Is there a good way to extract the same information that uname does from a compressed kernel image? I want this to be able to check the dog tags of kernel sitting in dormant mtd's on an Embedded Linux system and compare it to the currently running kernel.
For Linux image compressed with gzip, use this:
dd if=arch/arm/boot/zImage bs=1 skip=$(LC_ALL=C grep -a -b -o $'\x1f\x8b\x08\x00\x00\x00\x00\x00' arch/arm/boot/zImage | head -n 1 | cut -d ':' -f 1) | zcat | grep -a 'Linux version'
For Linux image compressed with xz, use this:
dd if=arch/arm/boot/zImage bs=1 skip=$(LC_ALL=C grep -a -b -o $'\xFD\x37\x7A\x58\x5A\x00' arch/arm/boot/zImage | head -n 1 | cut -d ':' -f 1) | xzcat | grep -a 'Linux version'
Because the image file contains data after the end of the compressed stream, you'll get an error you can ignore.
The string constant appears to be part of the frozen userspace visible kernel API:
http://git.kernel.org/?p=linux/kernel/git/torvalds/linux.git;a=blob;f=init/version.c;hb=HEAD#l40