On Mon, Feb 24, 2014 at 12:32:39PM -0500, Vince Weaver wrote:
> I do note that
> perf_callchain_user();
>
> Does
> fp = (void __user *)regs->bp;
>
> ...
>
> bytes = copy_from_user_nmi(&frame, fp, sizeof(frame));
>
>
> And in my particular executable RBP has nothing to do with a frame
> pointer, but is instead being used as a general purpose register.
>
> Am I missing something here? Though in that case I'm not sure why this
> wouldn't be easier to trigger.
Ah, in case the frame doesn't actually exist we would expect to fault
and get the fixup treatment, returning a short copy (the return value
being bytes _NOT_ copied).
When that happens;
if (bytes != 0)
break;
At which point we'll terminate the stack frame iteration.
This is where we rely on being able to take a fault from NMI context,
the fault iret will re-enable NMIs, necessitating all the magic Steve
did.
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