Re: [PATCH 20/35] arm64: mte: Add in-kernel MTE helpers

[Catalin Marinas]

On Thu, Aug 27, 2020 at 11:31:56AM +0100, Vincenzo Frascino wrote:
> On 8/27/20 10:38 AM, Catalin Marinas wrote:
> > On Fri, Aug 14, 2020 at 07:27:02PM +0200, Andrey Konovalov wrote:
> >> +void * __must_check mte_set_mem_tag_range(void *addr, size_t size, u8 tag)
> >> +{
> >> +	void *ptr = addr;
> >> +
> >> +	if ((!system_supports_mte()) || (size == 0))
> >> +		return addr;
> >> +
> >> +	tag = 0xF0 | (tag & 0xF);
> >> +	ptr = (void *)__tag_set(ptr, tag);
> >> +	size = ALIGN(size, MTE_GRANULE_SIZE);
> > 
> > I think aligning the size is dangerous. Can we instead turn it into a
> > WARN_ON if not already aligned? At a quick look, the callers of
> > kasan_{un,}poison_memory() already align the size.
> 
> The size here is used only for tagging purposes and if we want to tag a
> subgranule amount of memory we end up tagging the granule anyway. Why do you
> think it can be dangerous?

In principle, I don't like expanding the size unless you are an
allocator. Since this code doesn't control the placement of the object
it was given, a warn seems more appropriate.

> >> +/*
> >> + * Assign allocation tags for a region of memory based on the pointer tag
> >> + *   x0 - source pointer
> >> + *   x1 - size
> >> + *
> >> + * Note: size is expected to be MTE_GRANULE_SIZE aligned
> >> + */
> >> +SYM_FUNC_START(mte_assign_mem_tag_range)
> >> +	/* if (src == NULL) return; */
> >> +	cbz	x0, 2f
> >> +	/* if (size == 0) return; */
> > 
> > You could skip the cbz here and just document that the size should be
> > non-zero and aligned. The caller already takes care of this check.
> 
> I would prefer to keep the check here, unless there is a valid reason, since
> allocate(0) is a viable option hence tag(x, 0) should be as well. The caller
> takes care of it in one place, today, but I do not know where the API will be
> used in future.

That's why I said just document it in the comment above the function.

The check is also insufficient if the size is not aligned to an MTE
granule, so it's not really consistent. This function should end with a
subs followed by b.gt as cbnz will get stuck in a loop for unaligned
size.

-- 
Catalin