Re: stack_used() not accurate?

["Mike Snitzer" <snitzer at gmail dot com>]

On Mon, Jun 2, 2008 at 9:12 PM, Mike Snitzer <snitzer@gmail.com> wrote:
> On Mon, Jun 2, 2008 at 7:06 PM, Jim Keniston <jkenisto@us.ibm.com> wrote:
>> On Mon, 2008-06-02 at 18:27 -0400, Mike Snitzer wrote:
>>> On Mon, Jun 2, 2008 at 5:44 PM, Jim Keniston <jkenisto@us.ibm.com> wrote:
>> ...
>>> >
>>> > Sorry, I haven't been following this thread for a while, so maybe this
>>> > has already been mentioned.  But keep in mind that on i386, when your
>>> > breakpoint trap happens in kernel code, esp and ss aren't saved on the
>>> > stack.  So regs->esp and regs->ss contain the top of the pre-trap stack,
>>> > and the pre-trap stack pointer is &regs->esp, not regs->esp.
>>>
>>> OK, so something like this for x86?
>>>
>>> function stack_used_new:long() %{
>>>         long free = THREAD_SIZE;
>>>         if (CONTEXT->regs) {
>>>                 long curbase = (long)task_stack_page(current);
>>>                 long *sp = (long *) &(REG_SP(CONTEXT->regs));
>>>                 free = (long)sp - (curbase + sizeof(struct thread_info));
>>>         }
>>>         THIS->__retvalue = THREAD_SIZE - free;
>>> %}
>>>
>>> The results look much more like I'd expect...
>>
>> Your calculation of free looks correct (although I'd make sp a long
>> rather than a long* to simplify things a bit).
>
> Yes, just need a cast to (long), thanks.
>
>> But your return value seems to include the size of the struct
>> thread_info, which seems wrong.  I'd think you'd want
>>        THIS->__retvalue = (curbase + THREAD_SIZE) - sp;
>> or
>>        THIS->__retvalue = THREAD_SIZE - (sp & (THREAD_SIZE-1));
>> That seems more in keeping with the original implementation.
>
> Your 2nd suggestion always returns 4096.

Err, sorry; both of your suggestions yield identical results (I had a
typo in the code and stopped thinking momentarily).

Mike