Re: Hopefully Simple Question on Debugging Kernel Modules

From: Scott Long <scottl_at_samsco.org>
Date: Mon, 23 Feb 2009 10:10:07 -0700
John Baldwin wrote:
> On Friday 20 February 2009 6:40:56 pm David Christensen wrote:
>> I'm sure this is a simple question but the answer is alluding my Google
>> search capabilities.  My driver is being loaded as a kernel module and
>> is failing with the following error:
>>
>> Fatal trap 12: page fault while in kernel mode
>> cpuid = 0; apic id = 00
>> fault virtual address   = 0xfffffffe40abe9dc
>> fault code              = supervisor write data, page not present
>> instruction pointer     = 0x8:0xffffffff920b638f
>> stack pointer           = 0x10:0xffffffff9212bb10
>> frame pointer           = 0x10:0xffffffff9212bbb0
>> code segment            = base 0x0, limit 0xfffff, type 0x1b
>>                         = DPL 0, pres 1, long 1, def32 0, gran 1
>> processor eflags        = interrupt enabled, resume, IOPL = 0
>> current process         = 12 (irq268: bce0)
>> [thread pid 12 tid 100166 ]
>> Stopped at      bce_intr+0x8df: addl    $0x1,0x2c854(%r12,%rax,4)
>> db>
>>
>> I simply need to find the offending source line in my driver.  Not sure 
>> how I've managed to get the driver running at all without this but it's 
>> time to do things the right way.  I have KDB/DDB/GDB built into my 
>> -CURRENT kernel already.  It'd be great to find the source line while in
>> the kernel debugger but I'm also fine with rebooting the system to 
>> identify the line number.
> 
> Just use gdb on bce.ko (built with debug symbols):
> 
> gdb /path/to/if_bce.ko
> (gdb) l *bce_intr+0x8df
> 
> If you get a crashdump you can run kgdb on it and just walk up to the relevant 
> stack frame and use 'l' there to get a listing.
> 

One thing that I've never figured out is how debugging symbols are 
handled in module builds these days.  If I go to /sys/modules/bce and
do 'make', it generates a .ko and explicitly strips it.  I wind up
having to re-run the link command by hand so I get symbols.  What is
the correct way to do this?  Note that I'm not interested in answers
that involve "go to /usr/src and run make buildkernel" =-)


Scott
Received on Mon Feb 23 2009 - 16:10:13 UTC

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