Re: [PATCH] kbuild: Enable -Wincompatible-function-pointer-types-strict in W=1

From: Nick Desaulniers <ndesaulniers@google.com>
To: Nathan Chancellor <nathan@kernel.org>
Cc: masahiroy@kernel.org, keescook@chromium.org,
	samitolvanen@google.com,  nicolas@fjasle.eu, trix@redhat.com,
	linux-kbuild@vger.kernel.org,  llvm@lists.linux.dev,
	patches@lists.linux.dev
Subject: Re: [PATCH] kbuild: Enable -Wincompatible-function-pointer-types-strict in W=1
Date: Thu, 5 Oct 2023 11:38:41 -0700	[thread overview]
Message-ID: <CAKwvOdnzi9g1sz6C_CBFfaqRu7HQ4ZfBphpyBrgV1C=bviR_Uw@mail.gmail.com> (raw)
In-Reply-To: <20231002-enable-wincompatible-function-pointer-types-strict-w-1-v1-1-808ab955d42d@kernel.org>

On Mon, Oct 2, 2023 at 3:20 PM Nathan Chancellor <nathan@kernel.org> wrote:
>
> -Wincompatible-function-pointer-types-strict aims to catch clang kernel
> Control Flow Integrity (kCFI) violations at build time (rather than run
> time) by validating function pointer assignments against the expected
> prototype, similar to the existing -Wincompatible-function-pointer-types
> that is considered a hard error in the kernel. The -strict variant
> requires the types to match exactly, as opposed to just matching in
> terms of ABI compatibility. This is primarily visible with int/unsigned
> int in lieu of enum types or vice versa.
>
> The tree is not completely clean, so this warning cannot currently be
> enabled unconditionally. However, there are only warnings in one
> subsystem ('drivers/counter'), so it is really close. In order to
> benefit from CI infrastructure that tests with W=1, enable this warning
> at that level, so that new instances have a chance of being caught and
> fixed during development.
>
> This should eventually be a hard error in a similar manner as
> Wincompatible-function-pointer-types but some subsystems test
> with W=1 + CONFIG_WERROR=n, so it would be rude to break their builds
> when they do not care about warnings outside of their subsystem.
>
> Link: https://github.com/ClangBuiltLinux/linux/issues/1750
> Signed-off-by: Nathan Chancellor <nathan@kernel.org>

Sounds like we're pretty close to just enabling this by default.
Getting it on in W=1 for a cycle will help prevent any backslide.
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>

> ---
>  scripts/Makefile.extrawarn | 1 +
>  1 file changed, 1 insertion(+)
>
> diff --git a/scripts/Makefile.extrawarn b/scripts/Makefile.extrawarn
> index 2fe6f2828d37..9662f7fd6e39 100644
> --- a/scripts/Makefile.extrawarn
> +++ b/scripts/Makefile.extrawarn
> @@ -108,6 +108,7 @@ KBUILD_CFLAGS += $(call cc-option, -Wformat-overflow)
>  KBUILD_CFLAGS += $(call cc-option, -Wformat-truncation)
>  KBUILD_CFLAGS += $(call cc-option, -Wstringop-overflow)
>  KBUILD_CFLAGS += $(call cc-option, -Wstringop-truncation)
> +KBUILD_CFLAGS += $(call cc-option, -Wincompatible-function-pointer-types-strict)
>
>  KBUILD_CPPFLAGS += -Wundef
>  KBUILD_CPPFLAGS += -DKBUILD_EXTRA_WARN1
>
> ---
> base-commit: ce9ecca0238b140b88f43859b211c9fdfd8e5b70
> change-id: 20231002-enable-wincompatible-function-pointer-types-strict-w-1-4a56b99b8c6f
>
> Best regards,
> --
> Nathan Chancellor <nathan@kernel.org>
>

-- 
Thanks,
~Nick Desaulniers