fsverity: optimize fsverity_prepare_setattr() on non-verity files

Make fsverity_prepare_setattr() an inline function that does the
IS_VERITY() check, then (if needed) calls __fsverity_prepare_setattr()
to do the real work.  This reduces the overhead on non-verity files.
Signed-off-by: Eric Biggers <ebiggers@google.com>
Acked-by: Dave Chinner <dchinner@redhat.com>
Link: https://lore.kernel.org/r/20221214224304.145712-3-ebiggers@kernel.org

fs/verity/open.c

@@ -337,26 +337,16 @@ int __fsverity_file_open(struct inode *inode, struct file *filp)
 }
 EXPORT_SYMBOL_GPL(__fsverity_file_open);
 
-/**
- * fsverity_prepare_setattr() - prepare to change a verity inode's attributes
- * @dentry: dentry through which the inode is being changed
- * @attr: attributes to change
- *
- * Verity files are immutable, so deny truncates.  This isn't covered by the
- * open-time check because sys_truncate() takes a path, not a file descriptor.
- *
- * Return: 0 on success, -errno on failure
- */
-int fsverity_prepare_setattr(struct dentry *dentry, struct iattr *attr)
+int __fsverity_prepare_setattr(struct dentry *dentry, struct iattr *attr)
 {
-	if (IS_VERITY(d_inode(dentry)) && (attr->ia_valid & ATTR_SIZE)) {
+	if (attr->ia_valid & ATTR_SIZE) {
 		pr_debug("Denying truncate of verity file (ino %lu)\n",
 			 d_inode(dentry)->i_ino);
 		return -EPERM;
 	}
 	return 0;
 }
-EXPORT_SYMBOL_GPL(fsverity_prepare_setattr);
+EXPORT_SYMBOL_GPL(__fsverity_prepare_setattr);
 
 /**
  * fsverity_cleanup_inode() - free the inode's verity info, if present

include/linux/fsverity.h

@@ -149,7 +149,7 @@ int fsverity_get_digest(struct inode *inode,
 /* open.c */
 
 int __fsverity_file_open(struct inode *inode, struct file *filp);
-int fsverity_prepare_setattr(struct dentry *dentry, struct iattr *attr);
+int __fsverity_prepare_setattr(struct dentry *dentry, struct iattr *attr);
 void fsverity_cleanup_inode(struct inode *inode);
 
 /* read_metadata.c */
@@ -198,10 +198,10 @@ static inline int __fsverity_file_open(struct inode *inode, struct file *filp)
 	return -EOPNOTSUPP;
 }
 
-static inline int fsverity_prepare_setattr(struct dentry *dentry,
-					   struct iattr *attr)
+static inline int __fsverity_prepare_setattr(struct dentry *dentry,
+					     struct iattr *attr)
 {
-	return IS_VERITY(d_inode(dentry)) ? -EOPNOTSUPP : 0;
+	return -EOPNOTSUPP;
 }
 
 static inline void fsverity_cleanup_inode(struct inode *inode)
@@ -274,4 +274,22 @@ static inline int fsverity_file_open(struct inode *inode, struct file *filp)
 	return 0;
 }
 
+/**
+ * fsverity_prepare_setattr() - prepare to change a verity inode's attributes
+ * @dentry: dentry through which the inode is being changed
+ * @attr: attributes to change
+ *
+ * Verity files are immutable, so deny truncates.  This isn't covered by the
+ * open-time check because sys_truncate() takes a path, not a file descriptor.
+ *
+ * Return: 0 on success, -errno on failure
+ */
+static inline int fsverity_prepare_setattr(struct dentry *dentry,
+					   struct iattr *attr)
+{
+	if (IS_VERITY(d_inode(dentry)))
+		return __fsverity_prepare_setattr(dentry, attr);
+	return 0;
+}
+
 #endif	/* _LINUX_FSVERITY_H */