kerndat: check whether ipv6 is supported or not (v2)

v2: use a cached value to dump ipv6 interface addesses
    call get_ipv6() from kerndat_init_rst too
Signed-off-by: Andrew Vagin <avagin@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

include/kerndat.h

@@ -24,6 +24,7 @@ struct kerndat_s {
 	bool has_memfd;
 	bool has_fdinfo_lock;
 	unsigned long task_size;
+	bool ipv6;
 };
 
 extern struct kerndat_s kdat;

kerndat.c

@@ -333,6 +333,21 @@ out:
 	return exit_code;
 }
 
+static int get_ipv6()
+{
+	if (access("/proc/sys/net/ipv6", F_OK) < 0) {
+		if (errno == ENOENT) {
+			pr_debug("ipv6 is disabled\n");
+			kdat.ipv6 = false;
+			return 0;
+		}
+		pr_perror("Unable to access /proc/sys/net/ipv6");
+		return -1;
+	}
+	kdat.ipv6 = true;
+	return 0;
+}
+
 int kerndat_init(void)
 {
 	int ret;
@@ -348,6 +363,8 @@ int kerndat_init(void)
 		ret = kerndat_fdinfo_has_lock();
 	if (!ret)
 		ret = get_task_size();
+	if (!ret)
+		ret = get_ipv6();
 
 	kerndat_lsm();
 
@@ -371,6 +388,8 @@ int kerndat_init_rst(void)
 		ret = kerndat_has_memfd_create();
 	if (!ret)
 		ret = get_task_size();
+	if (!ret)
+		ret = get_ipv6();
 
 	kerndat_lsm();
 

net.c

@@ -25,6 +25,8 @@
 #include "pstree.h"
 #include "string.h"
 #include "sysctl.h"
+#include "kerndat.h"
+
 #include "protobuf.h"
 #include "protobuf/netdev.pb-c.h"
 
@@ -641,10 +643,8 @@ static inline int dump_route(struct cr_imgset *fds)
 		return -1;
 
 	/* If ipv6 is disabled, "ip -6 route dump" dumps all routes */
-	if (access("/proc/sys/net/ipv6/", F_OK)) {
-		pr_debug("ipv6 is disabled\n");
+	if (!kdat.ipv6)
 		return 0;
-	}
 
 	img = img_from_set(fds, CR_FD_ROUTE6);
 	if (run_ip_tool("-6", "route", "save", -1, img_raw_fd(img), 0))