[Yocto] QEMU kernel debugging with GDB

This is a post about aarch64 kernel debugging using qemu and gdb in yocto environment

kernel configuration to debug kernel with gdb

To include debugging information in the vmlinux binary image,set CONFIG_DEBUG_INFO to y. Usually it is not set as default.

CONFIG_DEBUG_INFO=y

kernel compilation option in the yocto environment

In ~~~temp/log.do_compile file, there is a compilation log as below. This information is used when specifying the path to the source code in gdb.

-fdebug-prefix-map=/mnt/sdb1/src/git/yocto/poky/build/tmp/work-shared/qemuarm64/kernel-source=/usr/src/kernel 

Look at the description about the debug-prefix-map option here

• This option is added by the code below.
  In conf/bitbake.conf

    KERNEL_SRC_PATH = "/usr/src/kernel"
  

  In conf/bitbake.conf

    STAGING_KERNEL_DIR = "${TMPDIR}/work-shared/${MACHINE}/kernel-source"
  

  In classes/kernel-arch.bbclass

   KERNEL_CC = "${CCACHE}${HOST_PREFIX}gcc ${HOST_CC_KERNEL_ARCH} -fuse-ld=bfd ${DEBUG_PREFIX_MAP} -fdebug-prefix-map=${STAGING_KERNEL_DIR}=${KERNEL_SRC_PATH}"
  

Run qemu

After building, run the command below in the build directory.
-s means -gdb tcp::1234. -S means Do not start CPU at startup(you must type ‘c’ in the monitor)

runqemu qemuarm64 qemuparams="-s -S"

Run gdb for aarch64

1. Run gdb

     $ ./aarch64-poky-linux-gdb
     ...
     ...
     For help, type "help".
     Type "apropos word" to search for commands related to "word".
     (gdb) 
   

2. Load symbol from vmlinux

   (gdb) add-symbol-file ~/src//git/yocto/poky/build/tmp/work/qemuarm64-poky-linux/linux-yocto/5.15.14+gitAUTOINC+72e4eafb6b_f77b2ba7d5-r0/linux-qemuarm64-standard-build/vmlinux
   

3. Connect to a target machine

    (gdb) target remote:1234
   

4. Set source path. Specify the path according to the debug-prefix-map setting specified during compilation. Specify the path as an absolute path.

    (gdb) set substitute-path /usr/src/kernel /home/chuljeon39a/src/git/yocto/poky/build/tmp/work/qemuarm64-poky-linux/linux-yocto/5.15.14+gitAUTOINC+72e4eafb6b_f77b2ba7d5-r0/linux-qemuarm64-standard-build/source/
   

5. Run layout subcommands

    (gdb) layout asm
    (gdb) layout src
    (gdb) layout split
   

6. set breakpoint at start_kernel

    (gdb) b start_kernel
   

7. Continue program being debugged

    (gdb) c
   

   

Debugging using eclipse cdt

If you use eclipse cdt, you can debug more conveniently in the GUI environment.

1. Execute eclipse
2. In menu, Run -> Debug Configurations…
3. Choose GDB QEMU Debugging and click the right mouse button. Then choose New Configuration.
4. On Main tab, set Name and C/C++ Application: to the path of vmlinux.
5. On Debugger tab, set GDB Client Setup to the path of GDB and set Remote Target to localhost 1234.
6. On Startup tab, set Load Symbols and Executable to the path of vmlinux.
   Set Set breakpoint at: to start_kernel.
7. On Source tab, set Path Mapping to the paths that were set as debug-prefix-map during compilation.
8. Click Apply and Debug
9. Click resume(F8) button or press ‘c’ at Debugger Console window. Then it stops at start_kernel

* To debug repeatedly,

1. Run qemu
2. Relaunch Debug
   Do debugging
3. Terminate gdb and qemu
4. repeat 1 ~ 3