can't execute any executable in shell except busybox commands..

[Chan Kim]

Hello, all

I built linux 5.4.0 and made initramfs using busybox. I added the initramfs
inside the linux kernel.
The init file in the intramfs executes shell. After kernel boots on a arm64
virtual machine, I'm in the busybox shell.
Now after I get file from outside world(using tftp), I cannot run the
executable except the commands provided by the busybox.
I guess there should be some people doing this all the time in this email
list.
How can I execute an executable file inside busybox? Of course the
executable was compiled using proper cross-compiler. 
(it's arm64 virtual machine and I can run the executable on ubuntu-20.04 on
the virtual machine. 
The difference is that now I'm trying to run it on vanilla 5.4.0 on the
virtual machine)
Thank you and any comment/advice will be welcomed.

Chan Kim