Check kernel compression algorithm

Most likely your zImage was compressed using LZMA compressor. You can check it in next files:

• in .config file (if you built kernel by yourself)
• in /boot/config-`uname -r` file (if you are using your distribution)
• in /proc/config.gz file (if CONFIG_IKCONFIG_PROC is enabled)

Look for CONFIG_KERNEL_* param:

$ cat .config | grep '^CONFIG_KERNEL_[^_]\+='

If you have CONFIG_KERNEL_LZMA=y set, it means LZMA compressor is used.

Unpack zImage

LZMA format has 5d 00 00 header signature. So one can find position of compressed Image file in zImage file this way:

$ grep -P -a -b -m 1 --only-matching '\x5D\x00\x00' zImage | cut -f 1 -d :

To extract compressed Image:

$ pos=$(grep -P -a -b -m 1 --only-matching '\x5D\x00\x00' zImage | cut -f 1 -d :)$ dd if=arch/arm/boot/zImage of=piggy.lzma bs=1 skip=$pos

Now make sure that piggy.lzma is actually LZMA archive:

$ file piggy.lzma 

piggy.lzma: LZMA compressed data, streamed

Decompress piggy.lzma:

$ unlzma -c piggy.lzma > Image

Find the Linux version

Now that you have unpacked Image, you can find the Linux version using strings tool:

$ strings Image | grep 'Linux version'

which should give you something like this:

Linux version 4.4.11-188843-g94c4bf5-dirty (joe@joe-laptop) (gcc version 4.8 (GCC) ) #1 SMP PREEMPT Thu May 26 20:55:27 EEST 2016