fs: ocfs2: fix possible null-pointer dereferences in ocfs2_xa_prepare_entry()

In ocfs2_xa_prepare_entry(), there is an if statement on line 2136 to
check whether loc->xl_entry is NULL:

    if (loc->xl_entry)

When loc->xl_entry is NULL, it is used on line 2158:

    ocfs2_xa_add_entry(loc, name_hash);
        loc->xl_entry->xe_name_hash = cpu_to_le32(name_hash);
        loc->xl_entry->xe_name_offset = cpu_to_le16(loc->xl_size);

and line 2164:

    ocfs2_xa_add_namevalue(loc, xi);
        loc->xl_entry->xe_value_size = cpu_to_le64(xi->xi_value_len);
        loc->xl_entry->xe_name_len = xi->xi_name_len;

Thus, possible null-pointer dereferences may occur.

To fix these bugs, if loc-xl_entry is NULL, ocfs2_xa_prepare_entry()
abnormally returns with -EINVAL.

These bugs are found by a static analysis tool STCheck written by us.

[akpm@linux-foundation.org: remove now-unused ocfs2_xa_add_entry()]
Link: http://lkml.kernel.org/r/20190726101447.9153-1-baijiaju1990@gmail.comSigned-off-by: Jia-Ju Bai <baijiaju1990@gmail.com>
Reviewed-by: Joseph Qi <joseph.qi@linux.alibaba.com>
Cc: Mark Fasheh <mark@fasheh.com>
Cc: Joel Becker <jlbec@evilplan.org>
Cc: Junxiao Bi <junxiao.bi@oracle.com>
Cc: Changwei Ge <gechangwei@live.cn>
Cc: Gang He <ghe@suse.com>
Cc: Jun Piao <piaojun@huawei.com>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/ocfs2/xattr.c

@@ -1490,18 +1490,6 @@ static int ocfs2_xa_check_space(struct ocfs2_xa_loc *loc,
 	return loc->xl_ops->xlo_check_space(loc, xi);
 }
 
-static void ocfs2_xa_add_entry(struct ocfs2_xa_loc *loc, u32 name_hash)
-{
-	loc->xl_ops->xlo_add_entry(loc, name_hash);
-	loc->xl_entry->xe_name_hash = cpu_to_le32(name_hash);
-	/*
-	 * We can't leave the new entry's xe_name_offset at zero or
-	 * add_namevalue() will go nuts.  We set it to the size of our
-	 * storage so that it can never be less than any other entry.
-	 */
-	loc->xl_entry->xe_name_offset = cpu_to_le16(loc->xl_size);
-}
-
 static void ocfs2_xa_add_namevalue(struct ocfs2_xa_loc *loc,
 				   struct ocfs2_xattr_info *xi)
 {
@@ -2133,29 +2121,31 @@ static int ocfs2_xa_prepare_entry(struct ocfs2_xa_loc *loc,
 	if (rc)
 		goto out;
 
-	if (loc->xl_entry) {
-		if (ocfs2_xa_can_reuse_entry(loc, xi)) {
-			orig_value_size = loc->xl_entry->xe_value_size;
-			rc = ocfs2_xa_reuse_entry(loc, xi, ctxt);
-			if (rc)
-				goto out;
-			goto alloc_value;
-		}
+	if (!loc->xl_entry) {
+		rc = -EINVAL;
+		goto out;
+	}
 
-		if (!ocfs2_xattr_is_local(loc->xl_entry)) {
-			orig_clusters = ocfs2_xa_value_clusters(loc);
-			rc = ocfs2_xa_value_truncate(loc, 0, ctxt);
-			if (rc) {
-				mlog_errno(rc);
-				ocfs2_xa_cleanup_value_truncate(loc,
-								"overwriting",
-								orig_clusters);
-				goto out;
-			}
+	if (ocfs2_xa_can_reuse_entry(loc, xi)) {
+		orig_value_size = loc->xl_entry->xe_value_size;
+		rc = ocfs2_xa_reuse_entry(loc, xi, ctxt);
+		if (rc)
+			goto out;
+		goto alloc_value;
+	}
+
+	if (!ocfs2_xattr_is_local(loc->xl_entry)) {
+		orig_clusters = ocfs2_xa_value_clusters(loc);
+		rc = ocfs2_xa_value_truncate(loc, 0, ctxt);
+		if (rc) {
+			mlog_errno(rc);
+			ocfs2_xa_cleanup_value_truncate(loc,
+							"overwriting",
+							orig_clusters);
+			goto out;
 		}
-		ocfs2_xa_wipe_namevalue(loc);
-	} else
-		ocfs2_xa_add_entry(loc, name_hash);
+	}
+	ocfs2_xa_wipe_namevalue(loc);
 
 	/*
 	 * If we get here, we have a blank entry.  Fill it.  We grow our