netdev - Re: Bug in short splice to socket?

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Message-ID: <909595.1685639680@warthog.procyon.org.uk>
Date: Thu, 01 Jun 2023 18:14:40 +0100
From: David Howells <dhowells@...hat.com>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: dhowells@...hat.com, Jakub Kicinski <kuba@...nel.org>,
    netdev@...r.kernel.org, "David S.
 Miller" <davem@...emloft.net>,
    Eric Dumazet <edumazet@...gle.com>, Paolo Abeni <pabeni@...hat.com>,
    Willem de Bruijn <willemdebruijn.kernel@...il.com>,
    David Ahern <dsahern@...nel.org>,
    Matthew Wilcox <willy@...radead.org>, Jens Axboe <axboe@...nel.dk>,
    linux-mm@...ck.org, linux-kernel@...r.kernel.org,
    Chuck Lever <chuck.lever@...cle.com>,
    Boris Pismenny <borisp@...dia.com>,
    John Fastabend <john.fastabend@...il.com>,
    Christoph Hellwig <hch@...radead.org>
Subject: Re: Bug in short splice to socket?

Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> > However, this might well cause a malfunction in UDP, for example.
> > MSG_MORE corks the current packet, so if I ask sendfile() say shove 32K
> > into a packet, if, say, 16K is read from the source and entirely
> > transcribed into the packet,
> 
> If you use splice() for UDP, I don't think you would normally expect
> to get all that well-defined packet boundaries.

Actually, it will.  Attempting to overfill a UDP packet with splice will get
you -EMSGSIZE.  It won't turn a splice into more than one UDP packet.

I wonder if the right solution actually is to declare that the problem is
userspace's.  If you ask it to splice Z amount of data and it can't manage
that because the source dries up prematurely, then make it so that you assume
it always passed MSG_MORE and returns a short splice to userspace.  Userspace
can retry the splice/sendfile or do an empty sendmsg() to cap the message (or
cancel it).  Perhaps flushing a short message is actually a *bad* idea.

The answer then might be to make TLS handle a zero-length send() and fix the
test cases.  Would the attached changes then work for you?

David
---
diff --git a/fs/splice.c b/fs/splice.c
index 3e06611d19ae..237688b0700b 100644
--- a/fs/splice.c
+++ b/fs/splice.c
@@ -956,13 +956,17 @@ ssize_t splice_direct_to_actor(struct file *in, struct splice_desc *sd,
 	 */
 	bytes = 0;
 	len = sd->total_len;
+
+	/* Don't block on output, we have to drain the direct pipe. */
 	flags = sd->flags;
+	sd->flags &= ~SPLICE_F_NONBLOCK;
 
 	/*
-	 * Don't block on output, we have to drain the direct pipe.
+	 * We signal MORE until we've read sufficient data to fulfill the
+	 * request and we keep signalling it if the caller set it.
 	 */
-	sd->flags &= ~SPLICE_F_NONBLOCK;
 	more = sd->flags & SPLICE_F_MORE;
+	sd->flags |= SPLICE_F_MORE;
 
 	WARN_ON_ONCE(!pipe_empty(pipe->head, pipe->tail));
 
@@ -978,14 +982,12 @@ ssize_t splice_direct_to_actor(struct file *in, struct splice_desc *sd,
 		sd->total_len = read_len;
 
 		/*
-		 * If more data is pending, set SPLICE_F_MORE
-		 * If this is the last data and SPLICE_F_MORE was not set
-		 * initially, clears it.
+		 * If we now have sufficient data to fulfill the request then
+		 * we clear SPLICE_F_MORE if it was not set initially.
 		 */
-		if (read_len < len)
-			sd->flags |= SPLICE_F_MORE;
-		else if (!more)
+		if (read_len >= len && !more)
 			sd->flags &= ~SPLICE_F_MORE;
+
 		/*
 		 * NOTE: nonblocking mode only applies to the input. We
 		 * must not do the output in nonblocking mode as then we
diff --git a/net/tls/tls_sw.c b/net/tls/tls_sw.c
index f63e4405cf34..5d48391da16c 100644
--- a/net/tls/tls_sw.c
+++ b/net/tls/tls_sw.c
@@ -995,6 +995,9 @@ static int tls_sw_sendmsg_locked(struct sock *sk, struct msghdr *msg,
 		}
 	}
 
+	if (!msg_data_left(msg) && eor)
+		goto copied;
+
 	while (msg_data_left(msg)) {
 		if (sk->sk_err) {
 			ret = -sk->sk_err;
diff --git a/tools/testing/selftests/net/tls.c b/tools/testing/selftests/net/tls.c
index e699548d4247..7df31583f2a4 100644
--- a/tools/testing/selftests/net/tls.c
+++ b/tools/testing/selftests/net/tls.c
@@ -377,7 +377,7 @@ static void chunked_sendfile(struct __test_metadata *_metadata,
 	char buf[TLS_PAYLOAD_MAX_LEN];
 	uint16_t test_payload_size;
 	int size = 0;
-	int ret;
+	int ret = 0;
 	char filename[] = "/tmp/mytemp.XXXXXX";
 	int fd = mkstemp(filename);
 	off_t offset = 0;
@@ -398,6 +398,9 @@ static void chunked_sendfile(struct __test_metadata *_metadata,
 		size -= ret;
 	}
 
+	if (ret < chunk_size)
+		EXPECT_EQ(send(self->fd, "", 0, 0), 0);
+
 	EXPECT_EQ(recv(self->cfd, buf, test_payload_size, MSG_WAITALL),
 		  test_payload_size);