Re: A question about PROT_NONE on ARM and ARM26

[Nicolas Pitre]

On Thu, 1 Jul 2004, Jamie Lokier wrote:

> Russell King wrote:
> > Ok, this could work, but there's one gotcha - TASK_SIZE-4 doesn't fit
> > in an 8-bit rotated constants, so we need 2 extra instructions:
> > 
> > __get_user_4:
> > 	mov	r1, #TASK_SIZE
> > 	sub	r1, r1, #4
> > 	cmp	r0, r1
> > 4:	ldrlet	r1, [r0]
> > 	movle	r0, #0
> > 	movle	pc, lr
> > 	...
> 
> One more possibility:
> 
> 	cmp	r0, #(TASK_SIZE - (1<<24))
> 
> I.e. just compare against the largest constant that can be
> represented.  For accesses to the last part of userspace, it's a
> penalty of 4 instructions -- but it might work out to be a net gain.

Maybe not.  The user stack is located at the top so any user buffer 
allocated on the stack would be penalized.


Nicolas

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