netdev - [PATCH] tcp: really ignore MSG_ZEROCOPY if no SO

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Date:   Thu,  6 Sep 2018 15:54:59 +0200
From:   Vincent Whitchurch <vincent.whitchurch@...s.com>
To:     davem@...emloft.net
Cc:     netdev@...r.kernel.org, willemb@...gle.com,
        Vincent Whitchurch <rabinv@...s.com>
Subject: [PATCH] tcp: really ignore MSG_ZEROCOPY if no SO_ZEROCOPY

According to the documentation in msg_zerocopy.rst, the SO_ZEROCOPY
flag was introduced because send(2) ignores unknown message flags and
any legacy application which was accidentally passing the equivalent of
MSG_ZEROCOPY earlier should not see any new behaviour.

Before commit f214f915e7db ("tcp: enable MSG_ZEROCOPY"), a send(2) call
which passed the equivalent of MSG_ZEROCOPY without setting SO_ZEROCOPY
would succeed.  However, after that commit, it fails with -ENOBUFS.  So
it appears that the SO_ZEROCOPY flag fails to fulfill its intended
purpose.  Fix it.

Fixes: f214f915e7db ("tcp: enable MSG_ZEROCOPY")
Signed-off-by: Vincent Whitchurch <vincent.whitchurch@...s.com>
---
 net/core/skbuff.c | 3 ---
 net/ipv4/tcp.c    | 2 +-
 2 files changed, 1 insertion(+), 4 deletions(-)

diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index c996c09d095f..b2c807f67aba 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -939,9 +939,6 @@ struct ubuf_info *sock_zerocopy_alloc(struct sock *sk, size_t size)
 
 	WARN_ON_ONCE(!in_task());
 
-	if (!sock_flag(sk, SOCK_ZEROCOPY))
-		return NULL;
-
 	skb = sock_omalloc(sk, 0, GFP_KERNEL);
 	if (!skb)
 		return NULL;
diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index b8af2fec5ad5..10c6246396cc 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -1185,7 +1185,7 @@ int tcp_sendmsg_locked(struct sock *sk, struct msghdr *msg, size_t size)
 
 	flags = msg->msg_flags;
 
-	if (flags & MSG_ZEROCOPY && size) {
+	if (flags & MSG_ZEROCOPY && size && sock_flag(sk, SOCK_ZEROCOPY)) {
 		if (sk->sk_state != TCP_ESTABLISHED) {
 			err = -EINVAL;
 			goto out_err;
-- 
2.11.0