[PATCH] block: make extroot mount preparation more robust

Ken Miller ken at miller.ec
Mon Apr 8 19:03:26 EDT 2019

The extroot mount preparation code for r/w rootfs overlay discovery, and
determining the user-defined /etc/config/fstab location within, would only
discover overlays residing on JFFS2 or UBIFS MTD storage.

This led to attempts at loading the uci fstab configuration without the required
/tmp/overlay directory prefix on devices with a non-MTD r/w rootfs overlay, and
thus failure to find any custom fstab /overlay extroot entries on PREINIT.
(example: the default openwrt eMMC partition layout on the zyxel nbg6817)

Futher, with UBIFS_EXTROOT enabled (fstools package default), and no MTD rootfs
partitions present, check_extroot would not attempt rootfs discovery on block
devices, such as the ext4 mmcblk rootfs overlay on the nbg6817.

With this patch:
1) main_extroot now attempts to load uci fstab configuration from an already
   mounted overlay, before defaulting to the prefix-less uci config dir when
   no MTD rootfs partitions are detected.
2) check_extroot now also attempts to find rootfs partitions on block devices
   when no MTD rootfs partitions are detected.
 block.c | 17 +++++------------
 1 file changed, 5 insertions(+), 12 deletions(-)

diff --git a/block.c b/block.c
index 39212d2..3dfc4a5 100644
--- a/block.c
+++ b/block.c
@@ -1301,7 +1301,7 @@ static int find_block_ubi_RO(libubi_t libubi, char *name, char *part, int plen)
 	return err;
 static int find_root_dev(char *buf, int len)
@@ -1332,8 +1332,6 @@ static int find_root_dev(char *buf, int len)
 	return -1;
 static int test_fs_support(const char *name)
 	char line[128], *p;
@@ -1363,25 +1361,20 @@ static int check_extroot(char *path)
 	struct probe_info *pr = NULL;
 	char devpath[32];
 	if (find_block_mtd("\"rootfs\"", devpath, sizeof(devpath))) {
 		int err = -1;
 		libubi_t libubi;
 		libubi = libubi_open();
 		err = find_block_ubi_RO(libubi, "rootfs", devpath, sizeof(devpath));
-		if (err)
-			return -1;
-	}
-	if (find_block_mtd("\"rootfs\"", devpath, sizeof(devpath))) {
-		if (find_root_dev(devpath, sizeof(devpath))) {
+		if (err && find_root_dev(devpath, sizeof(devpath))) {
 			ULOG_ERR("extroot: unable to determine root device\n");
 			return -1;
 	list_for_each_entry(pr, &devices, list) {
 		if (!strcmp(pr->dev, devpath)) {
@@ -1585,7 +1578,7 @@ static int main_extroot(int argc, char **argv)
-	return mount_extroot(NULL);
+	return mount_extroot("/tmp/overlay");
 static int main_mount(int argc, char **argv)

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