Basic tips

Finding ELF format

readelf -h hotplug 

└─$ xxd -c 1 -l 6 hotplug
00000000: 7f  .
00000001: 45  E
00000002: 4c  L
00000003: 46  F
00000004: 01  .
00000005: 01  .

If the last line (the sixed byte) is 01, according to ELF format, 01 is little endian and 02 is big endian.

Create file image with rootFS from scratch

dd if=/dev/zero of=myimage.img bs=1M count=500
mkdir -p MountPoint
mkfs.ext2 myimage.img
mount -t ext2 -o loop myimage.img MountPoint

cp rootfs.tar MountPoint
cd MountPoint
tar -xvf rootfs.tar
df -h

umount MountPoint
dd if=myimage.img bs=1k | gzip -v9 > rootfs.gz

DD

dd bs=1 skip=THEOFFSET count=THELENGTH if=INPUTFILENAME of=OUTPUTFILENAME
file output # to check ;)

To cut the "Linux kernel version 2.6.36" we need to do:

Get the skip offset: 3163712
Get the count size: 3226456 - 3163712 = 62.744

dd if=1C bs=1 skip=3163712 count=62744 of=kernel_out
file kernel_out
kernel_out: DIY-Thermocam raw data (Lepton 3.x), scale 7417-5248, spot sensor temperature 0.000000,

cat kernel_out | strings | less

cpio unpacking

cpio -idm — no-absolute-filenames < cpio

Reversing FirmwareMedium

OverlayFS

sudo docker run --privileged=true --name=firmware -p 2221:22 -p 8888:80 -p 4443:443 -p 2223:23 -it 8d319a850335

cd /tmp
mkdir lower upper workdir overlay

mount -t overlay -o lowerdir=/tmp/lower,upperdir=/tmp/upper,workdir=/tmp/workdir none /tmp/overlay

e.g.:
mount -t overlay -o lowerdir=/tmp/firmwarefs,upperdir=/,workdir=/tmp/workdir none /tmp/overlay
chroot /tmp/overlay /bin/sh

OverlayFSProgramster's Blog

sasquatch

Since file doesn't recognize it, the vendor probably used a custom SquashFS magic signature. I expect that unsquashfs is also giving you an error about not being able to find a valid superblock.

Give sasquatch a try; it's a modified version of unsquashfs that attempts to support such vendor hacks.

GitHub - devttys0/sasquatchGitHub

GDB server

target:> gdbserver localhost:2000 /bin/httpd
or
target:> ./gdbserver-7.12-mips-be-stripped localhost:5555 /bin/httpd

host  :> gdb /bin/httpd
(gdb) target remote 127.0.0.1:2000
continue
stepi
next
disas "function"

GitHub - hugsy/gdb-static: Public repository of statically compiled GDB and GDBServerGitHub

Trace apps with qemu-mipsel-static

sudo apt-get install qemu qemu-user qemu-user-static
sudo apt-get install gdb-multiarch
sudo apt-get install 'binfmt*'

$ sudo apt-get install libc6-mipsel-cross      # For MIPS-EL
$ sudo apt-get install libc6-armhf-armel-cross # For ARM

$ sudo apt-get install gcc-4.4-mipsel-linux-gnu # For MIPS-EL on Ubuntu 14.04
$ sudo apt-get install gcc-mipsel-linux-gnu     # For MIPS-EL on Ubuntu 16.04
$ sudo apt-get install gcc-arm-linux-gnueabihf  # For ARM

$ sudo mkdir /etc/qemu-binfmt
$ sudo ln -s /usr/mipsel-linux-gnu /etc/qemu-binfmt/mipsel # MIPSEL
$ sudo ln -s /usr/arm-linux-gnueabihf /etc/qemu-binfmt/arm # ARM

---execution---------
qemu-mipsel -strace ./myelf
or
qemu-mipsel-static -strace ./myelf


------GDB--------
$ qemu-mipsel -g 12345 ./a.out &

or

./qemu-mipsel-static -g 12345 /bin/httpd

$ gdb-multiarch ./a.out
(gdb) set arch mips
The target architecture is assumed to be mips
(gdb) set endian little
The target is assumed to be little endian
(gdb) target remote localhost:12345
Remote debugging using localhost:12345

(gdb) info functions

Non-debugging symbols:
0x00407360  _init
0x004073f0  _ftext
0x004075a0  CheckWanType
0x004078ac  check_dhcpc
0x00407d80  CheckPPPOE
0x00407ed0  random_xid
0x004082bc  send_discover
0x00408754  udhcp_checksum
0x00408884  get_raw_packet
0x00408c60  parsePacket
0x004096c8  printErr
0x004097f8  computeTCPChecksum
0x0040a100  sendPADT
0x0040a874  pktLogErrs
0x0040bca0  discovery
0x0040c020  openInterface
0x0040c3e4  sendPacket
0x0040c498  receivePacket

(...)