linux-kernel - [PATCH 3.17 075/141] Fix thinko in iov_iter_single_seg

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Message-Id: <20141119205153.127266902@linuxfoundation.org>
Date:	Wed, 19 Nov 2014 12:51:38 -0800
From:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:	linux-kernel@...r.kernel.org
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	stable@...r.kernel.org, Paul Mackerras <paulus@...ba.org>,
	Al Viro <viro@...iv.linux.org.uk>
Subject: [PATCH 3.17 075/141] Fix thinko in iov_iter_single_seg_count

3.17-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Paul Mackerras <paulus@...ba.org>

commit ad0eab9293485d1c06237e9249f6d4dfa3d93d4d upstream.

The branches of the if (i->type & ITER_BVEC) statement in
iov_iter_single_seg_count() are the wrong way around; if ITER_BVEC is
clear then we use i->bvec, when we should be using i->iov.  This fixes
it.

In my case, the symptom that this caused was that a KVM guest doing
filesystem operations on a virtual disk would result in one of qemu's
threads on the host going into an infinite loop in
generic_perform_write().  The loop would hit the copied == 0 case and
call iov_iter_single_seg_count() to reduce the number of bytes to try
to process, but because of the error, iov_iter_single_seg_count()
would just return i->count and the loop made no progress and continued
forever.

Signed-off-by: Paul Mackerras <paulus@...ba.org>
Signed-off-by: Al Viro <viro@...iv.linux.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

---
 mm/iov_iter.c |    4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

--- a/mm/iov_iter.c
+++ b/mm/iov_iter.c
@@ -699,9 +699,9 @@ size_t iov_iter_single_seg_count(const s
 	if (i->nr_segs == 1)
 		return i->count;
 	else if (i->type & ITER_BVEC)
-		return min(i->count, i->iov->iov_len - i->iov_offset);
-	else
 		return min(i->count, i->bvec->bv_len - i->iov_offset);
+	else
+		return min(i->count, i->iov->iov_len - i->iov_offset);
 }
 EXPORT_SYMBOL(iov_iter_single_seg_count);

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