[lxc-devel] [PATCH 2/2] Update absolute paths for overlay and aufs mounts

[Christian Brauner]

When using overlay and aufs mounts with lxc.mount.entry users have to specify
absolute paths for upperdir and workdir which will then get created
automatically by mount_entry_create_overlay_dirs() and
mount_entry_create_aufs_dirs() in conf.c. When we clone a container with
overlay or aufs lxc.mount.entry entries we need to update these absolute paths.
In order to do this we add the function update_union_mount_entry_paths() in
lxccontainer.c. The function updates the mounts in two locations:

        1) lxc_conf->mount_list

and

        2) lxc_conf->unexpanded_config (by calling clone_update_unexp_ovl_dir())

If we were to only update 2) we would end up with wrong upperdir and workdir
mounts as the absolute paths would still point to the container that serves as
the base for the clone. If we were to only update 1) we would end up with wrong
upperdir and workdir lxc.mount.entry entries in the clone's config as the
absolute paths in upperdir and workdir would still point to the container that
serves as the base for the clone. Updating both will get the job done.

NOTE: This function does not sanitize paths apart from removing trailing
slashes. (So when a user specifies //home//someone/// it will be cleaned to
//home//someone. This is the minimal path cleansing which is also done by
lxc_container_new().) But the mount_entry_create_overlay_dirs() and
mount_entry_create_aufs_dirs() functions both try to be extremely strict about
when to create upperdirs and workdirs. They will only accept sanitized paths,
i.e. they require /home/someone. I think this is a (safety) virtue and we
should consider sanitizing paths in general. In short:
update_union_mount_entry_paths() does update all absolute paths to the new
container but mount_entry_create_overlay_dirs() and
mount_entry_create_aufs_dirs() will still refuse to create upperdir and workdir
when the updated path is unclean. This happens easily when e.g. a user calls
lxc-clone -o OLD -n NEW -P //home//chb///.

Signed-off-by: Christian Brauner <christianvanbrauner at gmail.com>
---
 src/lxc/lxccontainer.c | 97 ++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 97 insertions(+)

diff --git a/src/lxc/lxccontainer.c b/src/lxc/lxccontainer.c
index 42e23e7..8edf6a2 100644
--- a/src/lxc/lxccontainer.c
+++ b/src/lxc/lxccontainer.c
@@ -2894,6 +2894,99 @@ static int create_file_dirname(char *path, struct lxc_conf *conf)
 	return ret;
 }
 
+/* When we clone a container with overlay or aufs lxc.mount.entry entries we
+*  need to update these absolute paths. In order to do this we add the function
+*  update_union_mount_entry_paths() in lxccontainer.c. The function operates on
+*  c->lxc_conf->unexpanded_config instead of the intuitively plausible
+*  c->lxc_conf->mount_list because the latter also contains mounts from other
+*  files as well as generic mounts. */
+static int update_union_mount_entry_paths(struct lxc_conf *lxc_conf,
+					  const char *lxc_path,
+					  const char *lxc_name,
+					  const char *newpath,
+					  const char *newname)
+{
+	char new_upper[MAXPATHLEN];
+	char new_work[MAXPATHLEN];
+	char old_upper[MAXPATHLEN];
+	char old_work[MAXPATHLEN];
+	char *cleanpath = NULL;
+	int i;
+	int fret = -1;
+	int ret = 0;
+	struct lxc_list *iterator;
+	const char *ovl_dirs[] = {"br", "upperdir", "workdir"};
+
+	cleanpath = strdup(newpath);
+	if (!cleanpath)
+		goto err;
+
+	remove_trailing_slashes(cleanpath);
+
+	for (i = 0; i < sizeof(ovl_dirs) / sizeof(ovl_dirs[0]); i++) {
+		if (!clone_update_unexp_ovl_dir(lxc_conf, lxc_path, newpath,
+						lxc_name, newname, ovl_dirs[i]))
+			goto err;
+	}
+
+	ret = snprintf(old_work, MAXPATHLEN, "workdir=%s/%s", lxc_path, lxc_name);
+	if (ret < 0 || ret >= MAXPATHLEN)
+		goto err;
+
+	ret = snprintf(new_work, MAXPATHLEN, "workdir=%s/%s", cleanpath, newname);
+	if (ret < 0 || ret >= MAXPATHLEN)
+		goto err;
+
+	lxc_list_for_each(iterator, &lxc_conf->mount_list) {
+		char *mnt_entry = NULL;
+		char *new_mnt_entry = NULL;
+		char *tmp = NULL;
+		char *tmp_mnt_entry = NULL;
+		mnt_entry = iterator->elem;
+
+		if (strstr(mnt_entry, "overlay"))
+			tmp = "upperdir";
+		else if (strstr(mnt_entry, "aufs"))
+			tmp = "br";
+
+		if (!tmp)
+			continue;
+
+			ret = snprintf(old_upper, MAXPATHLEN, "%s=%s/%s", tmp, lxc_path, lxc_name);
+			if (ret < 0 || ret >= MAXPATHLEN)
+				goto err;
+
+			ret = snprintf(new_upper, MAXPATHLEN, "%s=%s/%s", tmp, cleanpath, newname);
+			if (ret < 0 || ret >= MAXPATHLEN)
+				goto err;
+
+			if (strstr(mnt_entry, old_upper)) {
+				tmp_mnt_entry = lxc_string_replace(old_upper, new_upper, mnt_entry);
+			}
+
+			if (strstr(mnt_entry, old_work)) {
+				if (tmp_mnt_entry)
+					new_mnt_entry = lxc_string_replace(old_work, new_work, tmp_mnt_entry);
+			}
+
+			if (new_mnt_entry) {
+				free(iterator->elem);
+				iterator->elem = strdup(new_mnt_entry);
+			} else if (tmp_mnt_entry) {
+				free(iterator->elem);
+				iterator->elem = strdup(tmp_mnt_entry);
+			}
+
+			free(new_mnt_entry);
+			free(tmp_mnt_entry);
+	}
+
+	fret = 0;
+err:
+	free(cleanpath);
+	return fret;
+}
+
 static struct lxc_container *do_lxcapi_clone(struct lxc_container *c, const char *newname,
 		const char *lxcpath, int flags,
 		const char *bdevtype, const char *bdevdata, uint64_t newsize,
@@ -3009,6 +3102,10 @@ static struct lxc_container *do_lxcapi_clone(struct lxc_container *c, const char
 		}
 	}
 
+	// update absolute paths for union mount directories
+	if (update_union_mount_entry_paths(c2->lxc_conf, c->config_path, c->name, lxcpath, newname) < 0)
+		goto out;
+
 	// We've now successfully created c2's storage, so clear it out if we
 	// fail after this
 	storage_copied = 1;
-- 
2.6.2