Re: [PATCH 2/6] __wr_after_init: write rare for static allocation

[Igor Stoppa]

On 06/12/2018 06:44, Matthew Wilcox wrote:
> On Tue, Dec 04, 2018 at 02:18:01PM +0200, Igor Stoppa wrote:
> > +void *__wr_op(unsigned long dst, unsigned long src, __kernel_size_t len,
> > +	      enum wr_op_type op)
> > +{
> > +	temporary_mm_state_t prev;
> > +	unsigned long flags;
> > +	unsigned long offset;
> > +	unsigned long wr_poking_addr;
> > +
> > +	/* Confirm that the writable mapping exists. */
> > +	BUG_ON(!wr_ready);
> > +
> > +	if (WARN_ONCE(op >= WR_OPS_NUMBER, "Invalid WR operation.") ||
> > +	    WARN_ONCE(!is_wr_after_init(dst, len), "Invalid WR range."))
> > +		return (void *)dst;
> > +
> > +	offset = dst - (unsigned long)&__start_wr_after_init;
> > +	wr_poking_addr = wr_poking_base + offset;
> > +	local_irq_save(flags);
>
> Why not local_irq_disable()?  Do we have a use-case for wanting to access
> this from interrupt context?

No, not that I can think of. It was "just in case", but I can remove it.

> > +	/* XXX make the verification optional? */
>
> Well, yes.  It seems like debug code to me.

Ok, I was not sure about this, because text_poke() does it as part of 
its normal operations.

> > +	/* Randomize the poking address base*/
> > +	wr_poking_base = TASK_UNMAPPED_BASE +
> > +		(kaslr_get_random_long("Write Rare Poking") & PAGE_MASK) %
> > +		(TASK_SIZE - (TASK_UNMAPPED_BASE + wr_range));
>
> I don't think this is a great idea.  We want to use the same mm for both
> static and dynamic wr memory, yes?  So we should have enough space for
> all of ram, not splatter the static section all over the address space.
>
> On x86-64 (4 level page tables), we have a 64TB space for all of physmem
> and 128TB of user space, so we can place the base anywhere in a 64TB
> range.

I was actually wondering about the dynamic part.
It's still not clear to me if it's possible to write the code in a 
sufficiently generic way that it could work on all 64 bit architectures.
I'll start with x86-64 as you suggest.

--
igor