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

[Christian Schoenebeck <linux_oss@crudebyte.com>]

On Tuesday, October 24, 2023 1:37:04 AM CEST Dominique Martinet wrote:
> 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.

Or ... what about dropping struct's 'num_rings' member and using the constant
XEN_9PFS_NUM_RINGS everywhere instead? As this is really a compile-time value
after all.

> Signed-off-by: Dominique Martinet <asmadeus@codewreck.org>
> ---
> 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);
>