valgrind: Adjust suppression for MSG_TRUNC with NULL buffer

valgrind complains if we pass a NULL buffer to recv(), even if we use
MSG_TRUNC, in which case it's actually safe.  For a long time we've had
a valgrind suppression for this.  It singles out the recv() in
tcp_sock_consume(), the only place we use MSG_TRUNC.

However, tcp_sock_consume() only has a single caller, which makes it a
prime candidate for inlining.  If inlined, it won't appear on the stack and
valgrind won't match the correct suppression.

It appears that certain compiler versions (for example gcc-13.2.1 in
Fedora 39) will inline this function even with the -O0 we use for valgrind
builds.  This breaks the suppression leading to a spurious failure in the
tests.

There's not really any way to adjust the suppression itself without making
it overly broad (we don't want to match other recv() calls).  So, as a hack
explicitly prevent inlining of this function when we're making a valgrind
build.  To accomplish this add an explicit -DVALGRIND when making such a
build.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

Makefile

@@ -128,7 +128,7 @@ qrap: $(QRAP_SRCS) passt.h
 valgrind: EXTRA_SYSCALLS += rt_sigprocmask rt_sigtimedwait rt_sigaction	\
 			    getpid gettid kill clock_gettime mmap	\
 			    munmap open unlink gettimeofday futex
-valgrind: FLAGS:=-g -O0 $(filter-out -O%,$(FLAGS))
+valgrind: FLAGS:=-g -O0 $(filter-out -O%,$(FLAGS)) -DVALGRIND
 valgrind: all
 
 .PHONY: clean

tcp.c

@@ -2097,6 +2097,15 @@ static void tcp_conn_from_tap(struct ctx *c,
  *
  * Return: 0 on success, negative error code from recv() on failure
  */
+#ifdef VALGRIND
+/* valgrind doesn't realise that passing a NULL buffer to recv() is ok if using
+ * MSG_TRUNC.  We have a suppression for this in the tests, but it relies on
+ * valgrind being able to see the tcp_sock_consume() stack frame, which it won't
+ * if this gets inlined.  This has a single caller making it a likely inlining
+ * candidate, and certain compiler versions will do so even at -O0.
+ */
+ __attribute__((noinline))
+#endif /* VALGRIND */
 static int tcp_sock_consume(const struct tcp_tap_conn *conn, uint32_t ack_seq)
 {
 	/* Simply ignore out-of-order ACKs: we already consumed the data we

test/valgrind.supp

@@ -3,7 +3,6 @@
    passt_recv_MSG_TRUNC_into_NULL_buffer
    Memcheck:Param
    socketcall.recvfrom(buf)
-   fun:recv
    ...
-   fun:tcp_sock_consume*
+   fun:tcp_sock_consume
 }