[lxc-devel] [PATCH] rootfs pinning: make file hidden, don't delete it, encode pid

[Christian Seiler]

This makes the following changes to the rootfs pinning mechanism:

 - Contrary to a recent change, the file will not be deleted
   immediately after creation. This should make NFS happy.

 - To reduce nuissance for administrators, make the file hidden by
   default.

 - Delete the file at container shutdown. (i.e. clean up after oneself)

 - Because the file is now deleted, and a rootfs could be shared
   between containers, the filename now encodes the pid of the
   lxc-start process, so that the rw-hold on the filesystem will only
   be removed once the last container exits.

Signed-off-by: Christian Seiler <christian at iwakd.de>
---
 src/lxc/conf.c  |   24 +++++++++++++++---------
 src/lxc/conf.h  |    2 +-
 src/lxc/start.c |   10 +++++++++-
 src/lxc/start.h |    1 +
 4 files changed, 26 insertions(+), 11 deletions(-)

diff --git a/src/lxc/conf.c b/src/lxc/conf.c
index ecbcf41..4cd9462 100644
--- a/src/lxc/conf.c
+++ b/src/lxc/conf.c
@@ -695,15 +695,18 @@ static int mount_rootfs_block(const char *rootfs, const char *target)
 
 /*
  * pin_rootfs
- * if rootfs is a directory, then open ${rootfs}/lxc.hold for writing for
- * the duration of the container run, to prevent the container from marking
- * the underlying fs readonly on shutdown. unlink the file immediately so
- * no name pollution is happens
+ * if rootfs is a directory, then open ${rootfs}/.lxc-hold-$pid for writing
+ * for the duration of the container run, to prevent the container from
+ * marking the underlying fs readonly on shutdown. $pid is the pid of
+ * lxc-start. At shutdown the hold file will be removed again. The pid is
+ * encoded in the filename, so that if multiple containers use the same
+ * root filesystem, a hold will be kept open until the last container is
+ * stopped.
  * return -1 on error.
  * return -2 if nothing needed to be pinned.
  * return an open fd (>=0) if we pinned it.
  */
-int pin_rootfs(const char *rootfs)
+int pin_rootfs(const char *rootfs, char **filename_ptr)
 {
 	char absrootfs[MAXPATHLEN];
 	char absrootfspin[MAXPATHLEN];
@@ -725,16 +728,19 @@ int pin_rootfs(const char *rootfs)
 	if (!S_ISDIR(s.st_mode))
 		return -2;
 
-	ret = snprintf(absrootfspin, MAXPATHLEN, "%s/lxc.hold", absrootfs);
+	ret = snprintf(absrootfspin, MAXPATHLEN, "%s/.lxc-hold-%lu", absrootfs, (unsigned long)getpid());
 	if (ret >= MAXPATHLEN)
 		return -1;
 
+	if (filename_ptr) {
+		*filename_ptr = strdup(absrootfspin);
+		if (!*filename_ptr)
+			return -1;
+	}
+
 	process_lock();
 	fd = open(absrootfspin, O_CREAT | O_RDWR, S_IWUSR|S_IRUSR);
 	process_unlock();
-	if (fd < 0)
-		return fd;
-	(void)unlink(absrootfspin);
 	return fd;
 }
 
diff --git a/src/lxc/conf.h b/src/lxc/conf.h
index 84acce8..e3a962a 100644
--- a/src/lxc/conf.h
+++ b/src/lxc/conf.h
@@ -331,7 +331,7 @@ extern int detect_shared_rootfs(void);
 extern struct lxc_conf *lxc_conf_init(void);
 extern void lxc_conf_free(struct lxc_conf *conf);
 
-extern int pin_rootfs(const char *rootfs);
+extern int pin_rootfs(const char *rootfs, char **filename_ptr);
 
 extern int lxc_create_network(struct lxc_handler *handler);
 extern void lxc_delete_network(struct lxc_handler *handler);
diff --git a/src/lxc/start.c b/src/lxc/start.c
index 7538403..26c71c4 100644
--- a/src/lxc/start.c
+++ b/src/lxc/start.c
@@ -386,6 +386,14 @@ static void lxc_fini(const char *name, struct lxc_handler *handler)
 	if (run_lxc_hooks(name, "post-stop", handler->conf, handler->lxcpath, NULL))
 		ERROR("failed to run post-stop hooks for container '%s'.", name);
 
+	/* remove rootfs pin file after we shutdown this
+	 * container
+	 */
+	if (handler->pin_filename) {
+		unlink(handler->pin_filename);
+		free(handler->pin_filename);
+	}
+
 	/* reset mask set by setup_signal_fd */
 	if (sigprocmask(SIG_SETMASK, &handler->oldmask, NULL))
 		WARN("failed to restore sigprocmask");
@@ -708,7 +716,7 @@ int lxc_spawn(struct lxc_handler *handler)
 	 * marking it readonly.
 	 */
 
-	handler->pinfd = pin_rootfs(handler->conf->rootfs.path);
+	handler->pinfd = pin_rootfs(handler->conf->rootfs.path, &handler->pin_filename);
 	if (handler->pinfd == -1)
 		INFO("failed to pin the container's rootfs");
 
diff --git a/src/lxc/start.h b/src/lxc/start.h
index c35c5c4..345b4e0 100644
--- a/src/lxc/start.h
+++ b/src/lxc/start.h
@@ -50,6 +50,7 @@ struct lxc_handler {
 	struct lxc_operations *ops;
 	void *data;
 	int sv[2];
+	char *pin_filename;
 	int pinfd;
 	const char *lxcpath;
 	struct cgroup_process_info *cgroup;
-- 
1.7.10.4