net: nexthop: Add flag to assert that NHGRP reserved fields are zero

There are many unpatched kernel versions out there that do not initialize
the reserved fields of struct nexthop_grp. The issue with that is that if
those fields were to be used for some end (i.e. stop being reserved), old
kernels would still keep sending random data through the field, and a new
userspace could not rely on the value.

In this patch, use the existing NHA_OP_FLAGS, which is currently inbound
only, to carry flags back to the userspace. Add a flag to indicate that the
reserved fields in struct nexthop_grp are zeroed before dumping. This is
reliant on the actual fix from commit 6d745cd0e972 ("net: nexthop:
Initialize all fields in dumped nexthops").

Signed-off-by: Petr Machata <[e-mail]>
Reviewed-by: Ido Schimmel <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

include/uapi/linux/nexthop.h

···
 #define NHA_OP_FLAG_DUMP_STATS		BIT(0)
 #define NHA_OP_FLAG_DUMP_HW_STATS	BIT(1)
 
+/* Response OP_FLAGS. */
+#define NHA_OP_FLAG_RESP_GRP_RESVD_0	BIT(31)	/* Dump clears resvd fields. */
+
 enum {
 	NHA_UNSPEC,
 	NHA_ID,		/* u32; id for nexthop. id == 0 means auto-assign */

net/ipv4/nexthop.c

···
 }
 
 static int nla_put_nh_group(struct sk_buff *skb, struct nexthop *nh,
-			    u32 op_flags)
+			    u32 op_flags, u32 *resp_op_flags)
 {
 	struct nh_group *nhg = rtnl_dereference(nh->nh_grp);
 	struct nexthop_grp *p;
···
 	struct nlattr *nla;
 	u16 group_type = 0;
 	int i;
+
+	*resp_op_flags |= NHA_OP_FLAG_RESP_GRP_RESVD_0;
 
 	if (nhg->hash_threshold)
 		group_type = NEXTHOP_GRP_TYPE_MPATH;
···
 
 	if (nh->is_group) {
 		struct nh_group *nhg = rtnl_dereference(nh->nh_grp);
+		u32 resp_op_flags = 0;
 
 		if (nhg->fdb_nh && nla_put_flag(skb, NHA_FDB))
 			goto nla_put_failure;
-		if (nla_put_nh_group(skb, nh, op_flags))
+		if (nla_put_nh_group(skb, nh, op_flags, &resp_op_flags) ||
+		    nla_put_u32(skb, NHA_OP_FLAGS, resp_op_flags))
 			goto nla_put_failure;
 		goto out;
 	}
···
 	sz += nla_total_size(4); /* NHA_ID */
 
 	if (nh->is_group)
-		sz += nh_nlmsg_size_grp(nh);
+		sz += nh_nlmsg_size_grp(nh) +
+		      nla_total_size(4) +	/* NHA_OP_FLAGS */
+		      0;
 	else
 		sz += nh_nlmsg_size_single(nh);