Problem with GDB when debugging IRQ handlers

[Dmitry Eremin-Solenikov]

On 6/28/11, Catalin Marinas <catalin.marinas at arm.com> wrote:
> On Tue, Jun 28, 2011 at 03:20:45PM +0100, Catalin Marinas wrote:
>> On Tue, Jun 28, 2011 at 04:06:11PM +0400, Dmitry Eremin-Solenikov wrote:
>> > On 6/28/11, Russell King - ARM Linux <linux at arm.linux.org.uk> wrote:
>> > I did some checks. It seems, the problem isn't related to unwinder. At
>> > least
>> > it looks like kernel has all necessary unwinding subops. It looks like
>> > the
>> > problem is really related to the lack of necessary .cfi information. At
>> > least
>> > when i added .cfi_startproc/.cfi_endproc annotations to entry-armv.S
>> > code,
>> > gdb stopped decoding backtrace with the "previous frame identical to
>> > this frame"
>> > error. Unfortunately I don't have enough knowledge to add .cfi
>> > annotations to
>> > irq handlers.
>>
>> I think it may have stopped decoding because of some information it
>> reads from the stack doesn't look sane. But I wonder whether we could
>> get it looping again depending on the register values in the interrupted
>> context.
>>
>> > diff --git a/arch/arm/kernel/entry-armv.S b/arch/arm/kernel/entry-armv.S
>> > index e8d8856..d77f9d7 100644
>> > --- a/arch/arm/kernel/entry-armv.S
>> > +++ b/arch/arm/kernel/entry-armv.S
>> > @@ -28,6 +28,7 @@
>> >  #include "entry-header.S"
>> >  #include <asm/entry-macro-multi.S>
>> >
>> > +	.cfi_sections	.debug_frame
>> >  /*
>> >   * Interrupt handling.  Preserves r7, r8, r9
>> >   */
>> > @@ -113,6 +114,7 @@ ENDPROC(__und_invalid)
>> >
>> >  	.macro	svc_entry, stack_hole=0
>> >   UNWIND(.fnstart		)
>> > +	.cfi_startproc
>> >   UNWIND(.save {r0 - pc}		)
>> >  	sub	sp, sp, #(S_FRAME_SIZE + \stack_hole - 4)
>>
>> Could you add some directives like below in the svc_entry macro (after
>> "sub sp...", not sure if it matters) and check whether gdb behaves
>> better:
>>
>> 	.cfi_def_cfa_offset S_PC
>> 	.cfi_offset 14, -4
>
> Actually since the return address is in S_PC (which maybe gdb assumes it
> would be the saved LR), this is probably not be correct. After SVC
> entry, we have he following structure on the stack:
>
> 	ORIG_r0
> 	CPSR
> 		<--- assuming this is the Call Frame Address (SP+S_PC+4)
> 	PC	<--- CFA - 4
> 	LR	<--- don't care
> 	SP	<--- CFA - 12
> 	...
>
>
> So we tell gdb about this with something like below (untested):
>
> 	.cfi_def_cfa_offset S_PC + 4
> 	.cfi_offset 14, -4
> 	.cfi_offset 13, -12

This brings "unknown CFA rule" gdb exception, but it seems I got your idea.

-- 
With best wishes
Dmitry