rtnl_route: handle multiple next hop entries inside RTA_MULTIPATH

* configure.ac (AC_CHECK_FUNCS): Add mempcpy.
* src/rtnl_route.c (decode_rta_multipath): Navigate through attribute
payload and print it as an array if there are more data in the payload
than the first struct rtnexthop.rtnh_len indicates.
* tests/nlattr_rtmsg.c [!HAVE_MEMPCPY] (mempcpy): Re-define
to strace_mempcpy.
[!HAVE_MEMPCPY] (mempcpy): New function.
(DEF_NLATTR_RTMSG_FUNCS): New macro for defining init/print functions.
(init_rtmsg_inet, print_rtmsg_inet): New functions, defined using
DEF_NLATTR_RTMSG_FUNCS macro.
(main): Update expected output, add a check for multiple
struct rtnexthop inside RTA_MULTIPATH.

Resolves: https://github.com/strace/strace/issues/195

3 files changed, 158 insertions, 42 deletions

diff --git a/configure.ac b/configure.ac
index 2771c0f82..e662c12f4 100644
--- a/configure.ac
+++ b/configure.ac
@@ -352,6 +352,7 @@ AC_CHECK_FUNCS(m4_normalize([
 	futimens
 	iconv_open
 	if_indextoname
+	mempcpy
 	open64
 	open_memstream
 	preadv
diff --git a/src/rtnl_route.c b/src/rtnl_route.c
index fd6be7361..e3d6d52ad 100644
--- a/src/rtnl_route.c
+++ b/src/rtnl_route.c
@@ -236,17 +236,42 @@ static const nla_decoder_t rtmsg_nla_decoders[] = {
 	[RTA_DPORT]		= decode_nla_u16
 };
 
+/*
+ * RTA_MULTIPATH payload is a list of struct rtnexthop-headed RTA_* netlink
+ * attributes:
+ *
+ * {RTA_MULTIPATH nlattr hdr} [ [{struct rtnexthop}, {RTA_* nlattr}],
+ *                              [{struct rtnexthop}, {RTA_* nlattr}], ... ]
+ */
 static bool
 decode_rta_multipath(struct tcb *const tcp,
 		     const kernel_ulong_t addr,
 		     const unsigned int len,
 		     const void *const opaque_data)
 {
+	bool is_array = false;
 	struct rtnexthop nh;
+	kernel_ulong_t cur = addr;
+	kernel_ulong_t left = len;
 
 	if (len < sizeof(nh))
 		return false;
-	else if (!umove_or_printaddr(tcp, addr, &nh)) {
+
+	while (!umove_or_printaddr(tcp, cur, &nh)) {
+		static const size_t offset = RTNH_ALIGN(sizeof(nh));
+		const unsigned int rtnh_len = MIN(left, nh.rtnh_len);
+
+		if (cur == addr && nh.rtnh_len < len) {
+			tprint_array_begin();
+			is_array = true;
+		}
+
+		if (cur > addr)
+			tprint_array_next();
+
+		if (rtnh_len > offset)
+			tprint_array_begin();
+
 		/* print the whole structure regardless of its rtnh_len */
 		tprint_struct_begin();
 		PRINT_FIELD_U(nh, rtnh_len);
@@ -259,18 +284,28 @@ decode_rta_multipath(struct tcb *const tcp,
 		PRINT_FIELD_IFINDEX(nh, rtnh_ifindex);
 		tprint_struct_end();
 
-		const unsigned short rtnh_len = MIN(len, nh.rtnh_len);
-		const size_t offset = RTNH_ALIGN(sizeof(nh));
 		if (rtnh_len > offset) {
 			tprint_array_next();
-			decode_nlattr(tcp, addr + offset, rtnh_len - offset,
+			decode_nlattr(tcp, cur + offset, rtnh_len - offset,
 				      rtnl_route_attrs, "RTA_???",
 				      rtmsg_nla_decoders,
 				      ARRAY_SIZE(rtmsg_nla_decoders),
 				      opaque_data);
 		}
+
+		if (rtnh_len > offset)
+			tprint_array_end();
+
+		if (RTNH_ALIGN(rtnh_len) >= left)
+			break;
+
+		cur += RTNH_ALIGN(rtnh_len);
+		left -= RTNH_ALIGN(rtnh_len);
 	}
 
+	if (is_array)
+		tprint_array_end();
+
 	return true;
 }
 
diff --git a/tests/nlattr_rtmsg.c b/tests/nlattr_rtmsg.c
index bddc13faf..cd477dd1d 100644
--- a/tests/nlattr_rtmsg.c
+++ b/tests/nlattr_rtmsg.c
@@ -15,43 +15,61 @@
 #include <linux/ip.h>
 #include <linux/rtnetlink.h>
 
-#define LWTUNNEL_ENCAP_NONE 0
-
-static void
-init_rtmsg(struct nlmsghdr *const nlh, const unsigned int msg_len)
+#if !defined HAVE_MEMPCPY
+# undef mempcpy
+# define mempcpy strace_mempcpy
+static void *
+mempcpy(void *dest, const void *src, size_t n)
 {
-	SET_STRUCT(struct nlmsghdr, nlh,
-		.nlmsg_len = msg_len,
-		.nlmsg_type = RTM_GETROUTE,
-		.nlmsg_flags = NLM_F_DUMP
-	);
-
-	struct rtmsg *const msg = NLMSG_DATA(nlh);
-	SET_STRUCT(struct rtmsg, msg,
-		.rtm_family = AF_UNIX,
-		.rtm_tos = IPTOS_LOWDELAY,
-		.rtm_table = RT_TABLE_DEFAULT,
-		.rtm_protocol = RTPROT_KERNEL,
-		.rtm_scope = RT_SCOPE_UNIVERSE,
-		.rtm_type = RTN_LOCAL,
-		.rtm_flags = RTM_F_NOTIFY
-	);
-}
+	memcpy(dest, src, n);
 
-static void
-print_rtmsg(const unsigned int msg_len)
-{
-	printf("{nlmsg_len=%u, nlmsg_type=RTM_GETROUTE, nlmsg_flags=NLM_F_DUMP"
-	       ", nlmsg_seq=0, nlmsg_pid=0}, {rtm_family=AF_UNIX"
-	       ", rtm_dst_len=0, rtm_src_len=0"
-	       ", rtm_tos=IPTOS_LOWDELAY"
-	       ", rtm_table=RT_TABLE_DEFAULT"
-	       ", rtm_protocol=RTPROT_KERNEL"
-	       ", rtm_scope=RT_SCOPE_UNIVERSE"
-	       ", rtm_type=RTN_LOCAL"
-	       ", rtm_flags=RTM_F_NOTIFY}",
-	       msg_len);
+	return dest + n;
 }
+#endif
+
+#define LWTUNNEL_ENCAP_NONE 0
+
+#define DEF_NLATTR_RTMSG_FUNCS(sfx_, af_)				\
+	static void							\
+	init_##sfx_(struct nlmsghdr *const nlh, const unsigned int msg_len) \
+	{								\
+		SET_STRUCT(struct nlmsghdr, nlh,			\
+			.nlmsg_len = msg_len,				\
+			.nlmsg_type = RTM_GETROUTE,			\
+			.nlmsg_flags = NLM_F_DUMP			\
+		);							\
+									\
+		struct rtmsg *const msg = NLMSG_DATA(nlh);		\
+		SET_STRUCT(struct rtmsg, msg,				\
+			.rtm_family = (af_),				\
+			.rtm_tos = IPTOS_LOWDELAY,			\
+			.rtm_table = RT_TABLE_DEFAULT,			\
+			.rtm_protocol = RTPROT_KERNEL,			\
+			.rtm_scope = RT_SCOPE_UNIVERSE,			\
+			.rtm_type = RTN_LOCAL,				\
+			.rtm_flags = RTM_F_NOTIFY			\
+		);							\
+	}								\
+									\
+	static void							\
+	print_##sfx_(const unsigned int msg_len)			\
+	{								\
+		printf("{nlmsg_len=%u, nlmsg_type=RTM_GETROUTE"		\
+		       ", nlmsg_flags=NLM_F_DUMP"			\
+		       ", nlmsg_seq=0, nlmsg_pid=0}, {rtm_family=" #af_	\
+		       ", rtm_dst_len=0, rtm_src_len=0"			\
+		       ", rtm_tos=IPTOS_LOWDELAY"			\
+		       ", rtm_table=RT_TABLE_DEFAULT"			\
+		       ", rtm_protocol=RTPROT_KERNEL"			\
+		       ", rtm_scope=RT_SCOPE_UNIVERSE"			\
+		       ", rtm_type=RTN_LOCAL"				\
+		       ", rtm_flags=RTM_F_NOTIFY}",			\
+		       msg_len);					\
+	}								\
+	/* End of DEF_NLATTR_RTMSG_FUNCS */
+
+DEF_NLATTR_RTMSG_FUNCS(rtmsg, AF_UNIX)
+DEF_NLATTR_RTMSG_FUNCS(rtmsg_inet, AF_INET)
 
 int
 main(void)
@@ -110,9 +128,9 @@ main(void)
 	TEST_NLATTR_OBJECT(fd, nlh0, hdrlen,
 			   init_rtmsg, print_rtmsg,
 			   RTA_MULTIPATH, pattern, nh,
-			   printf("{rtnh_len=%u, rtnh_flags=RTNH_F_DEAD"
+			   printf("[{rtnh_len=%u, rtnh_flags=RTNH_F_DEAD"
 				  ", rtnh_hops=%u"
-				  ", rtnh_ifindex=" IFINDEX_LO_STR "}",
+				  ", rtnh_ifindex=" IFINDEX_LO_STR "}]",
 				  nh.rtnh_len, nh.rtnh_hops));
 
 	char buf[RTNH_ALIGN(sizeof(nh)) + sizeof(nla)];
@@ -123,11 +141,73 @@ main(void)
 	TEST_NLATTR(fd, nlh0, hdrlen,
 		    init_rtmsg, print_rtmsg,
 		    RTA_MULTIPATH, sizeof(buf), buf, sizeof(buf),
-		    printf("{rtnh_len=%u, rtnh_flags=RTNH_F_DEAD"
+		    printf("[{rtnh_len=%u, rtnh_flags=RTNH_F_DEAD"
 			   ", rtnh_hops=%u, rtnh_ifindex=" IFINDEX_LO_STR "}"
-			   ", {nla_len=%u, nla_type=RTA_DST}",
+			   ", {nla_len=%u, nla_type=RTA_DST}]",
 			   nh.rtnh_len, nh.rtnh_hops, nla.nla_len));
 
+	static const struct in_addr gw_inet_addr = { .s_addr = BE32(0xdeadbeef) };
+	static const uint8_t via_inet6_addr[16] = {
+		0xde, 0xad, 0xfa, 0xce, 0xbe, 0xef, 0xca, 0xfe,
+		0xfe, 0xed, 0xba, 0x5e, 0x00, 0x00, 0xfa, 0xde };
+	static const struct rtvia rtvia = { .rtvia_family = AF_INET6 };
+	char buf2[2 * (RTNH_ALIGN(sizeof(nh)) + NLMSG_ALIGN(sizeof(nla))) +
+		       + NLMSG_ALIGN(sizeof(gw_inet_addr))
+		       + NLMSG_ALIGN(offsetof(struct rtvia, rtvia_addr)
+				     + sizeof(via_inet6_addr))];
+	char *pos = buf2;
+
+	nh.rtnh_len = RTNH_ALIGN(sizeof(nh)) + NLMSG_ALIGN(sizeof(nla))
+		      + NLMSG_ALIGN(sizeof(gw_inet_addr));
+	nh.rtnh_flags = 0xc0;
+	nla.nla_type = RTA_GATEWAY;
+	nla.nla_len = NLMSG_ALIGN(sizeof(nla))
+		      + NLMSG_ALIGN(sizeof(gw_inet_addr));
+	pos = mempcpy(pos, &nh, sizeof(nh));
+	pos = mempcpy(pos, &nla, sizeof(nla));
+	pos = mempcpy(pos, &gw_inet_addr, sizeof(gw_inet_addr));
+
+	nh.rtnh_len = RTNH_ALIGN(sizeof(nh)) + NLMSG_ALIGN(sizeof(nla))
+		      + NLMSG_ALIGN(offsetof(struct rtvia, rtvia_addr)
+				    + sizeof(via_inet6_addr));
+	nla.nla_type = RTA_VIA;
+	nla.nla_len = NLMSG_ALIGN(sizeof(nla))
+		      + NLMSG_ALIGN(offsetof(struct rtvia, rtvia_addr)
+				    + sizeof(via_inet6_addr));
+	pos = mempcpy(pos, &nh, sizeof(nh));
+	pos = mempcpy(pos, &nla, sizeof(nla));
+	pos = mempcpy(pos, &rtvia, sizeof(rtvia));
+	pos = mempcpy(pos, &via_inet6_addr, sizeof(via_inet6_addr));
+	TEST_NLATTR(fd, nlh0, hdrlen,
+		    init_rtmsg_inet, print_rtmsg_inet,
+		    RTA_MULTIPATH, sizeof(buf2), buf2, sizeof(buf2),
+		    printf("[[{rtnh_len=%u, rtnh_flags=RTNH_F_TRAP|0x80"
+			   ", rtnh_hops=%u, rtnh_ifindex=" IFINDEX_LO_STR "}"
+			   ", [{nla_len=%u, nla_type=RTA_GATEWAY}"
+			   ", inet_addr(\"222.173.190.239\")]]"
+			   ", [{rtnh_len=%u, rtnh_flags=RTNH_F_TRAP|0x80"
+			   ", rtnh_hops=%u, rtnh_ifindex=" IFINDEX_LO_STR "}"
+			   ", [{nla_len=%u, nla_type=RTA_VIA}"
+			   ", {rtvia_family=AF_INET6"
+			   ", inet_pton(AF_INET6"
+			   ", \"dead:face:beef:cafe:feed:ba5e:0:fade\""
+			   ", &rtvia_addr)}]]]",
+			   (uint32_t) (RTNH_ALIGN(sizeof(nh))
+			   + NLMSG_ALIGN(sizeof(nla))
+			   + NLMSG_ALIGN(sizeof(gw_inet_addr))),
+			   nh.rtnh_hops,
+			   (uint32_t) (NLMSG_ALIGN(sizeof(nla))
+			   + NLMSG_ALIGN(sizeof(gw_inet_addr))),
+			   (uint32_t) (RTNH_ALIGN(sizeof(nh))
+			   + NLMSG_ALIGN(sizeof(nla))
+			   + NLMSG_ALIGN(offsetof(struct rtvia, rtvia_addr)
+					 + sizeof(via_inet6_addr))),
+			   nh.rtnh_hops,
+			   (uint32_t) (NLMSG_ALIGN(sizeof(nla))
+			   + NLMSG_ALIGN(offsetof(struct rtvia, rtvia_addr)
+					 + sizeof(via_inet6_addr)))
+			   ));
+
 	static const struct rta_cacheinfo ci = {
 		.rta_clntref = 0xabcdefab,
 		.rta_lastuse = 0xbdadaedc,