Commit 27758d8c2583 ("kbuild: enable -Werror for hostprogs")
unconditionally enabled -Werror for the compiler, assembler, and linker
when building the host programs, as the build footprint of the host
programs is small (thus risk of build failures from warnings are low)
and that stage of the build may not have Kconfig values (thus
CONFIG_WERROR could not be used as a precondition).

While turning warnings into errors unconditionally happens in a few
places within the kernel, it can be disruptive to people who may be
building with newer compilers, such as while doing a bisect. While it is
possible to avoid this behavior by passing HOSTCFLAGS=-w or
HOSTCFLAGS=-Wno-error, it may not be the most intuitive for regular
users not intimately familiar with Kbuild.

Avoid being disruptive to the entire build by depending on the explicit
opt-in of CONFIG_WERROR or W=e to enable -Werror and the like while
building the host programs. While this means there is a small portion of
the build that does not have -Werror enabled (namely scripts/kconfig/*
and scripts/basic/fixdep), it is better than not having it altogether.

Fixes: 27758d8c2583 ("kbuild: enable -Werror for hostprogs")
Link: https://lore.kernel.org/20251005011100.1035272-1-safinaskar@gmail.com/
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
---
 scripts/Makefile.extrawarn | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/scripts/Makefile.extrawarn b/scripts/Makefile.extrawarn
index 1434cb6208cb..6af392f9cd02 100644
--- a/scripts/Makefile.extrawarn
+++ b/scripts/Makefile.extrawarn
@@ -223,9 +223,11 @@ KBUILD_USERCFLAGS	+= -Werror
 KBUILD_USERLDFLAGS	+= -Wl,--fatal-warnings
 KBUILD_RUSTFLAGS	+= -Dwarnings
 
-endif
-
-# Hostprog flags are used during build bootstrapping and can not rely on CONFIG_ symbols.
+# While hostprog flags are used during build bootstrapping (thus should not
+# depend on CONFIG_ symbols), -Werror is disruptive and should be opted into.
+# Only apply -Werror to hostprogs built after the initial Kconfig stage.
 KBUILD_HOSTCFLAGS	+= -Werror
 KBUILD_HOSTLDFLAGS	+= -Wl,--fatal-warnings
 KBUILD_HOSTRUSTFLAGS	+= -Dwarnings
+
+endif

---
base-commit: 2ea77fca84f07849aa995271271340d262d0c2e9
change-id: 20251006-kbuild-hostprogs-werror-fix-be6a3e123dfc

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

On Mon, 06 Oct 2025 14:39:56 -0700, Nathan Chancellor wrote:
> Commit 27758d8c2583 ("kbuild: enable -Werror for hostprogs")
> unconditionally enabled -Werror for the compiler, assembler, and linker
> when building the host programs, as the build footprint of the host
> programs is small (thus risk of build failures from warnings are low)
> and that stage of the build may not have Kconfig values (thus
> CONFIG_WERROR could not be used as a precondition).
> 
> [...]

Applied, thanks!

[1/1] scripts/Makefile.extrawarn: Respect CONFIG_WERROR / W=e for hostprogs
      https://git.kernel.org/kbuild/c/7ded7d37e5f5b

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

On Mon, Oct 6, 2025 at 11:40 PM Nathan Chancellor <nathan@kernel.org> wrote:
>
> Commit 27758d8c2583 ("kbuild: enable -Werror for hostprogs")
> unconditionally enabled -Werror for the compiler, assembler, and linker
> when building the host programs, as the build footprint of the host
> programs is small (thus risk of build failures from warnings are low)
> and that stage of the build may not have Kconfig values (thus
> CONFIG_WERROR could not be used as a precondition).
>
> While turning warnings into errors unconditionally happens in a few
> places within the kernel, it can be disruptive to people who may be
> building with newer compilers, such as while doing a bisect. While it is
> possible to avoid this behavior by passing HOSTCFLAGS=-w or
> HOSTCFLAGS=-Wno-error, it may not be the most intuitive for regular
> users not intimately familiar with Kbuild.
>
> Avoid being disruptive to the entire build by depending on the explicit
> opt-in of CONFIG_WERROR or W=e to enable -Werror and the like while
> building the host programs. While this means there is a small portion of
> the build that does not have -Werror enabled (namely scripts/kconfig/*
> and scripts/basic/fixdep), it is better than not having it altogether.
>
> Fixes: 27758d8c2583 ("kbuild: enable -Werror for hostprogs")
> Link: https://lore.kernel.org/20251005011100.1035272-1-safinaskar@gmail.com/
> Signed-off-by: Nathan Chancellor <nathan@kernel.org>

Acked-by: Miguel Ojeda <ojeda@kernel.org>

Cheers,
Miguel

On Tue, Oct 7, 2025 at 6:00 PM Miguel Ojeda
<miguel.ojeda.sandonis@gmail.com> wrote:
>
> Acked-by: Miguel Ojeda <ojeda@kernel.org>

I also tested that for Rust hostprogs it now respects `WERROR` and
`W=e` as expected:

Tested-by: Miguel Ojeda <ojeda@kernel.org> # Rust

Thanks!

Cheers,
Miguel

On Mon, Oct 06, 2025 at 02:39:56PM -0700, Nathan Chancellor wrote:
> Commit 27758d8c2583 ("kbuild: enable -Werror for hostprogs")
> unconditionally enabled -Werror for the compiler, assembler, and linker
> when building the host programs, as the build footprint of the host
> programs is small (thus risk of build failures from warnings are low)
> and that stage of the build may not have Kconfig values (thus
> CONFIG_WERROR could not be used as a precondition).
> 
> While turning warnings into errors unconditionally happens in a few
> places within the kernel, it can be disruptive to people who may be
> building with newer compilers, such as while doing a bisect. While it is
> possible to avoid this behavior by passing HOSTCFLAGS=-w or
> HOSTCFLAGS=-Wno-error, it may not be the most intuitive for regular
> users not intimately familiar with Kbuild.
> 
> Avoid being disruptive to the entire build by depending on the explicit
> opt-in of CONFIG_WERROR or W=e to enable -Werror and the like while
> building the host programs. While this means there is a small portion of
> the build that does not have -Werror enabled (namely scripts/kconfig/*
> and scripts/basic/fixdep), it is better than not having it altogether.
> 
> Fixes: 27758d8c2583 ("kbuild: enable -Werror for hostprogs")
> Link: https://lore.kernel.org/20251005011100.1035272-1-safinaskar@gmail.com/

Reported-by: Askar Safin <safinaskar@gmail.com>
Closes: https://lore.kernel.org/20251005011100.1035272-1-safinaskar@gmail.com/

> Signed-off-by: Nathan Chancellor <nathan@kernel.org>

Reviewed-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>


Thanks and sorry for the trouble!

> ---
>  scripts/Makefile.extrawarn | 8 +++++---
>  1 file changed, 5 insertions(+), 3 deletions(-)
> 
> diff --git a/scripts/Makefile.extrawarn b/scripts/Makefile.extrawarn
> index 1434cb6208cb..6af392f9cd02 100644
> --- a/scripts/Makefile.extrawarn
> +++ b/scripts/Makefile.extrawarn
> @@ -223,9 +223,11 @@ KBUILD_USERCFLAGS	+= -Werror
>  KBUILD_USERLDFLAGS	+= -Wl,--fatal-warnings
>  KBUILD_RUSTFLAGS	+= -Dwarnings
>  
> -endif
> -
> -# Hostprog flags are used during build bootstrapping and can not rely on CONFIG_ symbols.
> +# While hostprog flags are used during build bootstrapping (thus should not
> +# depend on CONFIG_ symbols), -Werror is disruptive and should be opted into.
> +# Only apply -Werror to hostprogs built after the initial Kconfig stage.
>  KBUILD_HOSTCFLAGS	+= -Werror
>  KBUILD_HOSTLDFLAGS	+= -Wl,--fatal-warnings
>  KBUILD_HOSTRUSTFLAGS	+= -Dwarnings
> +
> +endif
> 
> ---
> base-commit: 2ea77fca84f07849aa995271271340d262d0c2e9
> change-id: 20251006-kbuild-hostprogs-werror-fix-be6a3e123dfc
> 
> Best regards,
> --  
> Nathan Chancellor <nathan@kernel.org>
>