[PATCH v2 1/4] Add an internal helper for binding to a dynamically-assigned port

[Chuck Lever <chuck.lever@oracle.com>]

Create a helper function akin to bindresvport(3) that instead binds
to a dynamically assigned port using the rules in RFC 6335 Section 6
to avoid all IANA-assigned service port numbers.

This is intended to remain an internal helper for the time being, so
this commit provides no header declaration.

All internal bindresvport(3) call sites manufacture an INADDR_ANY-
type address to pass to bind(2), so the helper handles that as well,
to avoid code duplication. This means that callers do not need to
pass in a sockaddr. Only an open socket is required.

BugLink: https://bugzilla.linux-nfs.org/show_bug.cgi?id=320
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
---
 src/Makefile.am   |    5 +-
 src/binddynport.c |  133 +++++++++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 136 insertions(+), 2 deletions(-)
 create mode 100644 src/binddynport.c

diff --git a/src/Makefile.am b/src/Makefile.am
index fba2aa4..932414d 100644
--- a/src/Makefile.am
+++ b/src/Makefile.am
@@ -15,8 +15,9 @@ lib_LTLIBRARIES = libtirpc.la
 libtirpc_la_LDFLAGS = @LDFLAG_NOUNDEFINED@ -no-undefined -lpthread
 libtirpc_la_LDFLAGS += -version-info @LT_VERSION_INFO@
 
-libtirpc_la_SOURCES = auth_none.c auth_unix.c authunix_prot.c bindresvport.c clnt_bcast.c \
-        clnt_dg.c clnt_generic.c clnt_perror.c clnt_raw.c clnt_simple.c \
+libtirpc_la_SOURCES = auth_none.c auth_unix.c authunix_prot.c \
+        binddynport.c bindresvport.c \
+        clnt_bcast.c clnt_dg.c clnt_generic.c clnt_perror.c clnt_raw.c clnt_simple.c \
         clnt_vc.c rpc_dtablesize.c getnetconfig.c getnetpath.c getrpcent.c \
         getrpcport.c mt_misc.c pmap_clnt.c pmap_getmaps.c pmap_getport.c \
         pmap_prot.c pmap_prot2.c pmap_rmt.c rpc_prot.c rpc_commondata.c \
diff --git a/src/binddynport.c b/src/binddynport.c
new file mode 100644
index 0000000..6fa18e0
--- /dev/null
+++ b/src/binddynport.c
@@ -0,0 +1,133 @@
+/*
+ * Copyright (c) 2018, Oracle America, Inc.
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ * - Redistributions of source code must retain the above copyright notice,
+ *   this list of conditions and the following disclaimer.
+ * - Redistributions in binary form must reproduce the above copyright notice,
+ *   this list of conditions and the following disclaimer in the documentation
+ *   and/or other materials provided with the distribution.
+ * - Neither the name of "Oracle America, Inc." nor the names of its
+ *   contributors may be used to endorse or promote products derived
+ *   from this software without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE
+ * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+ * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+ * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+ * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
+ * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+ * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+ * POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#include <sys/types.h>
+#include <sys/socket.h>
+
+#include <netdb.h>
+#include <netinet/in.h>
+
+#include <stdlib.h>
+#include <unistd.h>
+#include <errno.h>
+#include <string.h>
+
+#include <rpc/rpc.h>
+
+#include "reentrant.h"
+#include "rpc_com.h"
+
+extern pthread_mutex_t port_lock;
+
+/*
+ * Dynamic port range as defined in RFC 6335 Section 6.
+ * This range avoids all IANA-assigned service port
+ * numbers.
+ */
+enum {
+	LOWPORT		= 49152,
+	ENDPORT		= 65534,
+	NPORTS		= ENDPORT - LOWPORT + 1,
+};
+
+/*
+ * Bind a socket to a dynamically-assigned IP port.
+ *
+ * @fd is an open but unbound socket.
+ *
+ * On each call, a port number is chosen at random from
+ * within the dynamic/private port range, even if the
+ * caller has CAP_NET_ADMIN_BIND.
+ *
+ * Returns 0 on success, -1 on failure. errno may be
+ * set to a non-determinant value.
+ *
+ * This function is re-entrant.
+ */
+int __binddynport(int fd)
+{
+	struct sockaddr_storage ss;
+#ifdef INET6
+	struct sockaddr_in6 *sin6;
+#endif
+	struct sockaddr_in *sin;
+	in_port_t port, *portp;
+	struct sockaddr *sap;
+	socklen_t salen;
+	unsigned int seed;
+	int i, res;
+
+	if (__rpc_sockisbound(fd))
+		return 0;
+
+	res = -1;
+	sap = (struct sockaddr *)(void *)&ss;
+	salen = sizeof(ss);
+	memset(sap, 0, salen);
+
+	mutex_lock(&port_lock);
+
+	if (getsockname(fd, sap, &salen) == -1)
+		goto out;
+
+	switch (ss.ss_family) {
+	case AF_INET:
+		sin = (struct sockaddr_in *)(void *)&ss;
+		portp = &sin->sin_port;
+		salen = sizeof(struct sockaddr_in);
+		break;
+#ifdef INET6
+	case AF_INET6:
+		sin6 = (struct sockaddr_in6 *)(void *)&ss;
+		portp = &sin6->sin6_port;
+		salen = sizeof(struct sockaddr_in6);
+		break;
+#endif
+	default:
+		goto out;
+	}
+
+	seed = time(NULL);
+	port = (rand_r(&seed) % NPORTS) + LOWPORT;
+	for (i = 0; i < NPORTS; ++i) {
+		*portp = htons(port++);
+		res = bind(fd, sap, salen);
+		if (res >= 0) {
+			res = 0;
+			break;
+		}
+		if (errno != EADDRINUSE)
+			break;
+		if (port > ENDPORT)
+			port = LOWPORT;
+	}
+
+out:
+	mutex_unlock(&port_lock);
+	return res;
+}