initrd insmod: error inserting '/lib/foo.ko' : -1 operation not permitted

Hi, I am trying to add one driver to my kernel. I have the kernel source and use make all build all the modules and bzImage. I already have initrd.img in my machine. I extract it to a tmp folder. Then I copy the driver module into the lib folder. I edited the init file to insert the module when machine boot. After that, I recreated the initrd.img by cpio and gzip. When machine boot, I got "insmod: error inserting '/lib/foo.ko' : -1 operation not permitted" error. Did I missed any step? What caused the error? Thanks.