Remote gdb debugging does not stop at breakpoints

Question

I'm having problems with debugging my modified Linux-3.11.0 kernel. For debugging the code, I use:

GDB 7.6
QEMU 1.6.5
Linux kernel 3.11.0

My system has Intel(R) Core(TM) i7-2760QM CPU @ 2.40GHz running Ubuntu 12.10

I compiled the modified kernel with a configuration file that I created using 'make menuconfig'. (with modifications in the "kernel hacking" category)

Some of the .config file lines (that I find relevant) are quoted here:

CONFIG_DEBUG_INFO=y 
CONFIG_DEBUG_KERNEL=y
CONFIG_MAGIC_SYSRQ=y
CONFIG_DEBUG_FS=y
CONFIG_HAVE_HW_BREAKPOINT=y
CONFIG_HAVE_MIXED_BREAKPOINTS_REGS=y

I use qemu with the "-S" option to boot my kernel, and using gdb I connect to qemu and resume it's execution:

(gdb) target remote :1234
(gdb) continue

When setting breakpoints, for an example: break schedule I get a confirmation message such as:

Breakpoint 1 at 0xffffffff81736400: file kernel/sched/core.c, line 2509.

So till here everything seems fine!

The problem is that gdb and qemu does not stop at the breakpoints! They both simply continue running...

I tried to set hardware breakpoints as suggested in some places, but I get the message:

No hardware breakpoint support in the target.

which is weird because I have the CONFIG_HAVE_HW_BREAKPOINT set.

I also unset CONFIG_DEBUG_RODATA as suggested in other places,but still no results...

I tried to mess around with the .config file as suggested here: gdbserver inside qemu does not stop on breakpoints but I couldn't figure out the relevant differences between the working .config file and my .config file... (There are too much differences!)

Can anyone please help me?

Thank You!!

How do you tell qemu that the running kernel is being debugged ? Where do you set the port "1234" ?. I've not used qemu before but, if you can, try enabling KGDB/KDB and running your kernel over a serial link. — srd
I use the -s and -S flags. Qemu start and wait for gdb to connect on the default port - 1234. Only when I press continue in the gdb side Qemu resumes its execution. So the connection between gdb and Qemu is fine... — Noamiko

Noamiko Noamiko · Accepted Answer · 2013-09-23T11:37:07

OK, I found the problem so I'm posting here for other people: Apparently this is a very delicate procedure... My config file was fine. All I needed to do is to define the breakpoints only after I connect to qemu, and not after loading the vmlinux. These breakpoints have to be HARDWARE breakpoints... otherwise they don't break!

Remote gdb debugging does not stop at breakpoints

2 Answers