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

[Jan Kiszka <jan.kiszka@web.de>]

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Neo Jia wrote:
> On 4/25/07, Jan Kiszka <jan.kiszka@web.de> wrote:
>> Neo Jia wrote:
>> > On 4/25/07, Jan Kiszka <jan.kiszka@web.de> wrote:
>> >> Neo Jia wrote:
>> >> > On 4/25/07, Jan Kiszka <jan.kiszka@web.de> 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.
>>
> 
> Could you show me the procedure you did? I just want to use this to
> learn kernel code.

Did you redirect kgdb to ttyS1?

Jan


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