On 2/16/21 1:21 AM, Pavel Tatashin wrote:

On Mon, Feb 15, 2021 at 2:34 PM Ard Biesheuvel [off-list ref] wrote:

quoted

On Mon, 15 Feb 2021 at 20:30, Pavel Tatashin [off-list ref] wrote:

quoted

quoted

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.

quoted

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.

An explicit check for the flip over and providing two different start
addresses points would be required in order to use the new framework.

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