restore: copy special cpuset props recursively

The symptom of this bug was that users restoring tasks to a nested cgroup where
the top level group was created by criu (and not previously configured) e.g.
cpuset:/lxc/u1 would get an ENOSPC. criu would try to copy the special
properties into /lxc/u1 directly and (silently) fail, and then tried to copy
the task into the cg and fail with ENOSPC:

ENOSPC Attempted  to  write(2)  an empty cpuset.cpus or cpuset.mems setting to
       a cpuset that has tasks attached.

Fixing the silent failure to a loud failure, it gave EACCES:

EACCES Attempted to add, using write(2), a CPU or memory node to a cpuset, when
       that CPU or memory node was not already in its parent.

So, we need to copy the the special props down the entire tree. Additionally,
we shouldn't copy props directly from the top, since some intermediate point in
the tree could add restrictions. We first walk back up the tree to find the
first point where the props are empty, and then copy that parent's props all
the way down.
Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

cgroup.c

@@ -868,42 +868,106 @@ static const char *special_cpuset_props[] = {
 	NULL,
 };
 
-static int copy_special_cg_props(const char *controller, const char *path)
+static int find_value(const char *controller, const char *path, const char *prop,
+			char *value, char *prefix, char *missing_path)
 {
-	int cg = get_service_fd(CGROUP_YARD);
+	char fpath[PATH_MAX], my_path[PATH_MAX];
+	char *pos;
+	int ret, cg;
 
+	cg = get_service_fd(CGROUP_YARD);
+	strncpy(my_path, path, PATH_MAX);
+
+	while (1) {
+		FILE *f;
+
+		pos = strrchr(my_path, '/');
+		if (pos) {
+			*pos = 0;
+			snprintf(prefix, PATH_MAX, "%s/%s", controller, my_path);
+		} else {
+			snprintf(fpath, PATH_MAX, "%s", controller);
+		}
+		snprintf(fpath, PATH_MAX, "%s/%s", prefix, prop);
+
+		f = fopenat(cg, fpath, "r");
+		if (!f)
+			return -1;
+
+		ret = fscanf(f, "%1024s", value);
+		fclose(f);
+
+		if (ret > 0) {
+			strcpy(missing_path, path + (pos - my_path));
+			return 0;
+		} else if (!pos) {
+			pr_err("didn't find a value to propogate in the root!\n");
+			return -1;
+		}
+	}
+
+	return -1;
+}
+
+static int copy_recursive(const char *prefix, char *path, const char *prop, const char *value)
+{
+	char fpath[PATH_MAX];
+	char *pos;
+	int ret, out, cg;
+
+	cg = get_service_fd(CGROUP_YARD);
+
+	/* skip the first / */
+	pos = path + 1;
+	while (1) {
+		pos = strchr(pos, '/');
+
+		if (pos)
+			*pos = 0;
+		snprintf(fpath, PATH_MAX, "%s/%s/%s", prefix, path, prop);
+		if (pos) {
+			*pos = '/';
+			pos++;
+		}
+
+		out = openat(cg, fpath, O_RDWR);
+		if (out < 0) {
+			pr_perror("couldn't open cg prop at %s\n", fpath);
+			return -1;
+		}
+
+		ret = write(out, value, strlen(value));
+		close(out);
+
+		if (ret < 0) {
+			pr_perror("copying property %s to %s failed\n", prop, fpath);
+			return -1;
+		}
+
+		if (!pos)
+			break;
+	}
+
+	return 0;
+}
+
+static int copy_special_cg_props(const char *controller, char *path)
+{
 	pr_info("copying special cg props for %s\n", controller);
 
 	if (strstr(controller, "cpuset")) {
 		int i;
 
 		for (i = 0; special_cpuset_props[i]; i++) {
-			char fpath[PATH_MAX], buf[1024];
+			char buf[1024], prefix[PATH_MAX], missing_path[PATH_MAX];
 			const char *prop = special_cpuset_props[i];
-			int ret;
-			FILE *f;
-
-			snprintf(fpath, PATH_MAX, "%s/%s", controller, prop);
-			f = fopenat(cg, fpath, "r");
-			if (!f)
+			if (find_value(controller, path, prop, buf, prefix, missing_path) < 0)
 				return -1;
 
-			ret = fscanf(f, "%1024s", buf);
-			fclose(f);
-
-			if (ret <= 0) {
-				continue;
-			}
-
-			pr_info("copying %s for %s/%s\n", buf, controller, prop);
-
-			snprintf(fpath, PATH_MAX, "%s/%s/%s", controller, path, prop);
-			f = fopenat(cg, fpath, "w");
-			if (!f)
+			if (copy_recursive(prefix, missing_path, prop, buf) < 0) {
+				pr_err("copying prop %s failed\n", prop);
 				return -1;
-
-			fprintf(f, "%s", buf);
-			fclose(f);
+			}
 		}
 	}