[PATCH] cppcheck,valgrind: Don't pass NULL to recv() with MSG_TRUNC

[David Gibson <david@gibson.dropbear.id.au>]

Usually, of course, it's invalid to pass a NULL buffer to recv().  However,
it's acceptable when using MSG_TRUNC, because that suppresses actually
writing to the buffer.  So, we pass NULL in tcp_sock_consume().

Unfortunately, checker tools aren't always aware of that special case: we
already have a suppression for cppcheck to cover this.  valgrind-3.22.0
(present in Fedora 39) has a similar problem, generating a spurious warning
here.

We could generate another suppression for valgrind, however, it so happens
that we already have tcp_buf_discard ready to hand.  If we pass this
instead of NULL it makes both cppcheck and valgrind happy.  We're still
using the MSG_TRUNC flag, the kernel doesn't actually have to copy data,
so we should still have the performance benefits of it.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
---
 tcp.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/tcp.c b/tcp.c
index cfcd40a..f51d27a 100644
--- a/tcp.c
+++ b/tcp.c
@@ -2106,8 +2106,13 @@ static int tcp_sock_consume(const struct tcp_tap_conn *conn, uint32_t ack_seq)
 	if (SEQ_LE(ack_seq, conn->seq_ack_from_tap))
 		return 0;
 
-	/* cppcheck-suppress [nullPointer, unmatchedSuppression] */
-	if (recv(conn->sock, NULL, ack_seq - conn->seq_ack_from_tap,
+	/* Since we're using MSG_TRUNC, it's allowed to pass NULL instead of a
+	 * real buffer.  However some checker tools (including at least some
+	 * versions of cppcheck and valgrind) aren't aware of that special case.
+	 * We so happen to have a convenient discard buffer, so we might as well
+	 * pass it to avoid spurious complaints from those tools.
+	 */
+	if (recv(conn->sock, tcp_buf_discard, ack_seq - conn->seq_ack_from_tap,
 		 MSG_DONTWAIT | MSG_TRUNC) < 0)
 		return -errno;
 
-- 
@@ -2106,8 +2106,13 @@ static int tcp_sock_consume(const struct tcp_tap_conn *conn, uint32_t ack_seq)
 	if (SEQ_LE(ack_seq, conn->seq_ack_from_tap))
 		return 0;
 
-	/* cppcheck-suppress [nullPointer, unmatchedSuppression] */
-	if (recv(conn->sock, NULL, ack_seq - conn->seq_ack_from_tap,
+	/* Since we're using MSG_TRUNC, it's allowed to pass NULL instead of a
+	 * real buffer.  However some checker tools (including at least some
+	 * versions of cppcheck and valgrind) aren't aware of that special case.
+	 * We so happen to have a convenient discard buffer, so we might as well
+	 * pass it to avoid spurious complaints from those tools.
+	 */
+	if (recv(conn->sock, tcp_buf_discard, ack_seq - conn->seq_ack_from_tap,
 		 MSG_DONTWAIT | MSG_TRUNC) < 0)
 		return -errno;
 
-- 
2.41.0