[PATCH 3/3] 9p/net: xen: fix false positive printf format overflow warning

[Dominique Martinet]

Use a local variable to make the compiler happy about this warning:
net/9p/trans_xen.c: In function ‘xen_9pfs_front_changed’:
net/9p/trans_xen.c:444:39: warning: ‘%d’ directive writing between 1 and 11 bytes into a region of size 8 [-Wformat-overflow=]
  444 |                 sprintf(str, "ring-ref%d", i);
      |                                       ^~
In function ‘xen_9pfs_front_init’,
    inlined from ‘xen_9pfs_front_changed’ at net/9p/trans_xen.c:516:8,
    inlined from ‘xen_9pfs_front_changed’ at net/9p/trans_xen.c:504:13:
net/9p/trans_xen.c:444:30: note: directive argument in the range [-2147483644, 2147483646]
  444 |                 sprintf(str, "ring-ref%d", i);
      |                              ^~~~~~~~~~~~
net/9p/trans_xen.c:444:17: note: ‘sprintf’ output between 10 and 20 bytes into a destination of size 16
  444 |                 sprintf(str, "ring-ref%d", i);
      |                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
net/9p/trans_xen.c: In function ‘xen_9pfs_front_changed’:
net/9p/trans_xen.c:450:45: warning: ‘%d’ directive writing between 1 and 11 bytes into a region of size 2 [-Wformat-overflow=]
  450 |                 sprintf(str, "event-channel-%d", i);
      |                                             ^~
In function ‘xen_9pfs_front_init’,
    inlined from ‘xen_9pfs_front_changed’ at net/9p/trans_xen.c:516:8,
    inlined from ‘xen_9pfs_front_changed’ at net/9p/trans_xen.c:504:13:
net/9p/trans_xen.c:450:30: note: directive argument in the range [-2147483644, 2147483646]
  450 |                 sprintf(str, "event-channel-%d", i);
      |                              ^~~~~~~~~~~~~~~~~~
net/9p/trans_xen.c:450:17: note: ‘sprintf’ output between 16 and 26 bytes into a destination of size 16
  450 |                 sprintf(str, "event-channel-%d", i);
      |                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

There is no change in logic: there only are a constant number of rings,
and there also already is a BUILD_BUG_ON that checks if that constant
goes over 9 as anything bigger would no longer fit the event-channel-%d
destination size.

In theory having that size as part of the struct means it could be
modified by another thread and makes the compiler lose track of possible
values for 'i' here, using a local variable makes it happy.

Signed-off-by: Dominique Martinet <asmadeus@xxxxxxxxxxxxx>
---
While looking at warnings with W=1, I noticed this one in net/9p.

This is silly but shouldn't hurt, num_rings is never changed so there is
no risk of introducing a race here, it's just helping the compiler a
bit.

net/9p is also now warning-free at W=1

 net/9p/trans_xen.c | 13 +++++++------
 1 file changed, 7 insertions(+), 6 deletions(-)

diff --git a/net/9p/trans_xen.c b/net/9p/trans_xen.c
index 1fffe2bed5b0..79e91f31a84a 100644
--- a/net/9p/trans_xen.c
+++ b/net/9p/trans_xen.c
@@ -382,7 +382,7 @@ static int xen_9pfs_front_init(struct xenbus_device *dev)
 	struct xenbus_transaction xbt;
 	struct xen_9pfs_front_priv *priv = dev_get_drvdata(&dev->dev);
 	char *versions, *v;
-	unsigned int max_rings, max_ring_order, len = 0;
+	unsigned int num_rings, max_rings, max_ring_order, len = 0;
 
 	versions = xenbus_read(XBT_NIL, dev->otherend, "versions", &len);
 	if (IS_ERR(versions))
@@ -408,15 +408,15 @@ static int xen_9pfs_front_init(struct xenbus_device *dev)
 	if (p9_xen_trans.maxsize > XEN_FLEX_RING_SIZE(max_ring_order))
 		p9_xen_trans.maxsize = XEN_FLEX_RING_SIZE(max_ring_order) / 2;
 
-	priv->num_rings = XEN_9PFS_NUM_RINGS;
-	priv->rings = kcalloc(priv->num_rings, sizeof(*priv->rings),
+	num_rings = priv->num_rings = XEN_9PFS_NUM_RINGS;
+	priv->rings = kcalloc(num_rings, sizeof(*priv->rings),
 			      GFP_KERNEL);
 	if (!priv->rings) {
 		kfree(priv);
 		return -ENOMEM;
 	}
 
-	for (i = 0; i < priv->num_rings; i++) {
+	for (i = 0; i < num_rings; i++) {
 		priv->rings[i].priv = priv;
 		ret = xen_9pfs_front_alloc_dataring(dev, &priv->rings[i],
 						    max_ring_order);
@@ -434,10 +434,11 @@ static int xen_9pfs_front_init(struct xenbus_device *dev)
 	if (ret)
 		goto error_xenbus;
 	ret = xenbus_printf(xbt, dev->nodename, "num-rings", "%u",
-			    priv->num_rings);
+			    num_rings);
 	if (ret)
 		goto error_xenbus;
-	for (i = 0; i < priv->num_rings; i++) {
+
+	for (i = 0; i < num_rings; i++) {
 		char str[16];
 
 		BUILD_BUG_ON(XEN_9PFS_NUM_RINGS > 9);
-- 
2.41.0