[PATCH v2 1/4] exfat: advance valid_size to EOF for append writes

[Chi Zhiling]

From: Chi Zhiling <chizhiling@xxxxxxxxxx>

For an append write, the write position is recalculated by
generic_write_checks(). Therefore, the write position must be obtained
after generic_write_checks() returns.

Using the original write position to advance valid_size may cause the
valid_size update to be skipped, potentially exposing stale data.

Also move the truncate_pagecache() check below the recalculated
position, so that pos is never read before it is assigned.

Signed-off-by: Chi Zhiling <chizhiling@xxxxxxxxxx>
---
 fs/exfat/file.c | 18 ++++++++----------
 1 file changed, 8 insertions(+), 10 deletions(-)

diff --git a/fs/exfat/file.c b/fs/exfat/file.c
index a2a9ee1a2004..b30aec858da6 100644
--- a/fs/exfat/file.c
+++ b/fs/exfat/file.c
@@ -854,8 +854,7 @@ static ssize_t exfat_file_write_iter(struct kiocb *iocb, struct iov_iter *iter)
 	struct file *file = iocb->ki_filp;
 	struct inode *inode = file_inode(file);
 	struct exfat_inode_info *ei = EXFAT_I(inode);
-	loff_t pos = iocb->ki_pos;
-	loff_t valid_size;
+	loff_t pos, valid_size;
 	int err;
 
 	if (unlikely(exfat_forced_shutdown(inode->i_sb)))
@@ -863,11 +862,6 @@ static ssize_t exfat_file_write_iter(struct kiocb *iocb, struct iov_iter *iter)
 
 	inode_lock(inode);
 
-	if (pos > i_size_read(inode))
-		truncate_pagecache(inode, i_size_read(inode));
-
-	valid_size = ei->valid_size;
-
 	ret = generic_write_checks(iocb, iter);
 	if (ret <= 0)
 		goto unlock;
@@ -878,6 +872,11 @@ static ssize_t exfat_file_write_iter(struct kiocb *iocb, struct iov_iter *iter)
 		goto unlock;
 	}
 
+	pos = iocb->ki_pos;
+	if (pos > i_size_read(inode))
+		truncate_pagecache(inode, i_size_read(inode));
+
+	valid_size = ei->valid_size;
 	if (pos > valid_size) {
 		ret = exfat_extend_valid_size(inode, pos);
 		if (ret < 0 && ret != -ENOSPC) {
@@ -887,6 +886,8 @@ static ssize_t exfat_file_write_iter(struct kiocb *iocb, struct iov_iter *iter)
 		}
 		if (ret < 0)
 			goto unlock;
+
+		pos = valid_size;
 	}
 
 	if (iocb->ki_flags & IOCB_DIRECT)
@@ -899,9 +900,6 @@ static ssize_t exfat_file_write_iter(struct kiocb *iocb, struct iov_iter *iter)
 
 	inode_unlock(inode);
 
-	if (pos > valid_size)
-		pos = valid_size;
-
 	if (iocb->ki_pos > pos) {
 		ssize_t err = generic_write_sync(iocb, iocb->ki_pos - pos);
 
-- 
2.53.0