Hardware breakpoint in GDB +QEMU missing start_kernel

Question

I am trying to debug a kernel running on QEMU with GDB.

The kernel has been compiled with these options:

CONFIG_DEBUG_INFO=y
CONFIG_GDB_SCRIPTS=y

I launch the kernel in qemu with the following command:

qemu-system-x86_64 -s -S -kernel arch/x86_64/boot/bzImage

In a separate terminal, I launch GDB from the same path and issue these commands in sequence:

 gdb ./vmlinux
(gdb) target remote localhost:1234
(gdb) hbreak start_kernel
(gdb) c

I did not provide a rootfs, as I am not interested in a full working system as of now, just the kernel. I also tried combinations of hbreak/break.

The kernel just boots and reaches a kernel panic as rootfs cannot be found... expected. I want it to stop at start_kernel and then step through the code.

observation: if I set an immediate breakpoint, it works and stops, but not on start_kernel / startup_64 / main

Is it possible that qemu is not calling all these functions, or is it being masked in some way?

Kernel: 4.13.4 
GDB: GNU gdb (Ubuntu 7.7.1-0ubuntu5~14.04.3) 7.7.1
GCC: gcc (Ubuntu 4.8.4-2ubuntu1~14.04.3) 4.8.4    
system: ubuntu 14.04 LTS

NOTE: This exact same procedure worked with kernel 3.2.93, but does not work with 4.13.4, so I guess some more configurations are needed. I could not find resources online which enabled this debug procedure for kernel 4.0 and up, so as of now I am continuing with 3.2, any and all inputs on this are welcome.

Answer 1

I ran into the same problem and found the solution from the linux kernel newbies mailing list.

You should disable KASLR in your kernel command line with nokaslr option, or disable kernel option "Randomize the kernel memory sections" inside "Processor type and features" when you build your kernel image.