overlayfs: Implement splice-read

Implement splice-read for overlayfs by passing the request down a layer
rather than going through generic_file_splice_read() which is going to be
changed to assume that ->read_folio() is present on buffered files.

Signed-off-by: David Howells <[email protected]>
Acked-by: Christian Brauner <[email protected]>
cc: Christoph Hellwig <[email protected]>
cc: Jens Axboe <[email protected]>
cc: Al Viro <[email protected]>
cc: John Hubbard <[email protected]>
cc: David Hildenbrand <[email protected]>
cc: Matthew Wilcox <[email protected]>
cc: Miklos Szeredi <[email protected]>
cc: Amir Goldstein <[email protected]>
cc: [email protected]
cc: [email protected]
cc: [email protected]
cc: [email protected]
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Jens Axboe <[email protected]>

Author: dhowells

fs/overlayfs/file.c

@@ -419,6 +419,27 @@ static ssize_t ovl_write_iter(struct kiocb *iocb, struct iov_iter *iter)
 	return ret;
 }
 
+static ssize_t ovl_splice_read(struct file *in, loff_t *ppos,
+			       struct pipe_inode_info *pipe, size_t len,
+			       unsigned int flags)
+{
+	const struct cred *old_cred;
+	struct fd real;
+	ssize_t ret;
+
+	ret = ovl_real_fdget(in, &real);
+	if (ret)
+		return ret;
+
+	old_cred = ovl_override_creds(file_inode(in)->i_sb);
+	ret = vfs_splice_read(real.file, ppos, pipe, len, flags);
+	revert_creds(old_cred);
+	ovl_file_accessed(in);
+
+	fdput(real);
+	return ret;
+}
+
 /*
  * Calling iter_file_splice_write() directly from overlay's f_op may deadlock
  * due to lock order inversion between pipe->mutex in iter_file_splice_write()
@@ -695,7 +716,7 @@ const struct file_operations ovl_file_operations = {
 	.fallocate	= ovl_fallocate,
 	.fadvise	= ovl_fadvise,
 	.flush		= ovl_flush,
-	.splice_read    = generic_file_splice_read,
+	.splice_read    = ovl_splice_read,
 	.splice_write   = ovl_splice_write,
 
 	.copy_file_range	= ovl_copy_file_range,