Kernel Debugging

[Julian Elischer]

Daniel O'Connor wrote:
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> On Wed, 18 Aug 2004 15:41, Dennis George wrote:
> 
>>I am studying the kernel source of FreeBSD. I like to know the flow of
>>packets from NIC to different modules of Kernel and then to the
>>user-level. I studied the code and identified some of the functions through
>>which the kernel handles network packets..... But I want to check from
>>where the control goes to that function.......
>>
>>So I decided to debug the kernel.... since I have only one machine I am
>>not checking for kgdb.... I decided to work with DDB so compiled the
>>kernel with option DDB and with debugging enabled (-g option). After
>>installing the new kernel I rebooted the system and in the boot prompt I
>>gave "-d" option to  enter the debugging module..... My problem is that I
>>can't provide any breakpoints with this method... It gives me error
>>saying "symbol not found".....  Can anybody tell me where is the
>>problem................
> 
> 
> Maybe you should try vmware then you can pretend you have 2 machines :)
> 
> Not sure how to answer your actual question though.

In some releases of freebsd.. you can not see symbols at the moment that boot -d 
stops.
for some reason they have not been enabled yet.

so use nm to find the address of something that will be called and
use the address to set a breakpoint there..
when it stops you will have symbols.

I also would suggest vmware.. I've used it very successfully in the past..
you can use the nmdm device to set up s 'serial link' with
the virtual machine and debug across that.

this is from my web page:
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A screenshot of a FreeBSD system running under vmware, running X11, with its 
console output redirected out one serial port (with 'tip' on the other side of 
the nullmodem), and stopped in the kernel debugger, which is redirected to 
another serial port (actually the nullmodem driver again)with gdb (in the guise 
of xxgdb) showing the breakpoint and the current instruction pointer. You can 
also see that I was running ddb on the console port for a while too, all without 
disturbing the X11 display. Hopefully a more explicit description of how to do 
this will be forthcoming. The gif file is here.

and the URL for the gif is:
http://www.freebsd.org/~julian/VM-Debug_screenshot.gif

> 
> - -- 
> Daniel O'Connor software and network engineer
> for Genesis Software - http://www.gsoft.com.au
> "The nice thing about standards is that there
> are so many of them to choose from."
>   -- Andrew Tanenbaum
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