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Dealing with readonly filesystem on camera

mount -bind approach

In this example we override etc directory using ramdisk and mount -bind. Make sure that you have enough memory. In case of shortage RAM you can use NFS share for copying files.

$ mount | grep "type ramfs"
/dev/mem on /var type ramfs (rw,relatime)
/dev/mem2 on /utils type ramfs (rw,relatime)
$ cp -arv /etc/ /var
$ mount -o bind /var/etc /etc
$ passwd
Changing password for root
New password:
Retype password:
Password for root changed by root
$ cp /etc/passwd /etc/xmtelnetdpw

Now you can login via telnet using nonstandard root password.

OverlayFS

Make sure that you're using kernel 3.18 version or upper

# insmod overlay.ko
# mkdir -p /tmp/upper /tmp/work
# mount -t overlay none \
    -o lowerdir=/mnt/custom,upperdir=/tmp/upper,workdir=/tmp/work /mnt/custom
# touch /mnt/custom/test

Overlay rootfs on boot

Good example showed common case of setting up both main and additional overlay system with bootparams like this:

mem=42M console=ttyAMA0,115200 root=/dev/mtdblock2 rootfstype=cramfs mtdparts=hi_sfc:256K(boot),1472K(uImage),5760K(rootfs),512K(overlay) overlay=/dev/mtdblock3 overlayfstype=jffs2 init=/init