Re: ARM64 suggestion: reduce the compat address limit (currently 0x100000000)?

[Will Deacon]

Hi Jann,

On Wed, Jan 30, 2019 at 05:32:00PM +0100, Jann Horn wrote:
> At the moment, compat tasks running on ARM64 can allocate memory up to
> 0x100000000 (TASK_SIZE_32). Testing on an Android device (with an
> admittedly somewhat old kernel):

[...]

> ffff1000-100000000 rw-p 00000000 00:00 0
> 
> This means that mmap() allocations do not adhere to section 6.5.8 of
> C99 ("If the
> expression P points to an element of an array object and the
> expression Q points to the
> last element of the same array object, the pointer expression Q+1
> compares greater than
> P.") if you treat mmap() allocations as returning an array.

Oh, good point.

> In practice, I've also seen code that does things like computing a
> pointer that is out of bounds by a few bytes and then comparing it
> against the end of the array; while this is UB according to C99, it
> probably makes sense to try to avoid breaking such code.

Agreed, and since the current behaviour isn't something you can portably
rely on anyway, I think we're ok to change it.

> X86-64's compat code uses the limit 0xFFFFe000 (IA32_PAGE_OFFSET),
> which I think makes more sense. Would it make sense to do something
> like the following (untested)?

Can't we just go with 0x100000000 - PAGE_SIZE instead?

Will