Android KGDB Patch for Nexus 5X

To be able to use brakpoints you also need to set CONFIG_FORCE_PAGES=Y (which isn't done by the patch).

Kernel patch to get KGDB working on the Nexus 5X.

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

• git clone https://android.googlesource.com/kernel/msm
• git checkout android-msm-bullhead-3.10-nougat (should be commit b75eb01ab048d2b3b42c18585c5bbd7219c0536a which was the commit this patch was created on)
• Apply this patch.
• export ARCH=arm64 && export CROSS_COMPILE=<path_to_ndk_r17c>/toolchains/aarch64-linux-android-4.9/prebuilt/linux-x86_64/bin/aarch64-linux-android-
• make bullhead_kgdb_defconfig && make
• 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'
• Flash boot.img to the device.
• Open a shell (adb shell), su to root, then type: echo -n g > /proc/sysrq-trigger to make the kernel break
• On your host machine fire up GDB (you'll need a working version of GDB cross-compiled for ARM, ndk_r10e has a prebuilt binary included): <path_to_ndk_r10e>/toolchains/aarch64-linux-android-4.9/prebuilt/linux-x86_64/bin/aarch64-linux-android-gdb <path_to_kernel_repo>/vmlinux
• Inside GDB connect to device: (gdb) target remote /dev/ttyUSB0
• Note that sometimes the serial console needs to be open to be able to connect and sometimes it needs to be closed. Could not narrow it down why this happens, just try both.

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

More verbose explanation of the patch:

Add poll commands to Nexus 5X serial driver to make KGDB work.

Also fix compiling issue when deactivating CONFIG_MSM_WATCHDOG_V2. Which is necessary because the watchdog would trigger a reboot while KGDB breaks execution.

To compile with KGDB use bullhead_kgdb_defconfig or enable CONFIG_KGDB and CONFIG_KGDB_SERIAL_CONSOLE manually while deactivating CONFIG_MSM_WATCHDOG_V2.

You can also enable CONFIG_KGDB_KDB to debug directly from the serial console. If it is enable you can still also connect using an external GDB but you need to close the serial console first in this case. For some odd reason sometime connecting with an external debugger only works when the serial console is open sometimes only when it's closed and sometimes it doesn't matter. I thought it was kdb causing this but it seems that I was wrong.