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
---
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;
--
2.41.0