[PATCH net-next RFC 0/3] net: move .getsockopt away from __user buffers

[Breno Leitao <leitao@debian.org>]

Currently, .getsockopt callback cannot be called with kernel buffers
because it requires userspace addresses:

  int (*getsockopt)(struct socket *sock, int level,
		    int optname, char __user *optval, int __user *optlen);

This prevents kernel callers (io_uring, BPF, etc) from using getsockopt
on levels other than SOL_SOCKET, since they pass kernel pointers rather
than __user pointers.

Following Linus' suggestion [0], this series introduces a wrapper
around iov_iter (sockopt_t) and a temporary getsockopt_iter callback:

  typedef struct sockopt {
	  struct iov_iter iter;
	  int optlen;
  } sockopt_t;

Note: optlen was not suggested by Linus' but I believe it is needed, given
random values could be passed by protocols back to userspace.

And the callback becomes:

  int (*getsockopt_iter)(struct socket *sock, int level,
			 int optname, sockopt_t *opt);

The sockopt_t structure encapsulates:
- An iov_iter for reading/writing option data (works with both user
  and kernel buffers)
- An optlen field for buffer size (input) and returned data size
  (output)

The plan is to enable getsockopt to leverage kernel buffers initially,
but then move .setsockopt from sockptr_t into this as well.

This series:

 1. Adds the sockopt_t type and getsockopt_iter callback to proto_ops
 2. Adds do_sock_getsockopt_iter() helper that prefers getsockopt_iter
 3. Converts one protocol (netlink) to use getsockopt_iter as a proof of
    concept

This is what I have in mind for this work stream, to make it more
digestible:

 * Keep the temporary getsockopt_iter callback allows protocols to
   migrate gradually.
 * Once all protocols have been converted, getsockopt can be removed and
   getsockopt_iter renamed back to getsockopt with the new API.
 * Once the protocols are converted, the SOL_SOCKET limitation in
   io_uring_cmd_getsockopt() will be removed.
 * Covert setsockopt() to also use a similar strategy, moving it away
   from sockptr_t.
 * Remove sockptr_t in the front end (do_sock_getsockopt(),
   io_uring_cmd_getsockopt()) and start with sockopt_t (instead of
   sockptr_t) in __sys_getsockopt() and io_uring_cmd_getsockopt()

Link: https://lore.kernel.org/all/CAHk-=whmzrO-BMU=uSVXbuoLi-3tJsO=0kHj1BCPBE3F2kVhTA@mail.gmail.com/ [0]
---
Breno Leitao (3):
      net: add getsockopt_iter callback to proto_ops
      net: prefer getsockopt_iter in do_sock_getsockopt
      netlink: convert to getsockopt_iter

 include/linux/net.h      | 19 +++++++++++++++++++
 net/netlink/af_netlink.c | 22 ++++++++++++----------
 net/socket.c             | 42 +++++++++++++++++++++++++++++++++++++++---
 3 files changed, 70 insertions(+), 13 deletions(-)
---
base-commit: 4d310797262f0ddf129e76c2aad2b950adaf1fda
change-id: 20260130-getsockopt-9f36625eedcb

Best regards,
--  
Breno Leitao <leitao@debian.org>