kgdb-android

Kernel patches to get KGDB working on the Nexus 6 and Nexus 5x.

For background, please see associated blog post at http://www.contextis.com/resources/blog/kgdb-android-debugging-kernel-boss

1. Root your Nexus

2. Clone the MSM kernel from AOSP. Checkout to version that you need.
   Hint: stock kernels have a commit hash in their version string, like "3.10.73-gdc40906c97ae". Whole string after 'g' is a commit hash. So, you can checkout it with easy:

   git checkout dc40906c97ae
   

3. Go to root of your kernel source (kernel/msm/) and apply the patches:

       patch -bp 1 < "kgdboc.c-patch"
       patch -bp 1 < "msm_serial_hs_lite.c-patch"
   

4. Configure the kernel to enable kernel-debug feature.
   At this step you need to edit '...config' files. There are 2 ways to do.
   a) build the kernel one time, and change kbuild-generated '.config' file at the root of kernel source. Build kernel again, to reuse this configuration.
   b) change config file, that correspond to your device. For example "kernel/msm/arch/armv8/bullhead_defconfig", then just build kernel. How to select device

   • Lookup and remove (or set to ="n"):

     CONFIG_MSM_WATCHDOG_V2
     CONFIG_STRICT_MEMORY_RWX
     CONFIG_MSM_WATCHDOG_CTX_PRINT
     

   • Lookup and set to ="y" (or add if not exist):

     CONFIG_CONSOLE_POLL
     CONFIG_KGDB_SERIAL_CONSOLE
     CONFIG_KGDB
     

5. Deprecated compiler.
   You may fail with a freshly installed package of the old compiler, for example GCC4.9, which was going to your kernel. Check toolchain gcc content. If there is python wrapper, turn off deprecation-info printing, kbuild does not understand it's output.

6. Create your boot image, passing console arguments e.g. to update a stock image I used:
   abootimg -u boot.img -k zImage-dtb -c 'cmdline=console=ttyHSL0,115200,n8 kgdboc=ttyHSL0,115200 kgdbretry=4'

7. Boot your phone into the bootloader (adb reboot bootloader) and on your host run:

   fastboot oem config console enable

8. Reboot into bootloader again

9. Plug in your debug cable (see blog)

10. Boot your image e.g. fastboot boot boot.img

11. Open a shell (adb shell), su to root, then type:

    echo -n g > /proc/sysrq-trigger
    

12. Hit enter

13. On your host machine fire up GDB (you'll need a working version of GDB cross-compiled for ARM):

    arm-eabi-gdb ./vmlinux
    (gdb) set remoteflow off
    (gdb) set remotebaud 115200
    (gdb) target remote /dev/ttyUSB0

You should hit the KGDB breakpoint and be able to continue, examine memory, etc.