[PATCH] ovl: support stacked SEEK_HOLE/SEEK_DATA

[Amir Goldstein <amir73il@gmail.com>]

Overlay file f_pos is the master copy that is preserved
through copy up, but only real fs knows how to SEEK_HOLE/
SEEK_DATA. So we copy f_pos from overlay file, perform the seek
on real file and copy f_pos back to overlay file.

Fixes: d1d04ef8572b ("ovl: stack file ops")
Reported-by: Eddie Horng <eddiehorng.tw@gmail.com>
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
---

Miklos,

I verified no regressions with xfstests quick tests.
The improved generic/seek tests that I posted to fstests
are failing on master and passing with this fix.

Thanks,
Amir.

 fs/overlayfs/file.c | 27 +++++++++++++++++++++++----
 1 file changed, 23 insertions(+), 4 deletions(-)

diff --git a/fs/overlayfs/file.c b/fs/overlayfs/file.c
index 84dd957efa24..0d472940ce9e 100644
--- a/fs/overlayfs/file.c
+++ b/fs/overlayfs/file.c
@@ -145,11 +145,30 @@ 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 fd real;
+	const struct cred *old_cred;
+	ssize_t ret;
 
-	return generic_file_llseek_size(file, offset, whence,
-					realinode->i_sb->s_maxbytes,
-					i_size_read(realinode));
+	ret = ovl_real_fdget(file, &real);
+	if (ret)
+		return ret;
+
+	/*
+	 * Overlay file f_pos is the master copy that is preserved
+	 * through copy up, but only real fs knows how to SEEK_HOLE/
+	 * SEEK_DATA.
+	 */
+	real.file->f_pos = file->f_pos;
+
+	old_cred = ovl_override_creds(file_inode(file)->i_sb);
+	ret = vfs_llseek(real.file, offset, whence);
+	revert_creds(old_cred);
+
+	file->f_pos = real.file->f_pos;
+
+	fdput(real);
+
+	return ret;
 }
 
 static void ovl_file_accessed(struct file *file)
-- 
2.17.1