unix_stream_sendmsg() splices external pages directly into skb frags when
MSG_SPLICE_PAGES is set, but it does not propagate SKBFL_SHARED_FRAG
afterward.

That leaves later writers without the shared-frag marker even though the
skb still references externally owned pages.

Set the marker after a successful skb_splice_from_iter() call.

Fixes: a0dbf5f818f90 ("af_unix: Support MSG_SPLICE_PAGES")
Reported-by: Yiming Qian <redacted>
Reported-by: Can Liu <redacted>
Signed-off-by: Yiming Qian <redacted>
Signed-off-by: Can Liu <redacted>
---
 net/unix/af_unix.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c
index f7a9d55eee8a1..f2cd0f8ec0914 100644
--- a/net/unix/af_unix.c
+++ b/net/unix/af_unix.c

@@ -2458,6 +2458,7 @@ static int unix_stream_sendmsg(struct socket *sock, struct msghdr *msg,
 				goto out_free;
 
 			size = err;
+			skb_shinfo(skb)->flags |= SKBFL_SHARED_FRAG;
 			refcount_add(size, &sk->sk_wmem_alloc);
 		} else {
 			skb_put(skb, size - data_len);

-- 
2.34.1