Re: [PATCH v2 1/1] arm64: mm: correct the inside linear map boundaries during hotplug check

[Pavel Tatashin]

On Mon, Feb 15, 2021 at 2:34 PM Ard Biesheuvel <ardb@xxxxxxxxxx> wrote:
>
> On Mon, 15 Feb 2021 at 20:30, Pavel Tatashin <pasha.tatashin@xxxxxxxxxx> wrote:
> >
> > > Can't we simply use signed arithmetic here? This expression works fine
> > > if the quantities are all interpreted as s64 instead of u64
> >
> > I was thinking about that, but I do not like the idea of using sign
> > arithmetics for physical addresses. Also, I am worried that someone in
> > the future will unknowingly change it to unsigns or to phys_addr_t. It
> > is safer to have start explicitly set to 0 in case of wrap.
>
> memstart_addr is already a s64 for this exact reason.

memstart_addr is basically an offset and it must be negative. For
example, this would not work if it was not signed:
#define vmemmap ((struct page *)VMEMMAP_START - (memstart_addr >> PAGE_SHIFT))

However, on powerpc it is phys_addr_t type.

>
> Btw, the KASLR check is incorrect: memstart_addr could also be
> negative when running the 52-bit VA kernel on hardware that is only
> 48-bit VA capable.

Good point!

if (IS_ENABLED(CONFIG_ARM64_VA_BITS_52) && (vabits_actual != 52))
    memstart_addr -= _PAGE_OFFSET(48) - _PAGE_OFFSET(52);

So, I will remove IS_ENABLED(CONFIG_RANDOMIZE_BASE) again.

I am OK to change start_linear_pa, end_linear_pa to signed, but IMO
what I have now is actually safer to make sure that does not break
again in the future.