[PATCH v2 3/3] 9p/net: xen: fix false positive printf format overflow warning

[Dominique Martinet]

Use the constant 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 the constant directly here makes it work.

Signed-off-by: Dominique Martinet <asmadeus@xxxxxxxxxxxxx>
Message-ID: <20231023233704.1185154-4-asmadeus@xxxxxxxxxxxxx>
---
v1->v2:
- use constant directly instead of going through a local variable

 net/9p/trans_xen.c | 15 +++++++--------
 1 file changed, 7 insertions(+), 8 deletions(-)

diff --git a/net/9p/trans_xen.c b/net/9p/trans_xen.c
index 1fffe2bed5b0..dfdbe1ca5338 100644
--- a/net/9p/trans_xen.c
+++ b/net/9p/trans_xen.c
@@ -54,7 +54,6 @@ struct xen_9pfs_front_priv {
 	char *tag;
 	struct p9_client *client;
 
-	int num_rings;
 	struct xen_9pfs_dataring *rings;
 };
 
@@ -131,7 +130,7 @@ static int p9_xen_request(struct p9_client *client, struct p9_req_t *p9_req)
 	if (list_entry_is_head(priv, &xen_9pfs_devs, list))
 		return -EINVAL;
 
-	num = p9_req->tc.tag % priv->num_rings;
+	num = p9_req->tc.tag % XEN_9PFS_NUM_RINGS;
 	ring = &priv->rings[num];
 
 again:
@@ -279,7 +278,7 @@ static void xen_9pfs_front_free(struct xen_9pfs_front_priv *priv)
 	list_del(&priv->list);
 	write_unlock(&xen_9pfs_lock);
 
-	for (i = 0; i < priv->num_rings; i++) {
+	for (i = 0; i < XEN_9PFS_NUM_RINGS; i++) {
 		struct xen_9pfs_dataring *ring = &priv->rings[i];
 
 		cancel_work_sync(&ring->work);
@@ -408,15 +407,14 @@ 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),
+	priv->rings = kcalloc(XEN_9PFS_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 < XEN_9PFS_NUM_RINGS; i++) {
 		priv->rings[i].priv = priv;
 		ret = xen_9pfs_front_alloc_dataring(dev, &priv->rings[i],
 						    max_ring_order);
@@ -434,10 +432,11 @@ 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);
+			    XEN_9PFS_NUM_RINGS);
 	if (ret)
 		goto error_xenbus;
-	for (i = 0; i < priv->num_rings; i++) {
+
+	for (i = 0; i < XEN_9PFS_NUM_RINGS; i++) {
 		char str[16];
 
 		BUILD_BUG_ON(XEN_9PFS_NUM_RINGS > 9);
-- 
2.41.0