cg: Make lacking properties during dump non-fatal

Because different kernel versions have different cgroup properties, criu
shouldn't crash just because the properties statically listed aren't exact.
Instead, during dump, ignore properties the kernel doesn't have and continue.

Change-Id: I5a8b93d6a8a3a9664914f10cf8e2110340dd8b31
Signed-off-by: Garrison Bellack <gbellack@google.com>
Acked-by: Tycho Andersen <tycho.andersen@canonical.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

cgroup.c

@@ -298,34 +298,29 @@ static inline char *strip(char *str)
  * Currently this function only supports properties that have 1 value, under 100
  * chars
  */
-static int read_cgroup_prop(struct cgroup_prop *property, const char *fpath)
+static int read_cgroup_prop(struct cgroup_prop *property, const char *fullpath)
 {
-	char pbuf[PATH_MAX], buf[100];
+	char buf[100];
 	FILE *f;
 
-	if (snprintf(pbuf, PATH_MAX, "%s/%s", fpath, property->name) >= PATH_MAX) {
-		pr_err("snprintf output was truncated");
-		return -1;
-	}
-
-	f = fopen(pbuf, "r");
+	f = fopen(fullpath, "r");
 	if (!f) {
-		pr_err("Failed opening %s\n", pbuf);
 		property->value = NULL;
+		pr_perror("Failed opening %s\n", fullpath);
 		return -1;
 	}
 
 	memset(buf, 0, sizeof(buf));
 	if (fread(buf, sizeof(buf), 1, f) != 1) {
 		if (!feof(f)) {
-			pr_err("Failed scanning %s\n", pbuf);
+			pr_err("Failed scanning %s\n", fullpath);
 			fclose(f);
 			return -1;
 		}
 	}
 
 	if (fclose(f) != 0) {
-		pr_err("Failed closing %s\n", pbuf);
+		pr_err("Failed closing %s\n", fullpath);
 		return -1;
 	}
 
@@ -395,6 +390,7 @@ static int add_cgroup_properties(const char *fpath, struct cgroup_dir *ncd,
 				 struct cg_controller *controller)
 {
 	int i, j;
+	char buf[PATH_MAX];
 	struct cgroup_prop *prop;
 
 	for (i = 0; i < controller->n_controllers; ++i) {
@@ -402,16 +398,28 @@ static int add_cgroup_properties(const char *fpath, struct cgroup_dir *ncd,
 		const char **prop_arr = get_known_properties(controller->controllers[i]);
 
 		for (j = 0; prop_arr != NULL && prop_arr[j] != NULL; ++j) {
+			if (snprintf(buf, PATH_MAX, "%s/%s", fpath, prop_arr[j]) >= PATH_MAX) {
+				pr_err("snprintf output was truncated");
+				return -1;
+			}
+
+			if (access(buf, F_OK) < 0 && errno == ENOENT) {
+				pr_info("Couldn't open %s. This cgroup property may not exist on this kernel\n", buf);
+				continue;
+			}
+
 			prop = create_cgroup_prop(prop_arr[j]);
 			if (!prop) {
 				free_all_cgroup_props(ncd);
 				return -1;
 			}
-			if (read_cgroup_prop(prop, fpath) < 0) {
+
+			if (read_cgroup_prop(prop, buf) < 0) {
 				free_cgroup_prop(prop);
 				free_all_cgroup_props(ncd);
 				return -1;
 			}
+
 			pr_info("Dumping value %s from %s/%s\n", prop->value, fpath, prop->name);
 			list_add_tail(&prop->list, &ncd->properties);
 			ncd->n_properties++;