linux-kernel - Re: Usage of kgdb in v2.6.26

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Date:	Thu, 17 Jul 2008 08:14:32 -0500
From:	Jason Wessel <jason.wessel@...driver.com>
To:	ZC Miao <hellwolf.misty@...il.com>
CC:	Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Re: Usage of kgdb in v2.6.26



Do you have CONFIG_DEBUG_RODATA=y in your kernel config?

There is a presently a known problem where breakpoints will silently
fail to plant when this is turned on.

Jason.

ZC Miao wrote:
> I tried to use the kgdb from v2.6.26, but met few problems.
> I use :
> 1. gdb 6.8 (gdb-6.8-11.fc9.i386)
> 2. kernel v2.6.26 source code
> 3. Qemu 0.9.1 (qemu-0.9.1-5.fc9.i386)
> 4. Host is : 2.6.25.10-86.fc9.i686
> 5. Cpu : Intel(R) Core(TM)2 Duo CPU     T5450  @ 1.66GHz
>
>
> * Firstly I compile kernel 2.6.26 with kgdb support :
> CONFIG_KGDB=y
> CONFIG_KGDB_SERIAL_CONSOLE=y
>
> * Then I use qemu to boot my kernel along with a rootfs that I created,
> with a qemu option that makes a local tcp port a listening service as a
> serial port for the emulated system. The kernel arguments I gave is :
> nfsroot=192.168.200.1:/path/to/rootfs,udp,nolock ip=dhcp root=/dev/nfs
> ro single kgdboc=ttyS0,115200 kgdbwait
>
> * After the kernel loaded and boot for a while, the gdb receives a
> signal and hangs the kernel :
> kgdb_register_io_module (new_kgdb_io_ops=<value optimized out>)
>     at /data/home/hellwolf/mydoc/prog/linux/linux-2.6/kernel/kgdb.c:1674
> (gdb) 
>
> Question 1 :
> * If I type "b sys_read", and then "c", the kernel continues until I
> get :
>
> ---------------------------------------------------------------
> [New Thread 1]
> Cannot remove breakpoints because program is no longer writable.
> It might be running in another process.
> Further execution is probably impossible.
>
> Breakpoint 1, sys_read (fd=3, buf=0xbfdb31dc "", count=512)
>
> at /data/home/hellwolf/mydoc/prog/linux/linux-2.6/fs/read_write.c:360
> (gdb) 
> ---------------------------------------------------------------
>
> Why "program is no longer writable" happens here?
>
> Question 2 :
> * If I directly type "c" when kgdbwait triggered. And use Sys-Rq+G to
> interrupt the kernel, I will get :
> ---------------------------------------------------------------
> [New Thread -1]
>
> Program received signal SIGTRAP, Trace/breakpoint trap.
> [Switching to Thread -1]
> sysrq_handle_gdb (key=103, tty=0xc78f6400)
>     at /data/home/hellwolf/mydoc/prog/linux/linux-2.6/kernel/kgdb.c:167
> (gdb)  
> ---------------------------------------------------------------
> Here if I type "c sys_read" and "c", it never actually makes any break
> points, I will never receive a break from gdb.
>
>
> I don't know if I made anything wrong, or I missed any important thing,
> so I post here and would like to get some help from you.
>
> Thanks. 
>
> --
> ZC Miao (hellwolf.misty@...il.com)
> Blog http://hellwolf.cublog.cn
>
> gpg --keyserver pgp.mit.edu --recv-key 0x6B174C6F
>
> Most people deserve each other.
> 		-- Shirley
>
>
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