Re: Hopefully Simple Question on Debugging Kernel Modules

On Monday 23 February 2009 12:10:07 pm Scott Long wrote:
> 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" =-)

make DEBUG_FLAGS=-g is what I use.  The same thing works for userland tools 
and the kernel (usually we put 'makeoptions DEBUG_FLAGS=-g' in a kernel 
config so it is "automatic" for kernels though).

-- 
John Baldwin