[PATCH v1 4/4] overlayfs: fix race between llseek SEEK_END and write

[Eiichi Tsukata <devel@etsukata.com>]

Implement individual lock for SEEK_END for overlayfs which directly calls
generic_file_llseek_size().

Signed-off-by: Eiichi Tsukata <devel@etsukata.com>
---
 fs/overlayfs/file.c | 23 ++++++++++++++++++++---
 1 file changed, 20 insertions(+), 3 deletions(-)

diff --git a/fs/overlayfs/file.c b/fs/overlayfs/file.c
index 84dd957efa24..57bc6538eea8 100644
--- a/fs/overlayfs/file.c
+++ b/fs/overlayfs/file.c
@@ -146,10 +146,27 @@ static int ovl_release(struct inode *inode, struct file *file)
 static loff_t ovl_llseek(struct file *file, loff_t offset, int whence)
 {
 	struct inode *realinode = ovl_inode_real(file_inode(file));
+	loff_t ret;
 
-	return generic_file_llseek_size(file, offset, whence,
-					realinode->i_sb->s_maxbytes,
-					i_size_read(realinode));
+	switch (whence) {
+	default:
+		return generic_file_llseek_size(file, offset, whence,
+						realinode->i_sb->s_maxbytes,
+						i_size_read(realinode));
+	case SEEK_END:
+	case SEEK_DATA:
+	case SEEK_HOLE:
+		/*
+		 * protects against inode size race with write so that llseek
+		 * doesn't see inode size being updated in individual fs write
+		 */
+		inode_lock(realinode);
+		ret = generic_file_llseek_size(file, offset, whence,
+					       realinode->i_sb->s_maxbytes,
+					       i_size_read(realinode));
+		inode_unlock(realinode);
+		return ret;
+	}
 }
 
 static void ovl_file_accessed(struct file *file)
-- 
2.19.1