netlink: convert to getsockopt_iter

Convert AF_NETLINK's getsockopt implementation to use the new
getsockopt_iter callback with sockopt_t.

Key changes:
- Replace (char __user *optval, int __user *optlen) with sockopt_t *opt
- Use opt->optlen for buffer length (input) and returned size (output)
- Use copy_to_iter() instead of put_user()/copy_to_user()
- For NETLINK_LIST_MEMBERSHIPS: walk the groups bitmap and emit each
  u32 sequentially via copy_to_iter(), then set opt->optlen to the
  total size required (ALIGN(BITS_TO_BYTES(ngroups), sizeof(u32))).
  The wrapper writes opt->optlen back to userspace even on partial
  failure, preserving the existing API that lets userspace discover
  the needed allocation size.

Signed-off-by: Breno Leitao <leitao@debian.org>
Acked-by: Stanislav Fomichev <sdf@fomichev.me>
Link: https://patch.msgid.link/20260501-getsock_one-v1-1-810ce23ea70e@debian.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

1 files changed, 11 insertions, 10 deletions

diff --git a/net/netlink/af_netlink.c b/net/netlink/af_netlink.c
index 2aeb0680807d..db3be485b480 100644
--- a/net/netlink/af_netlink.c
+++ b/net/netlink/af_netlink.c
@@ -39,6 +39,7 @@
 #include <linux/fs.h>
 #include <linux/slab.h>
 #include <linux/uaccess.h>
+#include <linux/uio.h>
 #include <linux/skbuff.h>
 #include <linux/netdevice.h>
 #include <linux/rtnetlink.h>
@@ -1716,18 +1717,18 @@ static int netlink_setsockopt(struct socket *sock, int level, int optname,
 }
 
 static int netlink_getsockopt(struct socket *sock, int level, int optname,
-			      char __user *optval, int __user *optlen)
+			      sockopt_t *opt)
 {
 	struct sock *sk = sock->sk;
 	struct netlink_sock *nlk = nlk_sk(sk);
 	unsigned int flag;
 	int len, val;
+	u32 group;
 
 	if (level != SOL_NETLINK)
 		return -ENOPROTOOPT;
 
-	if (get_user(len, optlen))
-		return -EFAULT;
+	len = opt->optlen;
 	if (len < 0)
 		return -EINVAL;
 
@@ -1751,14 +1752,14 @@ static int netlink_getsockopt(struct socket *sock, int level, int optname,
 
 			idx = pos / sizeof(unsigned long);
 			shift = (pos % sizeof(unsigned long)) * 8;
-			if (put_user((u32)(nlk->groups[idx] >> shift),
-				     (u32 __user *)(optval + pos))) {
+			group = (u32)(nlk->groups[idx] >> shift);
+			if (copy_to_iter(&group, sizeof(u32),
+					 &opt->iter_out) != sizeof(u32)) {
 				err = -EFAULT;
 				break;
 			}
 		}
-		if (put_user(ALIGN(BITS_TO_BYTES(nlk->ngroups), sizeof(u32)), optlen))
-			err = -EFAULT;
+		opt->optlen = ALIGN(BITS_TO_BYTES(nlk->ngroups), sizeof(u32));
 		netlink_unlock_table();
 		return err;
 	}
@@ -1784,8 +1785,8 @@ static int netlink_getsockopt(struct socket *sock, int level, int optname,
 	len = sizeof(int);
 	val = test_bit(flag, &nlk->flags);
 
-	if (put_user(len, optlen) ||
-	    copy_to_user(optval, &val, len))
+	opt->optlen = len;
+	if (copy_to_iter(&val, len, &opt->iter_out) != len)
 		return -EFAULT;
 
 	return 0;
@@ -2813,7 +2814,7 @@ static const struct proto_ops netlink_ops = {
 	.listen =	sock_no_listen,
 	.shutdown =	sock_no_shutdown,
 	.setsockopt =	netlink_setsockopt,
-	.getsockopt =	netlink_getsockopt,
+	.getsockopt_iter = netlink_getsockopt,
 	.sendmsg =	netlink_sendmsg,
 	.recvmsg =	netlink_recvmsg,
 	.mmap =		sock_no_mmap,