Re: [PATCH] Makefile.extrawarn: re-enable -Wformat for clang; take 2

[Nathan Chancellor]

On Thu, Sep 01, 2022 at 10:59:13AM -0700, Nick Desaulniers wrote:
> -Wformat was recently re-enabled for builds with clang, then quickly
> re-disabled, due to concerns stemming from the frequency of default
> argument promotion related warning instances.
> 
> commit 258fafcd0683 ("Makefile.extrawarn: re-enable -Wformat for clang")
> commit 21f9c8a13bb2 ("Revert "Makefile.extrawarn: re-enable -Wformat for clang"")
> 
> ISO WG14 has ratified N2562 to address default argument promotion
> explicitly for printf, as part of the upcoming ISO C2X standard.
> 
> The behavior of clang was changed in clang-16 to not warn for the cited
> cases in all language modes.
> 
> Add a version check, so that users of clang-16 now get the full effect
> of -Wformat. For older clang versions, re-enable flags under the
> -Wformat group that way users still get some useful checks related to
> format strings, without noisy default argument promotion warnings. I
> intentionally omitted -Wformat-y2k and -Wformat-security from being
> re-enabled, which are also part of -Wformat in clang-16.
> 
> Link: https://github.com/ClangBuiltLinux/linux/issues/378
> Link: https://github.com/llvm/llvm-project/issues/57102
> Link: https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2562.pdf
> Suggested-by: Justin Stitt <jstitt007@xxxxxxxxx>
> Suggested-by: Nathan Chancellor <nathan@xxxxxxxxxx>
> Suggested-by: Youngmin Nam <youngmin.nam@xxxxxxxxxxx>
> Signed-off-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>

Reviewed-by: Nathan Chancellor <nathan@xxxxxxxxxx>

> ---
> Linus, I figured I'd send this to you to see whether you'd prefer to
> apply it, or let it "ride the trains" up through the kbuild tree? I do
> have another series I'm working on to improve the compiler version
> checks
> <https://lore.kernel.org/llvm/20220831184408.2778264-4-ndesaulniers@xxxxxxxxxx/>
> where I can/will improve the checks used here, but I'm also interested in
> having something that might backport cleanly to stable.
> 
>  scripts/Makefile.extrawarn | 12 ++++++++++++
>  1 file changed, 12 insertions(+)
> 
> diff --git a/scripts/Makefile.extrawarn b/scripts/Makefile.extrawarn
> index 0621c39a3955..6ae482158bc4 100644
> --- a/scripts/Makefile.extrawarn
> +++ b/scripts/Makefile.extrawarn
> @@ -47,7 +47,19 @@ else
>  
>  ifdef CONFIG_CC_IS_CLANG
>  KBUILD_CFLAGS += -Wno-initializer-overrides
> +# Clang before clang-16 would warn on default argument promotions.
> +ifeq ($(shell [ $(CONFIG_CLANG_VERSION) -lt 160000 ] && echo y),y)
> +# Disable -Wformat
>  KBUILD_CFLAGS += -Wno-format
> +# Then re-enable flags that were part of the -Wformat group that aren't
> +# problematic.
> +KBUILD_CFLAGS += -Wformat-extra-args -Wformat-invalid-specifier
> +KBUILD_CFLAGS += -Wformat-zero-length -Wnonnull
> +# Requires clang-12+.
> +ifeq ($(shell [ $(CONFIG_CLANG_VERSION) -ge 120000 ] && echo y),y)
> +KBUILD_CFLAGS += -Wformat-insufficient-args
> +endif
> +endif
>  KBUILD_CFLAGS += -Wno-sign-compare
>  KBUILD_CFLAGS += $(call cc-disable-warning, pointer-to-enum-cast)
>  KBUILD_CFLAGS += -Wno-tautological-constant-out-of-range-compare
> 
> base-commit: 2880e1a175b9f31798f9d9482ee49187f61b5539
> -- 
> 2.37.2.789.g6183377224-goog
>