Remounting as read-write a destination directory on device - linux

Question

How to remount as read-write a destination directory on device? (one folder) I need to replace file, but it's on "Read-only file system", not allow to change permissions. Path to folder: /etc/foo/bar. I need to remount /bar folder. Embedded Linux (busybox), Linux version 2.6.18_pro500
mount -o rw,remount [destination folder]
I tried following, with no success:
<root#elocal:/etc/foo/bar> ls -la
total 6
drwxr-xr-x 2 root 0 98 Jan 18 2011 .
drwxrwxr-x 7 root 0 105 Feb 10 2011 ..
-rw-r--r-- 1 root 0 1052 Jan 18 2011 file1
-rw-r--r-- 1 root 0 270 Jan 18 2011 file2
-rw-r--r-- 1 root 0 1088 Jan 18 2011 file3
-rw-r--r-- 1 root 0 270 Jan 18 2011 file4
mount -o rw,remount /etc/foo/bar
mount: can't find /etc/foo/bar in /proc/mounts
output mount command:
mount
rootfs on / type rootfs (rw)
/dev/root on / type squashfs (ro)
proc on /proc type proc (rw)
ramfs on /var type ramfs (rw)
sysfs on /sys type sysfs (rw)
tmpfs on /dev type tmpfs (rw)
devpts on /dev/pts type devpts (rw)
/dev/mtdblock4 on /nvram type jffs2 (rw)
output of cat /proc/mounts
cat /proc/mounts
rootfs / rootfs rw 0 0
/dev/root / squashfs ro 0 0
proc /proc proc rw 0 0
ramfs /var ramfs rw 0 0
sysfs /sys sysfs rw 0 0
tmpfs /dev tmpfs rw 0 0
devpts /dev/pts devpts rw 0 0
/dev/mtdblock4 /nvram jffs2 rw 0 0

Answer 1

Normally, you would use mount -oremount,rw / (/ is the mount point, not /etc/foo/bar).
However, this will not work in your case, per the df output,
rootfs / rootfs rw 0 0
/dev/root / squashfs ro 0 0
your rootfs is using squashfs, which is a read-only file system. See Wikipedia link. Basically, when the filesystem image is created on the build system, it is compressed. Once it is on the target system, it cannot be changed.
You will need to go back to the build system and change the contents and re-build the filesystem image.