Initramfs
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How to inject a separate initramfs file into a 2.6 vmlinux (as section .init.ramfs):
First turn the initramfs into an ELF, excluding anything but the file data:
objcopy -I binary -O elf32-i386 -B i386 --redefine-sym _binary_initramfs_start=.init.ramfs -N _binary_initramfs_end -N _binary_initramfs_size initramfs initramfs.elf
Note that the symbol names _binary_initramfs_{start,end,size} depend on the input filename. objcopy does some translation here, e.g. a period (.) in the filename will become an underscore (_) in the symbols.
Now remove any existing .init.ramfs section in vmlinux and add the section in initramfs.elf:
objcopy -I elf32-i386 -O elf32-i386 -R .init.ramfs --add-section .init.ramfs=initramfs.elf vmlinux vmlinux.withnewinitramfs
All done.