arm64: mm: check at build time that PAGE_OFFSET divides the VA space evenly

Commit 8439e62a1561 ("arm64: mm: use bit ops rather than arithmetic in
pa/va translations") changed the boundary check against PAGE_OFFSET from
an arithmetic comparison to a bit test. This means we now silently assume
that PAGE_OFFSET is a power of 2 that divides the kernel virtual address
space into two equal halves. So make that assumption explicit.

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

diff --git a/arch/arm64/mm/init.c b/arch/arm64/mm/init.c
index 22c6758..8c3d7dd 100644 (file)
--- a/arch/arm64/mm/init.c
+++ b/arch/arm64/mm/init.c
@@ -170,6 +170,13 @@ void __init arm64_memblock_init(void)
 {
        const s64 linear_region_size = -(s64)PAGE_OFFSET;
 
+       /*
+        * Ensure that the linear region takes up exactly half of the kernel
+        * virtual address space. This way, we can distinguish a linear address
+        * from a kernel/module/vmalloc address by testing a single bit.
+        */
+       BUILD_BUG_ON(linear_region_size != BIT(VA_BITS - 1));
+
        /*
         * Select a suitable value for the base of physical memory.
         */