How to expand the `bss`section and fill it zeros when 'objcopy'ing?

[PRC]

I want to create a raw binary file from a ELF image. I invoke the command:

mips-elf-objcopy -O binary -j ".text" -j ".data" -j ".bss" my.exe my.bin

But the size of the output `my.bin` is not the sum of sizes of ".text", ".data" and ".bss".
It is the sum of ".text" and ".data", excluding ".bss". 

How can I make objcopy expand the ".bss" section and fill it zeros?