While RE an embedded device I ran into a problem - I need to patch a check in the kernel which has no sources available. Unpacking the kernel and binary-patching went smoothly. However I cannot get over repacking the vmlinux into zImage/bzImage.

I extracted the kernel using extract-vmlinux (modified to show offset here):

$ ../../extract-vmlinux zImage > vmlinux
Found valid magic - decompressed using unlzma (tail -c+15967 "zImage" | unlzma)
Header at: head -c15966
Compressed at: tail -c+15967

$ binwalk zImage

15966         0x3E5E          LZMA compressed data, properties: 0x5D, dictionary size: 67108864 bytes, uncompressed size: -1 bytes

Next, without any modifications, I wanted to recompress it back and keep the header, as to my knowledge it has the decompression code:

$ cat <(head -c15966 zImage) <(lzma -z < vmlinux) > zImage-repacked
$ ls -l
-rw-r--r-- 1 user user 3032080 Jun  5 22:32 zImage
-rw-r--r-- 1 user user 3024065 Jun  6 00:04 zImage-repacked

$ binwalk zImage-repacked

15966         0x3E5E          LZMA compressed data, properties: 0x5D, dictionary size: 8388608 bytes, uncompressed size: -1 bytes

The image came ~8k short, which didn't worry me (as my compression might have been more aggressive). However the image is now broken:

$ ../../extract-vmlinux zImage-repacked > /dev/null
extract-vmlinux: Cannot find vmlinux.

I also found that the extract-linux script pipes everything after the offset to unlzma but there's 9441 bytes of something which unlzma ignores. When removed I'm getting a pure archive which matches the size saved just after it:

$ tail -c+15967 zImage | head -c-9441 | unlzma > /dev/null ; echo $?
$ tail -c+15967 zImage | head -c-9441 | unlzma | wc --bytes
$ tail -c+15967 zImage | head -c-9437 | tail -c4 | od -i --endian=little
0000000    13871216

I know the classical wisdom is "you have to recompile" but we're on RE - this is not possible ;)
How can I approach re-packing of the modified vmlinux back to LZMA-zImage?

  • So to anyone visiting from Google - this is MUCH harder than just throwing some lzma & cat. This requires a pretty extensive process but, contrary to what many claim, it is possible. I will be releasing a tool for exactly that purpose. I will provide more detailed answer later.
    – kiler129
    Jun 9 at 2:50

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