[PATCH net] fix a braino in cmsghdr_from_user_compat_to_kern()

[Al Viro]

	commit 547ce4cfb34c ("switch cmsghdr_from_user_compat_to_kern() to
copy_from_user()") missed one of the places where ucmlen should've been
replaced with cmsg.cmsg_len, now that we are fetching the entire struct
rather than doing it field-by-field.

	As the result, compat sendmsg() with several different-sized cmsg
attached started to fail with EINVAL.  Trivial to fix, fortunately.

Reported-by: Nick Bowler <nbowler@xxxxxxxxxx>
Tested-by: Nick Bowler <nbowler@xxxxxxxxxx>
Fixes: 547ce4cfb34c ("switch cmsghdr_from_user_compat_to_kern() to copy_from_user()")

Signed-off-by: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
---
diff --git a/net/compat.c b/net/compat.c
index 5e3041a2c37d..434838bef5f8 100644
--- a/net/compat.c
+++ b/net/compat.c
@@ -202,7 +202,7 @@ int cmsghdr_from_user_compat_to_kern(struct msghdr *kmsg, struct sock *sk,
 
 		/* Advance. */
 		kcmsg = (struct cmsghdr *)((char *)kcmsg + tmp);
-		ucmsg = cmsg_compat_nxthdr(kmsg, ucmsg, ucmlen);
+		ucmsg = cmsg_compat_nxthdr(kmsg, ucmsg, cmsg.cmsg_len);
 	}
 
 	/*