NFSD: Annotate struct pnfs_block_deviceaddr with __counted_by()

Add the __counted_by compiler attribute to the flexible array member
volumes to improve access bounds-checking via CONFIG_UBSAN_BOUNDS and
CONFIG_FORTIFY_SOURCE.

Use struct_size() instead of manually calculating the number of bytes to
allocate for a pnfs_block_deviceaddr with a single volume.
Signed-off-by: Thorsten Blum <thorsten.blum@toblux.com>
Reviewed-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Acked-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

fs/nfsd/blocklayout.c

@@ -147,8 +147,7 @@ nfsd4_block_get_device_info_simple(struct super_block *sb,
 	struct pnfs_block_deviceaddr *dev;
 	struct pnfs_block_volume *b;
 
-	dev = kzalloc(sizeof(struct pnfs_block_deviceaddr) +
-		      sizeof(struct pnfs_block_volume), GFP_KERNEL);
+	dev = kzalloc(struct_size(dev, volumes, 1), GFP_KERNEL);
 	if (!dev)
 		return -ENOMEM;
 	gdp->gd_device = dev;
@@ -255,8 +254,7 @@ nfsd4_block_get_device_info_scsi(struct super_block *sb,
 	const struct pr_ops *ops;
 	int ret;
 
-	dev = kzalloc(sizeof(struct pnfs_block_deviceaddr) +
-		      sizeof(struct pnfs_block_volume), GFP_KERNEL);
+	dev = kzalloc(struct_size(dev, volumes, 1), GFP_KERNEL);
 	if (!dev)
 		return -ENOMEM;
 	gdp->gd_device = dev;

fs/nfsd/blocklayoutxdr.h

@@ -47,7 +47,7 @@ struct pnfs_block_volume {
 
 struct pnfs_block_deviceaddr {
 	u32				nr_volumes;
-	struct pnfs_block_volume	volumes[];
+	struct pnfs_block_volume	volumes[] __counted_by(nr_volumes);
 };
 
 __be32 nfsd4_block_encode_getdeviceinfo(struct xdr_stream *xdr,