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Date: Sat, 13 Apr 2002 04:17:46 -0700 (PDT)
From: David Schultz <dschultz@uclink.Berkeley.EDU>
Reply-To: David Schultz <dschultz@uclink.Berkeley.EDU>
To: FreeBSD-gnats-submit@FreeBSD.org
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Subject: kern/37035: [PATCH] msdosfs_readdir() freezes after failed write
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>Number:         37035
>Category:       kern
>Synopsis:       [PATCH] msdosfs_readdir() freezes after failed write
>Confidential:   no
>Severity:       serious
>Priority:       low
>Responsible:    freebsd-bugs
>State:          open
>Quarter:        
>Keywords:       
>Date-Required:
>Class:          sw-bug
>Submitter-Id:   current-users
>Arrival-Date:   Sat Apr 13 04:20:01 PDT 2002
>Closed-Date:
>Last-Modified:
>Originator:     David Schultz
>Release:        FreeBSD 4.5-RELEASE-p2 i386
>Organization:
>Environment:
Problem affects all versions of FreeBSD from at least 4.0-RELEASE.
to 5.0-CURRENT
>Description:
After an attempt to write a buffer to a write-protected or otherwise
bad floppy, attempts to bread() the same block return success, but the
resulting buffer has b_count == b_resid.  Unlike the UFS code, which
correctly handles this situation, msdosfs_readdir() goes into an
infinite loop.
>How-To-Repeat:
Mount a write-protected DOS floppy, then...
/flp# cat > foo
(error)
/flp# ls
(lockup)
>Fix:
This patch causes msdosfs_readdir() to give up in the case where it would
otherwise spin forever.  There is still room for improvement; for example,
the buffer cache ought to give up when the floppy is forcibly unmounted.
However, this solves the actual lockup that many people complain about.

Index: msdosfs_vnops.c
===================================================================
RCS file: /home/ncvs/src/sys/fs/msdosfs/msdosfs_vnops.c,v
retrieving revision 1.119
diff -u -r1.119 msdosfs_vnops.c
--- msdosfs_vnops.c	2002/04/05 14:01:04	1.119
+++ msdosfs_vnops.c	2002/04/12 23:12:27
@@ -1603,7 +1603,16 @@
 			brelse(bp);
 			return (error);
 		}
+
+		/*
+		 * If a short read occurred, the buffer may contain
+		 * less valid data than expected.
+		 */
 		n = min(n, blsize - bp->b_resid);
+		if (n == 0) {
+			brelse(bp);
+			break;
+		}
 
 		/*
 		 * Convert from dos directory entries to fs-independent
>Release-Note:
>Audit-Trail:
>Unformatted:

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