exfat: fix appending discontinuous clusters to empty file

Eric Hong found that when using ftruncate to expand an empty file,
exfat_ent_set() will fail if discontinuous clusters are allocated.
The reason is that the empty file does not have a cluster chain,
but exfat_ent_set() attempts to append the newly allocated cluster
to the cluster chain. In addition, exfat_find_last_cluster() only
supports finding the last cluster in a non-empty file.

So this commit adds a check whether the file is empty. If the file
is empty, exfat_find_last_cluster() and exfat_ent_set() are no longer
called as they do not need to be called.

Fixes: f55c096f62f1 ("exfat: do not zero the extended part")
Reported-by: Eric Hong <[email protected]>
Signed-off-by: Yuezhang Mo <[email protected]>
Signed-off-by: Namjae Jeon <[email protected]>

file.c

@@ -37,13 +37,18 @@ static int exfat_cont_expand(struct inode *inode, loff_t size)
 	if (new_num_clusters == num_clusters)
 		goto out;
 
-	exfat_chain_set(&clu, ei->start_clu, num_clusters, ei->flags);
-	ret = exfat_find_last_cluster(sb, &clu, &last_clu);
-	if (ret)
-		return ret;
+	if (num_clusters) {
+		exfat_chain_set(&clu, ei->start_clu, num_clusters, ei->flags);
+		ret = exfat_find_last_cluster(sb, &clu, &last_clu);
+		if (ret)
+			return ret;
+
+		clu.dir = last_clu + 1;
+	} else {
+		last_clu = EXFAT_EOF_CLUSTER;
+		clu.dir = EXFAT_EOF_CLUSTER;
+	}
 
-	clu.dir = (last_clu == EXFAT_EOF_CLUSTER) ?
-			EXFAT_EOF_CLUSTER : last_clu + 1;
 	clu.size = 0;
 	clu.flags = ei->flags;
 
@@ -53,17 +58,19 @@ static int exfat_cont_expand(struct inode *inode, loff_t size)
 		return ret;
 
 	/* Append new clusters to chain */
-	if (clu.flags != ei->flags) {
-		exfat_chain_cont_cluster(sb, ei->start_clu, num_clusters);
-		ei->flags = ALLOC_FAT_CHAIN;
-	}
-	if (clu.flags == ALLOC_FAT_CHAIN)
-		if (exfat_ent_set(sb, last_clu, clu.dir))
-			goto free_clu;
-
-	if (num_clusters == 0)
+	if (num_clusters) {
+		if (clu.flags != ei->flags)
+			if (exfat_chain_cont_cluster(sb, ei->start_clu, num_clusters))
+				goto free_clu;
+
+		if (clu.flags == ALLOC_FAT_CHAIN)
+			if (exfat_ent_set(sb, last_clu, clu.dir))
+				goto free_clu;
+	} else
 		ei->start_clu = clu.dir;
 
+	ei->flags = clu.flags;
+
 out:
 #if LINUX_VERSION_CODE >= KERNEL_VERSION(6, 6, 0)
 #if LINUX_VERSION_CODE >= KERNEL_VERSION(6, 7, 0)