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Subject[PATCH 4.19 117/118] ovl: support stacked SEEK_HOLE/SEEK_DATA
Date
From: Amir Goldstein <amir73il@gmail.com>

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>

---
fs/overlayfs/file.c | 44 ++++++++++++++++++++++++++++++++++++++++----
1 file changed, 40 insertions(+), 4 deletions(-)

--- a/fs/overlayfs/file.c
+++ b/fs/overlayfs/file.c
@@ -146,11 +146,47 @@ static int ovl_release(struct inode *ino

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;

- return generic_file_llseek_size(file, offset, whence,
- realinode->i_sb->s_maxbytes,
- i_size_read(realinode));
+ /*
+ * 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;
+
+ /*
+ * 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)

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