Hi,

For some background, we're running an emulator that uses a null pointer 
value of 0xffff0000 and we want any accesses to that address to trap.

Weird :)

Do you anticipate any issues with the following change?

-#define TASK_SIZE_USER32 (0x0000000100000000UL - (1*PAGE_SIZE))
+#define TASK_SIZE_USER32 (0x00000000FFFF0000UL - (1*PAGE_SIZE))

Should be OK, for a 64kB kernel we will put the stack top at 0xFFFF0000
anyway.

Anton