[PATCH 2/4]: Blktap backend fixes

[Chris Lalancette <clalance@redhat.com>]

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(please ignore my [PATCH 2/5]; this is just a resend of the same thing, with a
proper subject)

This simple patch puts two fixes into the blktap backend.  The first is a change
of busid from an unsigned short to a u32 so we can accommodate the new expanded
format (which doesn't fit into 16-bits).  The second is a minor fix to
signal_tapdisk(); while developing, if the tools happen to set up the tapfds
wrong, you can get into a situation where the idx passed to signal_tapdisk() is
NULL, causing a kernel OOPS.  The fix for signal_tapdisk() just returns if the
idx is negative, preventing the crash.

Signed-off-by: Chris Lalancette <clalance@redhat.com>

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diff -r 5201a184f513 drivers/xen/blktap/blktap.c
--- a/drivers/xen/blktap/blktap.c	Fri Jun 20 17:43:16 2008 +0100
+++ b/drivers/xen/blktap/blktap.c	Mon Jun 23 17:50:28 2008 +0200
@@ -89,7 +89,7 @@ static int mmap_pages = MMAP_PAGES;
 /*Data struct handed back to userspace for tapdisk device to VBD mapping*/
 typedef struct domid_translate {
 	unsigned short domid;
-	unsigned short busid;
+	u32 busid;
 } domid_translate_t ;
 
 /*Data struct associated with each of the tapdisk devices*/
@@ -507,6 +507,13 @@ void signal_tapdisk(int idx)
 {
 	tap_blkif_t *info;
 	struct task_struct *ptask;
+
+	/*
+	 * if the userland tools set things up wrong, this could be negative;
+	 * just don't try to signal in this case
+	 */
+	if (idx < 0)
+		return;
 
 	info = tapfds[idx];
 	if ((idx < 0) || (idx > MAX_TAP_DEV) || !info)

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