Add a new getsockopt_iter callback to struct proto_ops that uses
sockopt_t, a type-safe wrapper around iov_iter. This provides a clean
interface for socket option operations that works with both user and
kernel buffers.

The sockopt_t type encapsulates an iov_iter and an optlen field.

The optlen field, although not suggested by Linus, serves as both input
(buffer size) and output (returned data size), allowing callbacks to
return a random values independent of the bytes written via
copy_to_iter(), so, keep it separated from iov_iter.count.

This is preparatory work for removing the SOL_SOCKET level restriction
from io_uring getsockopt operations.

Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Breno Leitao <leitao@debian.org>
---
 include/linux/net.h | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/include/linux/net.h b/include/linux/net.h
index f58b38ab37f8a..94f6c86769afc 100644
--- a/include/linux/net.h
+++ b/include/linux/net.h
@@ -23,9 +23,26 @@
 #include <linux/fs.h>
 #include <linux/mm.h>
 #include <linux/sockptr.h>
+#include <linux/uio.h>
 
 #include <uapi/linux/net.h>
 
+/**
+ * struct sockopt - socket option value container
+ * @iter: iov_iter for reading/writing option data
+ * @optlen: set by callback to indicate returned data size
+ *
+ * Type-safe wrapper for socket option data that works with both
+ * user and kernel buffers.
+ *
+ * The optlen field allows callbacks to return a specific length value
+ * independent of the bytes written via copy_to_iter().
+ */
+typedef struct sockopt {
+	struct iov_iter iter;
+	int optlen;
+} sockopt_t;
+
 struct poll_table_struct;
 struct pipe_inode_info;
 struct inode;
@@ -192,6 +209,8 @@ struct proto_ops {
 				      unsigned int optlen);
 	int		(*getsockopt)(struct socket *sock, int level,
 				      int optname, char __user *optval, int __user *optlen);
+	int		(*getsockopt_iter)(struct socket *sock, int level,
+				      int optname, sockopt_t *opt);
 	void		(*show_fdinfo)(struct seq_file *m, struct socket *sock);
 	int		(*sendmsg)   (struct socket *sock, struct msghdr *m,
 				      size_t total_len);

-- 
2.47.3