Re: [PATCH v2 6/6] kbuild: enable -Werror for hostprogs

[Nathan Chancellor <nathan@kernel.org>]

Hi Askar,

On Sun, Oct 05, 2025 at 04:10:47AM +0300, Askar Safin wrote:
> "Thomas Weißschuh" <thomas.weissschuh@linutronix.de>:
> > Enable -Werror unconditionally
> 
> Please, drop this patch.

This is already merged into Linus's tree so it cannot be dropped, it
would need to be reverted. However...

> Never enable -Werror unconditionally for any part of build.
> 
> I often bisect Linux. And to do this, I often need to build very old
> Linux commits (with modern compiler). Unconditional -Werror will make
> this impossible.
> 
> For example, recently I found this regression:
> 
> https://lore.kernel.org/regressions/197f290e30b.eaadc7bc7913.7315623184036672946@zohomail.com/T/#u
> 
> The regression caused by commits happened in 2019.
> 
> So to bisect it, I had to build 2019 trees using modern compiler.

I do my fair share of bisecting old Linux trees with a modern compiler,
so I do understand that pain. While it is easy enough to avoid this
behavior with HOSTCFLAGS=-Wno-error or HOSTCFLAGS=-w and there are other
places that the kernel enables -Werror unconditionally, I do not want
this to be a major pain point for random people doing bisected,
especially when the host tools are fairly battle tested so warnings may
not be a big deal.

In looking further into this, I noticed that in its current state,
-Werror is not getting applied to scripts/basic/fixup or
scripts/kconfig/*.o files (seems $(include-y) happens to late?), which
was one of the reasons to avoid making it depend on CONFIG_WERROR or
W=e. If that's the case, we could probably make it opt in like the rest
of the warnings for the kernel, which should be a fair compromise
between wanting to make potential issues more obvious while not
impacting people who build old sources with new compilers.

Thomas and Nicolas, thoughts?

Cheers,
Nathan

diff --git a/scripts/Makefile.extrawarn b/scripts/Makefile.extrawarn
index 1434cb6208cb..1a29598cf7cb 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 can not use on
+# CONFIG_ symbols), -Werror should be opted into, so only apply -Werror to
+# hostprogs built after the initial Kconfig bootstrap.
 KBUILD_HOSTCFLAGS	+= -Werror
 KBUILD_HOSTLDFLAGS	+= -Wl,--fatal-warnings
 KBUILD_HOSTRUSTFLAGS	+= -Dwarnings
+
+endif