[BusyBox] losetup+mount can only do ro mounts. util-linux ones work :(

[Denis Vlasenko]

I am trying to do rootfs-on-a-loopdevice trick.
With busybox, i can successfully do:

mount -t vfat /dev/somedev /new_root
modprobe loop
losetup /dev/loop/0 /new_root/diskimg
mount /dev/loop/0 /new_root2

This last mount gives me -EPERM.
I substituted util-linux's mount and it says
"/dev/loop/0 is ro, mounting readonly" - ??!
After that I also subst'ed lostup with util-linux
one - and it works rw now. Bug in busybox?
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vda