[Qemu-devel] Re: How to debug Linux kernel on qemu with kgdb?

[Jan Kiszka]

Neo Jia wrote:
> On 4/25/07, Jan Kiszka <address@hidden> wrote:
>> Neo Jia wrote:
>> > On 4/25/07, Jan Kiszka <address@hidden> wrote:
>> >> Neo Jia wrote:
>> >> > hi,
>> >> >
>> >> > I am trying to use debug kgdb patched linux kernel on my qemu. Both
>> >> > the native and target platform are IA32. I am wondering if there is
>> >> > anyone can show me the procedure?
>> >>
>> >> Yep, see
>> https://mail.gna.org/public/xenomai-core/2006-09/msg00202.html
>> >>
>> >> (BTW, I think that kgdb bug is still unfixed - I never got a
>> feedback.)
>> >
>> > I can connect gdb through /dev/pts/XX. My qemu is lanuched by
>> >
>> > qemu -nographic -hda linux.img -kernel
>> > ./2.6.15.5-kgdb/vmlinuz-2.6.15.5-kgdb -serial pty -append "kgdbwait
>> > console=ttyS0 root=/dev/hda sb=0x220,5,1,5 ide2=noprobe ide3=noprobe
>> > ide4=noprobe ide5=noprobe"
>> >
>> > Do you know where can I get the console output?
>> >
>>
>> Use "... -serial stdio -serial pty ..." and attached kgdb to the second
>> serial port (I think to recall that is default anyway). The first one is
>> then used for the kernel console.
>>
>> > I would like to work out kgdb + qemu to debug linux kernel.
>>
>> ??? So you really want to debug the kernel when kgdb is applied, ie.
>> actually debug kgdb? If you only intend to debug the kernel itself, qemu
>> -s + gdb is enough.
> 
> Jan,
> 
> I just would like to debug kernel itself. I have tried qemu -s + gdb
> but it keeps
> getting "apic_timer_interrupt" when I am using "n" command.
> 
> The following is the output:
> 
>> gdb vmlinux
> GNU gdb 6.5.50.20060621-cvs
> Copyright (C) 2006 Free Software Foundation, Inc.
> GDB is free software, covered by the GNU General Public License, and you
> are
> welcome to change it and/or distribute copies of it under certain
> conditions.
> Type "show copying" to see the conditions.
> There is absolutely no warranty for GDB.  Type "show warranty" for details.
> This GDB was configured as "i686-pc-linux-gnu"...Using host
> libthread_db library "/lib/tls/libthread_db.so.1".
> 
> (gdb) target remote localhost:1234
> Remote debugging using localhost:1234
> 0x0000fff0 in ?? ()
> warning: shared library handler failed to enable breakpoint
> (gdb) c
> Continuing.
> 
> Program received signal SIGINT, Interrupt.
> default_idle () at include/asm/bitops.h:252
> 252             return ((1UL << (nr & 31)) & (addr[nr >> 5])) != 0;
> (gdb) b sys_ex
> sys_execve      sys_exit        sys_exit_group
> (gdb) b sys_execve
> Breakpoint 1 at 0xc0101ac1: file arch/i386/kernel/process.c, line 791.
> (gdb) c
> Continuing.
> 
> Breakpoint 1, sys_execve (regs=
>      {ebx = 135197704, ecx = 135197864, edx = 135244936, esi =
> 135197704, edi = 135197704, ebp = -1079176984, eax = 11, xds = 123,
> xes = 123, orig_eax = 11, eip = -1208835017, xcs = 115, eflags = 582,
> esp = -1079177012, xss = 123})
>    at arch/i386/kernel/process.c:791
> 791             filename = getname((char __user *) regs.ebx);
> (gdb) n
> 0xc0103666 in apic_timer_interrupt () at include/asm/current.h:9
> 9       {

That's normal. You stopped the kernel in sys_execve with interrupts
enabled. When continuing, qemu replays any pending interrupts, here the
timer. Thus your next instruction is not in sys_execve, but in the
interrupt handler. You can try finish then and hope that you will return
to the next line in sys_execve, or you put another breakpoint at the
next interesting spot and just issue continue.

Jan


PS: Using kgdb from inside qemu should work (at least it once did for
me), but it is really sloooow.

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