ext4: annotate struct ext4_xattr_inode_array with __counted_by()

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

Remove the now obsolete comment on the count field.

In ext4_expand_inode_array(), use struct_size() instead of offsetof()
and remove the local variable count. Increment the count field before
adding a new inode to the inodes array.

Compile-tested only.

Signed-off-by: Thorsten Blum <thorsten.blum@toblux.com>
Link: https://patch.msgid.link/20240730220200.410939-3-thorsten.blum@toblux.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

fs/ext4/xattr.c

@@ -2879,33 +2879,31 @@ ext4_expand_inode_array(struct ext4_xattr_inode_array **ea_inode_array,
 	if (*ea_inode_array == NULL) {
 		/*
 		 * Start with 15 inodes, so it fits into a power-of-two size.
-		 * If *ea_inode_array is NULL, this is essentially offsetof()
 		 */
-		(*ea_inode_array) =
-			kmalloc(offsetof(struct ext4_xattr_inode_array,
-					 inodes[EIA_MASK]),
-				GFP_NOFS);
+		(*ea_inode_array) = kmalloc(
+			struct_size(*ea_inode_array, inodes, EIA_MASK),
+			GFP_NOFS);
 		if (*ea_inode_array == NULL)
 			return -ENOMEM;
 		(*ea_inode_array)->count = 0;
 	} else if (((*ea_inode_array)->count & EIA_MASK) == EIA_MASK) {
 		/* expand the array once all 15 + n * 16 slots are full */
 		struct ext4_xattr_inode_array *new_array = NULL;
-		int count = (*ea_inode_array)->count;
 
-		/* if new_array is NULL, this is essentially offsetof() */
 		new_array = kmalloc(
-				offsetof(struct ext4_xattr_inode_array,
-					 inodes[count + EIA_INCR]),
-				GFP_NOFS);
+			struct_size(*ea_inode_array, inodes,
+				    (*ea_inode_array)->count + EIA_INCR),
+			GFP_NOFS);
 		if (new_array == NULL)
 			return -ENOMEM;
 		memcpy(new_array, *ea_inode_array,
-		       offsetof(struct ext4_xattr_inode_array, inodes[count]));
+		       struct_size(*ea_inode_array, inodes,
+				   (*ea_inode_array)->count));
 		kfree(*ea_inode_array);
 		*ea_inode_array = new_array;
 	}
-	(*ea_inode_array)->inodes[(*ea_inode_array)->count++] = inode;
+	(*ea_inode_array)->count++;
+	(*ea_inode_array)->inodes[(*ea_inode_array)->count - 1] = inode;
 	return 0;
 }
 

fs/ext4/xattr.h

@@ -129,8 +129,8 @@ struct ext4_xattr_ibody_find {
 };
 
 struct ext4_xattr_inode_array {
-	unsigned int count;		/* # of used items in the array */
-	struct inode *inodes[];
+	unsigned int count;
+	struct inode *inodes[] __counted_by(count);
 };
 
 extern const struct xattr_handler ext4_xattr_user_handler;