[v3 PATCH 1/1] booke/kprobe: make program exception to use one dedicated exception stack

[Kumar Gala]

On Jul 11, 2011, at 6:31 AM, Tiejun Chen wrote:

> When kprobe these operations such as store-and-update-word for SP(r1),
> 
> stwu r1, -A(r1)
> 
> The program exception is triggered, and PPC always allocate an exception frame
> as shown as the follows:
> 
> old r1 ----------
>         ...
>         nip
>         gpr[2] ~ gpr[31]
>         gpr[1] <--------- old r1 is stored.
>         gpr[0]
>       -------- <--------- pr_regs @offset 16 bytes
>       padding
>       STACK_FRAME_REGS_MARKER
>       LR
>       back chain
> new r1 ----------
> Then emulate_step() will emulate this instruction, 'stwu'. Actually its
> equivalent to:
> 1> Update pr_regs->gpr[1] = mem[old r1 + (-A)]
> 2> stw [old r1], mem[old r1 + (-A)]
> 
> Please notice the stack based on new r1 may be covered with mem[old r1
> +(-A)] when addr[old r1 + (-A)] < addr[old r1 + sizeof(an exception frame0].
> So the above 2# operation will overwirte something to break this exception
> frame then unexpected kernel problem will be issued.
> 
> So looks we have to implement independed interrupt stack for PPC program
> exception when CONFIG_BOOKE is enabled. Here we can use
> EXC_LEVEL_EXCEPTION_PROLOG to replace original NORMAL_EXCEPTION_PROLOG
> for program exception if CONFIG_BOOKE. Then its always safe for kprobe
> with independed exc stack from one pre-allocated and dedicated thread_info.
> Actually this is just waht we did for critical/machine check exceptions
> on PPC.
> 
> Signed-off-by: Tiejun Chen <tiejun.chen at windriver.com>
> ---

I'm still very confused why we need a unique stack frame for kprobe/program exceptions on book-e devices.

Can you explain this further.

- k