fs/ntfs3: Sequential field availability check in mi_enum_attr()

The code is slightly reformatted to consistently check field availability
without duplication.

Fixes: 556bdf27 ("ntfs3: Add bounds checking to mi_enum_attr()")
Signed-off-by: Konstantin Komarov <almaz.alexandrovich@paragon-software.com>

fs/ntfs3/record.c

@@ -237,6 +237,7 @@ struct ATTRIB *mi_enum_attr(struct mft_inode *mi, struct ATTRIB *attr)
 	}
 
 	/* Can we use the first field (attr->type). */
+	/* NOTE: this code also checks attr->size availability. */
 	if (off + 8 > used) {
 		static_assert(ALIGN(sizeof(enum ATTR_TYPE), 8) == 8);
 		return NULL;
@@ -257,10 +258,6 @@ struct ATTRIB *mi_enum_attr(struct mft_inode *mi, struct ATTRIB *attr)
 		return NULL;
 
 	asize = le32_to_cpu(attr->size);
-	if (asize < SIZEOF_RESIDENT) {
-		/* Impossible 'cause we should not return such attribute. */
-		return NULL;
-	}
 
 	/* Check overflow and boundary. */
 	if (off + asize < off || off + asize > used)
@@ -290,6 +287,10 @@ struct ATTRIB *mi_enum_attr(struct mft_inode *mi, struct ATTRIB *attr)
 	if (attr->non_res != 1)
 		return NULL;
 
+	/* Can we use memory including attr->nres.valid_size? */
+	if (asize < SIZEOF_NONRESIDENT)
+		return NULL;
+
 	t16 = le16_to_cpu(attr->nres.run_off);
 	if (t16 > asize)
 		return NULL;
@@ -316,7 +317,8 @@ struct ATTRIB *mi_enum_attr(struct mft_inode *mi, struct ATTRIB *attr)
 
 	if (!attr->nres.svcn && is_attr_ext(attr)) {
 		/* First segment of sparse/compressed attribute */
-		if (asize + 8 < SIZEOF_NONRESIDENT_EX)
+		/* Can we use memory including attr->nres.total_size? */
+		if (asize < SIZEOF_NONRESIDENT_EX)
 			return NULL;
 
 		tot_size = le64_to_cpu(attr->nres.total_size);
@@ -326,9 +328,6 @@ struct ATTRIB *mi_enum_attr(struct mft_inode *mi, struct ATTRIB *attr)
 		if (tot_size > alloc_size)
 			return NULL;
 	} else {
-		if (asize + 8 < SIZEOF_NONRESIDENT)
-			return NULL;
-
 		if (attr->nres.c_unit)
 			return NULL;