[PATCH v2] udp: Provide dummy iov in udp_peek_addr() to avoid Coverity warning

[Jon Maloy <jmaloy@redhat.com>]

udp_peek_addr() initialises struct msghdr without setting msg_iov,
leaving it implicitly NULL.  Coverity flags this as FORWARD_NULL,
believing recvmsg() will dereference the NULL pointer.

In practice, msg_iovlen being zero means the kernel never touches
msg_iov, so the warning is a false positive.  We now provide a
one-byte dummy iov to make msg_iov non-NULL, hence suppressing this
warning without changing the function's behaviour.

Signed-off-by: Jon Maloy <jmaloy@redhat.com>

----
v2: - Make the dummy iov conditional on an ANALYZER macro, so it has
      zero runtime cost in production builds.
    - Add a new 'analyzer' Makefile target (similar to 'valgrind')
      that defines ANALYZER via CPPFLAGS for use with static analysis
      builds.
---
 Makefile |  3 +++
 udp.c    | 11 +++++++++++
 2 files changed, 14 insertions(+)

diff --git a/Makefile b/Makefile
index 0a0a60b0..4dcf4cd1 100644
--- a/Makefile
+++ b/Makefile
@@ -122,6 +122,9 @@ passt-repair: $(PASST_REPAIR_SRCS) $(PASST_REPAIR_HEADERS) seccomp_repair.h
 pesto: BASE_CPPFLAGS += -DPESTO
 pesto: $(PESTO_SRCS) $(PESTO_HEADERS) seccomp_pesto.h
 
+analyzer: BASE_CPPFLAGS += -DANALYZER
+analyzer: all
+
 valgrind: EXTRA_SYSCALLS += rt_sigprocmask rt_sigtimedwait rt_sigaction	\
 			    rt_sigreturn getpid gettid kill clock_gettime \
 			    mmap|mmap2 munmap open unlink gettimeofday futex \
diff --git a/udp.c b/udp.c
index c28d6ee2..36c8c070 100644
--- a/udp.c
+++ b/udp.c
@@ -734,9 +734,20 @@ static int udp_peek_addr(int s, union sockaddr_inany *src,
 {
 	char sastr[SOCKADDR_STRLEN], dstr[INANY_ADDRSTRLEN];
 	char cmsg[PKTINFO_SPACE];
+#ifdef ANALYZER
+	char dummy;
+	struct iovec iov = {
+		.iov_base = &dummy,
+		.iov_len = sizeof(dummy),
+	};
+#endif /* ANALYZER */
 	struct msghdr msg = {
 		.msg_name = src,
 		.msg_namelen = sizeof(*src),
+#ifdef ANALYZER
+		.msg_iov = &iov,
+		.msg_iovlen = 1,
+#endif /* ANALYZER */
 		.msg_control = cmsg,
 		.msg_controllen = sizeof(cmsg),
 	};
-- 
2.52.0