Re: [PATCH v2 1/5] Makefile: add -fno-builtin-stpcpy

[Nathan Chancellor <natechancellor@gmail.com>]

On Wed, Aug 19, 2020 at 12:16:50PM -0700, Nick Desaulniers wrote:
> LLVM implemented a recent "libcall optimization" that lowers calls to
> `sprintf(dest, "%s", str)` where the return value is used to
> `stpcpy(dest, str) - dest`. This generally avoids the machinery involved
> in parsing format strings. This optimization was introduced into
> clang-12. Because the kernel does not provide an implementation of
> stpcpy, we observe linkage failures for almost all targets when building
> with ToT clang.
> 
> The interface is unsafe as it does not perform any bounds checking.
> Disable this "libcall optimization" via `-fno-builtin-stpcpy`.
> 
> Cc: stable@vger.kernel.org # 4.4
> Link: https://bugs.llvm.org/show_bug.cgi?id=47162
> Link: https://github.com/ClangBuiltLinux/linux/issues/1126
> Link: https://reviews.llvm.org/D85963
> Reported-by: Sami Tolvanen <samitolvanen@google.com>
> Suggested-by: Dávid Bolvanský <david.bolvansky@gmail.com>
> Suggested-by: Kees Cook <keescook@chromium.org>
> Reviewed-by: Kees Cook <keescook@chromium.org>
> Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>

Reviewed-by: Nathan Chancellor <natechancellor@gmail.com>
Tested-by: Nathan Chancellor <natechancellor@gmail.com>

> ---
>  Makefile | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/Makefile b/Makefile
> index 9cac6fde3479..e523dc8d30e0 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -578,6 +578,7 @@ ifneq ($(LLVM_IAS),1)
>  CLANG_FLAGS	+= -no-integrated-as
>  endif
>  CLANG_FLAGS	+= -Werror=unknown-warning-option
> +CLANG_FLAGS	+= -fno-builtin-stpcpy
>  KBUILD_CFLAGS	+= $(CLANG_FLAGS)
>  KBUILD_AFLAGS	+= $(CLANG_FLAGS)
>  export CLANG_FLAGS
> -- 
> 2.28.0.297.g1956fa8f8d-goog
>