mount: handle tracefs more gracefully

See the comment for details, but basically tracefs is automounted by the
kernel, so we can just mount debugfs with MS_REC and get the right result.

v2: rebase on criu-dev
v3: don't use a new fstype->flags, just always set MS_REC in debugfs'
    ->parse
Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>

criu/mount.c

@@ -1678,6 +1678,22 @@ out:
 	return ret;
 }
 
+static int debugfs_parse(struct mount_info *pm)
+{
+	/* tracefs is automounted underneath debugfs sometimes, and the
+	 * kernel's overmounting protection prevents us from mounting debugfs
+	 * first without tracefs, so let's always mount debugfs MS_REC.
+	 */
+	pm->flags |= MS_REC;
+
+	return 0;
+}
+
+static int tracefs_parse(struct mount_info *pm)
+{
+	return 1;
+}
+
 static int cgroup_parse(struct mount_info *pm)
 {
 	if (!(root_ns_mask & CLONE_NEWCGROUP))
@@ -1779,6 +1795,11 @@ static struct fstype fstypes[] = {
 	}, {
 		.name = "debugfs",
 		.code = FSTYPE__DEBUGFS,
+		.parse = debugfs_parse,
+	}, {
+		.name = "tracefs",
+		.code = FSTYPE__TRACEFS,
+		.parse = tracefs_parse,
 	}, {
 		.name = "cgroup",
 		.code = FSTYPE__CGROUP,

criu/proc_parse.c

@@ -1387,13 +1387,22 @@ struct mount_info *parse_mountinfo(pid_t pid, struct ns_id *nsid, bool for_dump)
 
 		if (new->fstype->parse) {
 			ret = new->fstype->parse(new);
-			if (ret) {
+			if (ret < 0) {
 				pr_err("Failed to parse FS specific data on %s\n",
 						new->mountpoint);
 				mnt_entry_free(new);
 				new = NULL;
 				goto end;
 			}
+
+			if (ret > 0) {
+				pr_info("\tskipping fs mounted at %s\n", new->mountpoint + 1);
+				mnt_entry_free(new);
+				new = NULL;
+				ret = 0;
+				goto end;
+
+			}
 		}
 end:
 		if (fsname)

images/mnt.proto

@@ -19,6 +19,8 @@ enum fstype {
 	FUSE			= 15;
 	AUTO			= 16;
 	OVERLAYFS		= 17;
+	// AUTOFS		= 18; RESERVED in criu-dev branch
+	TRACEFS			= 19;
 };
 
 message mnt_entry {