kcm: Fix splice support

Flags passed in for splice() syscall should not end up in
skb_recv_datagram(). As SPLICE_F_NONBLOCK == MSG_PEEK, kernel gets
confused: skb isn't unlinked from a receive queue, while strp_msg::offset
and strp_msg::full_len are updated.

Unbreak the logic a bit more by mapping both O_NONBLOCK and
SPLICE_F_NONBLOCK to MSG_DONTWAIT. This way we align with man splice(2) in
regard to errno EAGAIN:

   SPLICE_F_NONBLOCK was specified in flags or one of the file descriptors
   had been marked as nonblocking (O_NONBLOCK), and the operation would
   block.

Fixes: 5121197 ("kcm: close race conditions on sk_receive_queue")
Fixes: 9168735 ("kcm: Splice support")
Signed-off-by: Michal Luczaj <mhal@rbox.co>
Link: https://patch.msgid.link/20250725-kcm-splice-v1-1-9a725ad2ee71@rbox.co
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: mmhal

net/kcm/kcmsock.c

@@ -19,6 +19,7 @@
 #include <linux/rculist.h>
 #include <linux/skbuff.h>
 #include <linux/socket.h>
+#include <linux/splice.h>
 #include <linux/uaccess.h>
 #include <linux/workqueue.h>
 #include <linux/syscalls.h>
@@ -1029,6 +1030,11 @@ static ssize_t kcm_splice_read(struct socket *sock, loff_t *ppos,
 	ssize_t copied;
 	struct sk_buff *skb;
 
+	if (sock->file->f_flags & O_NONBLOCK || flags & SPLICE_F_NONBLOCK)
+		flags = MSG_DONTWAIT;
+	else
+		flags = 0;
+
 	/* Only support splice for SOCKSEQPACKET */
 
 	skb = skb_recv_datagram(sk, flags, &err);