My place at the game. #aik #gnaget #kamratgängetdrickabärs #kgdb #kgdbsthlm (på/i Friends Arena)

Following [Fotis post](http://fotis.loukos.me/blog/wp-trackback.php?p=25), some useful tips to use kdgb with a Linux kernel running inside Virtualbox **Compiling and launching the kernel** - if you use gdb >=4.5 to compile, adding ``#pragma GCC optimize(0)`` in your .c file (for example, after all the ``#include`` in ``arch/x86/kernel/irq.c``) is really helpful. Since it does not seem possible to fully compile the kernel without any optimization, you can avoid optimization of a couple of files - in the ``make menuconfig``, don't select the kdb options, only the kgdb one - append ``vga=794`` to the kernel options for a bigger screen **Guest configuration** To make the SysRq key work in Virtualbox console, let's replace it with the F12 key: setkeycodes 58 99 echo 1 > /proc/sys/kernel/sysrq echo ttyS0,115200 > /sys/module/kgdboc/parameters/kgdboc **GDB tips** Getting a backtrace of every threads (for easy grepping) set pagination off set logging on gdb.1.txt thread apply all bt 30 If the system is locked, hitting Alt+SysRq+G will break within ``kgdb_breakpoint`` and not within the lock. Steps to get a proper trace if kgdb interrupted our lock: (will work for sure if irq.c has been compiled without optimization, see above) - system is deadlocked - run Alt+SysRq+9, Alt+SysRq+D, Alt+SysRq+L to know some information about this lock... - Alt+SysRq+G to make gdb break (within ``kgdb_breakpoint``) The actual code (trapped within the lock) has been interrupted. Let's get information about the previous context gdb> bt 30 #0 kgdb_breakpoint () at kernel/debug/debug_core.c:960 ... #15 0xffffffff81004567 in do_IRQ (regs=0xffff88041fc03e58) at arch/x86/kernel/irq.c:190 ... gdb> frame 15 Here the structure regs hold the state of the registers before the interruption. gdb> p/a *regs $11 = { ... ip = 0xffffffff8114f1c3 , ... gdb> hbr 0xffffffff8114f1c3 gdb> cont Continuing. Breakpoint gdb> bt 30