sysctl: Make CTL_READ nonfatal

Some entries might be missing and that should not cause
CRIU to stop dumping when we know the entries are safe
to unuse.
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

include/sysctl.h

@@ -5,6 +5,7 @@ struct sysctl_req {
 	char	*name;
 	void	*arg;
 	int	type;
+	int	flags;
 };
 
 extern int sysctl_op(struct sysctl_req *req, size_t nr_req, int op);
@@ -30,4 +31,9 @@ enum {
 #define CTL_LEN(t)	((t) >> CTL_SHIFT)
 #define CTL_TYPE(t)	((t) & ((1 << CTL_SHIFT) - 1))
 
+/*
+ * Some entries might be missing mark them as optional.
+ */
+#define CTL_FLAGS_OPTIONAL	1
+
 #endif /* __CR_SYSCTL_H__ */

net.c

@@ -104,6 +104,7 @@ static int ipv4_conf_op(char *tgt, int *conf, int op, NetnsEntry **netns)
 		req[ri].name = path[i];
 		req[ri].arg = &conf[i];
 		req[ri].type = CTL_32;
+		req[ri].flags = 0;
 		ri++;
 	}
 

sysctl.c

@@ -137,6 +137,8 @@ static int __sysctl_op(int dir, struct sysctl_req *req, int op)
 
 	fd = openat(dir, req->name, flags);
 	if (fd < 0) {
+		if (errno == ENOENT && (req->flags & CTL_FLAGS_OPTIONAL))
+			return 0;
 		pr_perror("Can't open sysctl %s", req->name);
 		return -1;
 	}