arm64: mm: treat memstart_addr as a signed quantity

Commit c031a4213c ("arm64: kaslr: randomize the linear region")
implements randomization of the linear region, by subtracting a random
multiple of PUD_SIZE from memstart_addr. This causes the virtual mapping
of system RAM to move upwards in the linear region, and at the same time
causes memstart_addr to assume a value which may be negative if the offset
of system RAM in the physical space is smaller than its offset relative to
PAGE_OFFSET in the virtual space.

Since memstart_addr is effectively an offset now, redefine its type as s64
so that expressions involving shifting or division preserve its sign.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/include/asm/memory.h

@@ -135,7 +135,7 @@
 #include <linux/bitops.h>
 #include <linux/mmdebug.h>
 
-extern phys_addr_t		memstart_addr;
+extern s64			memstart_addr;
 /* PHYS_OFFSET - the physical address of the start of memory. */
 #define PHYS_OFFSET		({ VM_BUG_ON(memstart_addr & 1); memstart_addr; })
 

arch/arm64/mm/init.c

@@ -54,7 +54,7 @@
  * executes, which assigns it its actual value. So use a default value
  * that cannot be mistaken for a real physical address.
  */
-phys_addr_t memstart_addr __read_mostly = ~0ULL;
+s64 memstart_addr __read_mostly = -1;
 phys_addr_t arm64_dma_phys_limit __read_mostly;
 
 #ifdef CONFIG_BLK_DEV_INITRD
@@ -181,7 +181,7 @@ void __init arm64_memblock_init(void)
 	 * linear mapping. Take care not to clip the kernel which may be
 	 * high in memory.
 	 */
-	memblock_remove(max(memstart_addr + linear_region_size, __pa(_end)),
+	memblock_remove(max_t(u64, memstart_addr + linear_region_size, __pa(_end)),
 			ULLONG_MAX);
 	if (memblock_end_of_DRAM() > linear_region_size)
 		memblock_remove(0, memblock_end_of_DRAM() - linear_region_size);