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

From: Igor Stoppa
Date: Sun Dec 09 2018 - 17:24:10 EST

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

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.