[PATCH] stack: replace "o" output with "r" input constraint

[Kees Cook <keescook@chromium.org>]

From: Nick Desaulniers <ndesaulniers@google.com>

"o" isn't a common asm() constraint to use; it triggers an assertion in
assert-enabled builds of LLVM that it's not recognized when targeting
aarch64 (though it appears to fall back to "m"). I've fixed this in LLVM
13 now, but there isn't really a good reason to be using "o" in particular
here. To avoid causing build issues for those using assert-enabled builds
of earlier LLVM versions, the constraint needs changing.

Instead, if the point is to retain the __builtin_alloca(), we can make ptr
appear to "escape" via being an input to an empty inline asm block. This
is preferable anyways, since otherwise this looks like a dead store.

While the use of "r" was considered in
https://lore.kernel.org/lkml/202104011447.2E7F543@keescook/
it was only tested as an output (which looks like a dead store, and
wasn't sufficient). Use "r" as an input constraint instead, which
behaves correctly across compilers and architectures:
https://godbolt.org/z/E9cd411ob

Link: https://reviews.llvm.org/D100412
Link: https://bugs.llvm.org/show_bug.cgi?id=49956
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
Tested-by: Kees Cook <keescook@chromium.org>
Fixes: 39218ff4c625 ("stack: Optionally randomize kernel stack offset each syscall")
Signed-off-by: Kees Cook <keescook@chromium.org>
---
 include/linux/randomize_kstack.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/linux/randomize_kstack.h b/include/linux/randomize_kstack.h
index fd80fab663a9..bebc911161b6 100644
--- a/include/linux/randomize_kstack.h
+++ b/include/linux/randomize_kstack.h
@@ -38,7 +38,7 @@ void *__builtin_alloca(size_t size);
 		u32 offset = raw_cpu_read(kstack_offset);		\
 		u8 *ptr = __builtin_alloca(KSTACK_OFFSET_MAX(offset));	\
 		/* Keep allocation even after "ptr" loses scope. */	\
-		asm volatile("" : "=o"(*ptr) :: "memory");		\
+		asm volatile("" :: "r"(ptr) : "memory");		\
 	}								\
 } while (0)
 
-- 
2.25.1