fork: allow arch-override of VMAP stack alignment

In some cases, an architecture might wish its stacks to be aligned to a
boundary larger than THREAD_SIZE. For example, using an alignment of
double THREAD_SIZE can allow for stack overflows smaller than
THREAD_SIZE to be detected by checking a single bit of the stack
pointer.

This patch allows architectures to override the alignment of VMAP'd
stacks, by defining THREAD_ALIGN. Where not defined, this defaults to
THREAD_SIZE, as is the case today.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Reviewed-by: Will Deacon <will.deacon@arm.com>
Tested-by: Laura Abbott <labbott@redhat.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: James Morse <james.morse@arm.com>
Cc: linux-kernel@vger.kernel.org

include/linux/thread_info.h

@@ -38,6 +38,10 @@ enum {
 
 #ifdef __KERNEL__
 
+#ifndef THREAD_ALIGN
+#define THREAD_ALIGN	THREAD_SIZE
+#endif
+
 #ifdef CONFIG_DEBUG_STACK_USAGE
 # define THREADINFO_GFP		(GFP_KERNEL_ACCOUNT | __GFP_NOTRACK | \
 				 __GFP_ZERO)