ovl: support stacked SEEK_HOLE/SEEK_DATA

commit 9e46b840c7053b5f7a245e98cd239b60d189a96c upstream.

Overlay file f_pos is the master copy that is preserved
through copy up and modified on read/write, but only real
fs knows how to SEEK_HOLE/SEEK_DATA and real fs may impose
limitations that are more strict than ->s_maxbytes for specific
files, so we use the real file to perform seeks.

We do not call real fs for SEEK_CUR:0 query and for SEEK_SET:0
requests.

Fixes: d1d04ef8572b ("ovl: stack file ops")
Reported-by: Eddie Horng <eddiehorng.tw@gmail.com>
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/fs/overlayfs/file.c b/fs/overlayfs/file.c
index 82fb91c..00338b8 100644 (file)
--- a/fs/overlayfs/file.c
+++ b/fs/overlayfs/file.c
@@ -146,11 +146,47 @@ 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));
+       struct inode *inode = file_inode(file);
+       struct fd real;
+       const struct cred *old_cred;
+       ssize_t ret;
+
+       /*
+        * The two special cases below do not need to involve real fs,
+        * so we can optimizing concurrent callers.
+        */
+       if (offset == 0) {
+               if (whence == SEEK_CUR)
+                       return file->f_pos;
+
+               if (whence == SEEK_SET)
+                       return vfs_setpos(file, 0, 0);
+       }
+
+       ret = ovl_real_fdget(file, &real);
+       if (ret)
+               return ret;
 
-       return generic_file_llseek_size(file, offset, whence,
-                                       realinode->i_sb->s_maxbytes,
-                                       i_size_read(realinode));
+       /*
+        * Overlay file f_pos is the master copy that is preserved
+        * through copy up and modified on read/write, but only real
+        * fs knows how to SEEK_HOLE/SEEK_DATA and real fs may impose
+        * limitations that are more strict than ->s_maxbytes for specific
+        * files, so we use the real file to perform seeks.
+        */
+       inode_lock(inode);
+       real.file->f_pos = file->f_pos;
+
+       old_cred = ovl_override_creds(inode->i_sb);
+       ret = vfs_llseek(real.file, offset, whence);
+       revert_creds(old_cred);
+
+       file->f_pos = real.file->f_pos;
+       inode_unlock(inode);
+
+       fdput(real);
+
+       return ret;
 }
 
 static void ovl_file_accessed(struct file *file)