: <199912230540.VAA80061@freefall.freebsd.org>
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gnats-admin@FreeBSD.org wrote:
> 
> Thank you very much for your problem report.
> It has the internal identification `kern/15639'.
> The individual assigned to look at your
> report is: freebsd-bugs.
> 
> >Category:       kern
> >Responsible:    freebsd-bugs
> >Synopsis:       msdosfs can't read large (3.8gig) files
> >Arrival-Date:   Wed Dec 22 21:40:00 PST 1999

I've since found a fix for this problem.  The bug
is in 'msdosfs_vnops.c', in the function
'msdosfs_read'.

Early in that function, it computes fileSize minus
requestedOffset, and stores the difference into a
signed int.  With very large files, that int can
overflow.  A better approach is to
make 'diff' an unsigned long, use a direct comparison
for the initial test, and then be cautious about comparing
'diff' to the signed 'n':

                if (dep->de_FileSize <= uio->uio_offset)
                        return (0);
                /* Note: diff can be as large as 4gig! */
                diff = dep->de_FileSize - uio->uio_offset;
                /* Avoid setting n to a negative value */
                if ((diff < n) && (diff < LONG_MAX))
                        n = diff;

This change allowed me to successfully copy a 3.8gig file from
an MSDOS-formatted hard disk.

				- Tim Kientzle


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