Subject: [bpf PATCH v2 8/8] bpf: sockmap/tls, fix pop data with SK_DROP return code

From: John Fastabend <[email-masked]>
Date: Sat, 11 Jan 2020 06:12:06 +0000

When user returns SK_DROP we need to reset the number of copied bytes
to indicate to the user the bytes were dropped and not sent. If we
don't reset the copied arg sendmsg will return as if those bytes were
copied giving the user a positive return value.

This works as expected today except in the case where the user also
pops bytes. In the pop case the sg.size is reduced but we don't correctly
account for this when copied bytes is reset. The popped bytes are not
accounted for and we return a small positive value potentially confusing
the user.

The reason this happens is due to a typo where we do the wrong comparison
when accounting for pop bytes. In this fix notice the if/else is not
needed and that we have a similar problem if we push data except its not
visible to the user because if delta is larger the sg.size we return a
negative value so it appears as an error regardless.

Cc: [email-masked]
Fixes: 7246d8ed4dcce ("bpf: helper to pop data from messages")
Acked-by: Jonathan Lemon <[email-masked]>
Signed-off-by: John Fastabend <[email-masked]>
---
 net/ipv4/tcp_bpf.c | 5 +----
 net/tls/tls_sw.c   | 5 +----
 2 files changed, 2 insertions(+), 8 deletions(-)

diff --git a/net/ipv4/tcp_bpf.c b/net/ipv4/tcp_bpf.c
index e6b08b5a0895..8a01428f80c1 100644
--- a/net/ipv4/tcp_bpf.c
+++ b/net/ipv4/tcp_bpf.c
@@ -315,10 +315,7 @@ static int tcp_bpf_send_verdict(struct sock *sk, struct sk_psock *psock,
 		 */
 		delta = msg->sg.size;
 		psock->eval = sk_psock_msg_verdict(sk, psock, msg);
-		if (msg->sg.size < delta)
-			delta -= msg->sg.size;
-		else
-			delta = 0;
+		delta -= msg->sg.size;
 	}
 
 	if (msg->cork_bytes &&
diff --git a/net/tls/tls_sw.c b/net/tls/tls_sw.c
index 21c7725d17ca..159d49dab403 100644
--- a/net/tls/tls_sw.c
+++ b/net/tls/tls_sw.c
@@ -809,10 +809,7 @@ static int bpf_exec_tx_verdict(struct sk_msg *msg, struct sock *sk,
 	if (psock->eval == __SK_NONE) {
 		delta = msg->sg.size;
 		psock->eval = sk_psock_msg_verdict(sk, psock, msg);
-		if (delta < msg->sg.size)
-			delta -= msg->sg.size;
-		else
-			delta = 0;
+		delta -= msg->sg.size;
 	}
 	if (msg->cork_bytes && msg->cork_bytes > msg->sg.size &&
 	    !enospc && !full_record) {
-- 
2.17.1