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: 5121197ecc5d ("kcm: close race conditions on sk_receive_queue")
Fixes: 91687355b927 ("kcm: Splice support")
Signed-off-by: Michal Luczaj <redacted>
---
 net/kcm/kcmsock.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/net/kcm/kcmsock.c b/net/kcm/kcmsock.c
index 24aec295a51cf737912f1aefe81556bd9f23331e..c05047dad62d7e201c950ab98af6dc7f0d48276c 100644
--- a/net/kcm/kcmsock.c
+++ b/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>

@@ -1030,6 +1031,11 @@ 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);

---
base-commit: c8f13134349b4385ae739f1efe403d5d3949ef92
change-id: 20250619-kcm-splice-01cb39a5997b

Best regards,
-- 
Michal Luczaj [off-list ref]