[PATCH v1] exfat: fix appending discontinuous clusters to empty file

[PATCH v1] exfat: fix appending discontinuous clusters to empty file

[Yuezhang.Mo]

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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 <erichong@qnap.com>
Closes: https://github.com/namjaejeon/linux-exfat-oot/issues/66
Signed-off-by: Yuezhang Mo <Yuezhang.Mo@sony.com>
---
 fs/exfat/file.c | 37 ++++++++++++++++++++++---------------
 1 file changed, 22 insertions(+), 15 deletions(-)

diff --git a/fs/exfat/file.c b/fs/exfat/file.c
index d25a96a148af..cc00f1a7a1e1 100644
--- a/fs/exfat/file.c
+++ b/fs/exfat/file.c
@@ -35,13 +35,18 @@ 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;
 
@@ -51,17 +56,19 @@ 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:
 	inode_set_mtime_to_ts(inode, inode_set_ctime_current(inode));
 	/* Expanded range not zeroed, do not update valid_size */
-- 
2.34.1


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From 7d15a2ed350e9c3bf6b3363af25255dee556a12b Mon Sep 17 00:00:00 2001
From: Yuezhang Mo <Yuezhang.Mo@sony.com>
Date: Fri, 16 Feb 2024 20:19:55 +0800
Subject: [PATCH v1] 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 <erichong@qnap.com>
Closes: https://github.com/namjaejeon/linux-exfat-oot/issues/66
Signed-off-by: Yuezhang Mo <Yuezhang.Mo@sony.com>
---
 fs/exfat/file.c | 37 ++++++++++++++++++++++---------------
 1 file changed, 22 insertions(+), 15 deletions(-)

diff --git a/fs/exfat/file.c b/fs/exfat/file.c
index d25a96a148af..cc00f1a7a1e1 100644
--- a/fs/exfat/file.c
+++ b/fs/exfat/file.c
@@ -35,13 +35,18 @@ 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;
 
@@ -51,17 +56,19 @@ 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:
 	inode_set_mtime_to_ts(inode, inode_set_ctime_current(inode));
 	/* Expanded range not zeroed, do not update valid_size */
-- 
2.34.1

Re: [PATCH v1] exfat: fix appending discontinuous clusters to empty file

[Namjae Jeon]

2024-02-17 15:38 GMT+09:00, Yuezhang.Mo@sony.com <Yuezhang.Mo@sony.com>:
> 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 <erichong@qnap.com>
> Closes: https://github.com/namjaejeon/linux-exfat-oot/issues/66
> Signed-off-by: Yuezhang Mo <Yuezhang.Mo@sony.com>
Applied it to #dev.
Thanks for your patch.