Intro

Things I often forget, what do I need to make Kernel changes effective. This is just a small summary of steps for myself, not a full guide.


The stuff

Modifications where done on the android kernel (5.4.61-android11), the downloading and building and running is straight forward. Why I did this is to help me out with some analysis I am doing on apps, and compiling the kernel and then run the emulator on an ARM based mac makes things just easier.

So I added a syscall, which gives me more control, I know there are kernel modules and the kernel has stuff like BPF and tracing but still. I just like to do my own stuff. So adding a syscall is easy:

Files to modify:

  • arch/arm/tools/syscall.tbl
  • (arch/x86/entry/syscalls/syscall_32.tbl) if you want x86 emulator
  • kernel/sys.c

If you wanna have your new code in a extra directory, which makes sense than you need a few more changes. (New directory is called Extension):

  • Kconfig (root dir)
  • Makefile (root dir)

New Files:

  • Extension/ext.c
  • include/Extension/ext.h
  • Extension/Kconfig
  • Extension/Makefile

The changes after all are pretty minor, here is the full git diff (shortened):

--- a/Kconfig
+++ b/Kconfig

 source "Documentation/Kconfig"
+
+source "Extension/Kconfig"

--- a/Makefile
+++ b/Makefile
 
-core-y         += kernel/ certs/ mm/ fs/ ipc/ security/ crypto/ block/
+core-y         += kernel/ certs/ mm/ fs/ ipc/ security/ crypto/ block/ Extension/

--- a/arch/x86/entry/syscalls/syscall_32.tbl
+++ b/arch/x86/entry/syscalls/syscall_32.tbl

 435    i386    clone3                  sys_clone3                      __ia32_sys_clone3
+436    i386    csys                    sys_csys                        __ia32_sys_csys

--- a/kernel/sys.c
+++ b/kernel/sys.c

+#include <Extension/code.h>
 
@@ -944,6 +946,19 @@ SYSCALL_DEFINE0(getegid)
 
+SYSCALL_DEFINE1(csys, char*, msg)
+{
+   // syscall implementation goes here
+   return 0;
+}
+

>> cat Extension/Kconfig
config EXTENSION
  bool "My super duper feature"
  default y
	help
    Extension for everything

>> cat Extension/Makefile
obj-y	+= code.o

And thats it, the code inside code.c and code.h is omitted its just some functions which can be used inside the syscall and some globals to store data between calls and so on.

Note that adding it to the core-y line is needed otherwise it will not be linked to the core. Also the y’s defines core, an m for example would stand for module.

Emulator

For running it in the emulator I use (but thats just what I use):

./emulator -verbose -avd <avd_device> -kernel /pathTo/bzImage -show-kernel
  -no-sim -wipe-data -no-audio -cores 2 -ranchu -no-boot-anim
  -port 4321 -no-window | tee emu_out

Dont forget to wipe data before running a new kernel otherwise it will reuse the snapshot.