[PATCH] kbuild: Disable clang's -Wformat-{overflow,truncation}-non-kprintf

[PATCH] kbuild: Disable clang's -Wformat-{overflow,truncation}-non-kprintf

[Nathan Chancellor]

Recently, clang added support for -Wformat-overflow and
-Wformat-truncation. When building the kernel, it was discovered that
clang's implementation of these warnings handles the '%p' specifier,
which differs from GCC's implementation. This results in false positive
warnings due to the kernel's various '%p' extensions. Fortunately, the
clang developers placed this warning difference into a separate flag,
allowing the kernel to turn off the warning for '%p' unconditionally.

This is not currently an issue for a normal build, as -Wformat-overflow
and -Wformat-truncation are unconditionally disabled, which includes
this sub-warning. However, ever since commit 6d4ab2e97dcf ("extrawarn:
enable format and stringop overflow warnings in W=1"), these warnings
are in W=1 and the goal is to enable them in the normal build once they
are all eliminated. Disable the warnings for W=1 to avoid false
positives. This block should move with -Wformat-overflow and
-Wformat-truncation when they are enabled for a normal build.

Link: https://github.com/ClangBuiltLinux/linux/issues/1923
Link: https://github.com/llvm/llvm-project/issues/64871
Link: https://github.com/llvm/llvm-project/pull/65969
Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111219
Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78512
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
---
 scripts/Makefile.extrawarn | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/scripts/Makefile.extrawarn b/scripts/Makefile.extrawarn
index 2fe6f2828d37..bad1baa2cfb4 100644
--- a/scripts/Makefile.extrawarn
+++ b/scripts/Makefile.extrawarn
@@ -106,6 +106,13 @@ KBUILD_CFLAGS += $(call cc-option, -Wunused-const-variable)
 KBUILD_CFLAGS += $(call cc-option, -Wpacked-not-aligned)
 KBUILD_CFLAGS += $(call cc-option, -Wformat-overflow)
 KBUILD_CFLAGS += $(call cc-option, -Wformat-truncation)
+# Clang checks for overflow/truncation with '%p', while GCC does not:
+# https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111219
+# The kernel has many extensions to '%p' that clang does not understand, so
+# always disable these warnings when '-Wformat-truncation' and
+# '-Wformat-overflow' are enabled.
+KBUILD_CFLAGS += $(call cc-disable-warning, format-overflow-non-kprintf)
+KBUILD_CFLAGS += $(call cc-disable-warning, format-truncation-non-kprintf)
 KBUILD_CFLAGS += $(call cc-option, -Wstringop-overflow)
 KBUILD_CFLAGS += $(call cc-option, -Wstringop-truncation)
 

---
base-commit: 8a749fd1a8720d4619c91c8b6e7528c0a355c0aa
change-id: 20231002-disable-wformat-truncation-overflow-non-kprintf-033e8d8b4de8

Best regards,
-- 
Nathan Chancellor <nathan@kernel.org>

Re: [PATCH] kbuild: Disable clang's -Wformat-{overflow,truncation}-non-kprintf

[Nick Desaulniers]

On Mon, Oct 2, 2023 at 12:52 PM Nathan Chancellor <nathan@kernel.org> wrote:
>
> Recently, clang added support for -Wformat-overflow and
> -Wformat-truncation. When building the kernel, it was discovered that
> clang's implementation of these warnings handles the '%p' specifier,
> which differs from GCC's implementation. This results in false positive
> warnings due to the kernel's various '%p' extensions. Fortunately, the
> clang developers placed this warning difference into a separate flag,
> allowing the kernel to turn off the warning for '%p' unconditionally.
>
> This is not currently an issue for a normal build, as -Wformat-overflow
> and -Wformat-truncation are unconditionally disabled, which includes
> this sub-warning. However, ever since commit 6d4ab2e97dcf ("extrawarn:
> enable format and stringop overflow warnings in W=1"), these warnings
> are in W=1 and the goal is to enable them in the normal build once they
> are all eliminated. Disable the warnings for W=1 to avoid false
> positives. This block should move with -Wformat-overflow and
> -Wformat-truncation when they are enabled for a normal build.
>
> Link: https://github.com/ClangBuiltLinux/linux/issues/1923
> Link: https://github.com/llvm/llvm-project/issues/64871
> Link: https://github.com/llvm/llvm-project/pull/65969
> Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111219
> Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78512
> Signed-off-by: Nathan Chancellor <nathan@kernel.org>

This should be the final piece of the puzzle...a long arduous puzzle.
Thanks Nathan!
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>

> ---
>  scripts/Makefile.extrawarn | 7 +++++++
>  1 file changed, 7 insertions(+)
>
> diff --git a/scripts/Makefile.extrawarn b/scripts/Makefile.extrawarn
> index 2fe6f2828d37..bad1baa2cfb4 100644
> --- a/scripts/Makefile.extrawarn
> +++ b/scripts/Makefile.extrawarn
> @@ -106,6 +106,13 @@ KBUILD_CFLAGS += $(call cc-option, -Wunused-const-variable)
>  KBUILD_CFLAGS += $(call cc-option, -Wpacked-not-aligned)
>  KBUILD_CFLAGS += $(call cc-option, -Wformat-overflow)
>  KBUILD_CFLAGS += $(call cc-option, -Wformat-truncation)
> +# Clang checks for overflow/truncation with '%p', while GCC does not:
> +# https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111219
> +# The kernel has many extensions to '%p' that clang does not understand, so
> +# always disable these warnings when '-Wformat-truncation' and
> +# '-Wformat-overflow' are enabled.
> +KBUILD_CFLAGS += $(call cc-disable-warning, format-overflow-non-kprintf)
> +KBUILD_CFLAGS += $(call cc-disable-warning, format-truncation-non-kprintf)
>  KBUILD_CFLAGS += $(call cc-option, -Wstringop-overflow)
>  KBUILD_CFLAGS += $(call cc-option, -Wstringop-truncation)
>
>
> ---
> base-commit: 8a749fd1a8720d4619c91c8b6e7528c0a355c0aa
> change-id: 20231002-disable-wformat-truncation-overflow-non-kprintf-033e8d8b4de8
>
> Best regards,
> --
> Nathan Chancellor <nathan@kernel.org>
>
>


-- 
Thanks,
~Nick Desaulniers

Re: [PATCH] kbuild: Disable clang's -Wformat-{overflow,truncation}-non-kprintf

[Nathan Chancellor]

On Mon, Oct 02, 2023 at 12:52:28PM -0700, Nathan Chancellor wrote:
> Recently, clang added support for -Wformat-overflow and
> -Wformat-truncation. When building the kernel, it was discovered that
> clang's implementation of these warnings handles the '%p' specifier,
> which differs from GCC's implementation. This results in false positive
> warnings due to the kernel's various '%p' extensions. Fortunately, the
> clang developers placed this warning difference into a separate flag,
> allowing the kernel to turn off the warning for '%p' unconditionally.
> 
> This is not currently an issue for a normal build, as -Wformat-overflow
> and -Wformat-truncation are unconditionally disabled, which includes
> this sub-warning. However, ever since commit 6d4ab2e97dcf ("extrawarn:
> enable format and stringop overflow warnings in W=1"), these warnings
> are in W=1 and the goal is to enable them in the normal build once they
> are all eliminated. Disable the warnings for W=1 to avoid false
> positives. This block should move with -Wformat-overflow and
> -Wformat-truncation when they are enabled for a normal build.
> 
> Link: https://github.com/ClangBuiltLinux/linux/issues/1923
> Link: https://github.com/llvm/llvm-project/issues/64871
> Link: https://github.com/llvm/llvm-project/pull/65969
> Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111219
> Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78512
> Signed-off-by: Nathan Chancellor <nathan@kernel.org>
> ---
>  scripts/Makefile.extrawarn | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/scripts/Makefile.extrawarn b/scripts/Makefile.extrawarn
> index 2fe6f2828d37..bad1baa2cfb4 100644
> --- a/scripts/Makefile.extrawarn
> +++ b/scripts/Makefile.extrawarn
> @@ -106,6 +106,13 @@ KBUILD_CFLAGS += $(call cc-option, -Wunused-const-variable)
>  KBUILD_CFLAGS += $(call cc-option, -Wpacked-not-aligned)
>  KBUILD_CFLAGS += $(call cc-option, -Wformat-overflow)
>  KBUILD_CFLAGS += $(call cc-option, -Wformat-truncation)
> +# Clang checks for overflow/truncation with '%p', while GCC does not:
> +# https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111219
> +# The kernel has many extensions to '%p' that clang does not understand, so
> +# always disable these warnings when '-Wformat-truncation' and
> +# '-Wformat-overflow' are enabled.
> +KBUILD_CFLAGS += $(call cc-disable-warning, format-overflow-non-kprintf)
> +KBUILD_CFLAGS += $(call cc-disable-warning, format-truncation-non-kprintf)
>  KBUILD_CFLAGS += $(call cc-option, -Wstringop-overflow)
>  KBUILD_CFLAGS += $(call cc-option, -Wstringop-truncation)
>  
> 
> ---
> base-commit: 8a749fd1a8720d4619c91c8b6e7528c0a355c0aa
> change-id: 20231002-disable-wformat-truncation-overflow-non-kprintf-033e8d8b4de8
> 
> Best regards,
> -- 
> Nathan Chancellor <nathan@kernel.org>
> 

I am going through my outstanding patches and I noticed this was not
picked up. Masahiro, would you like me to resend it? It still applies
cleanly to the current kbuild tree, although there will be a conflict in
-next because of commit 89741e7e42f6 ("Makefile: Enable
-Wstringop-overflow globally"), but there is not much that can be done
about that.

Cheers,
Nathan