From: Nick Desaulniers <ndesaulni...@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 <ndesaulni...@google.com> Tested-by: Kees Cook <keesc...@chromium.org> Fixes: 39218ff4c625 ("stack: Optionally randomize kernel stack offset each syscall") Signed-off-by: Kees Cook <keesc...@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
