[PATCH v2 0/6] kbuild: enable CONFIG_WERROR for more build steps

[PATCH v2 0/6] kbuild: enable CONFIG_WERROR for more build steps

[Thomas Weißschuh]

CONFIG_WERROR is useful for all build steps, not only compilation of C and
Rust sources linked into the kernel.

Also enable it for assembler and linker invocations and userprogs.

In addition unify the implementations of CONFIG_WERROR and W=e.

This partially implements features of Miguel's Rust -Werror series [0].
The changes to $(common_rust_flags) are not incorporated as it also modifies
Rust hostprog flags.

[0] https://lore.kernel.org/rust-for-linux/20240519211235.589325-2-ojeda@kernel.org

Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
---
Changes in v2:
- Unify CONFIG_WERROR with W=e
- Unconditionally enable -Werror for C and Rust hostprogs
- Link to v1: https://lore.kernel.org/r/20250812-kbuild-werror-v1-0-36c9ff653700@linutronix.de

---
Miguel Ojeda (1):
      kbuild: rust: move `-Dwarnings` handling to `Makefile.extrawarn`

Thomas Weißschuh (5):
      kbuild: align W=e with CONFIG_WERROR
      kbuild: unify W=e and CONFIG_WERROR
      kbuild: respect CONFIG_WERROR for linker and assembler
      kbuild: respect CONFIG_WERROR for userprogs
      kbuild: enable -Werror for hostprogs

 Makefile                   |  3 ---
 scripts/Makefile.extrawarn | 18 +++++++++++++-----
 2 files changed, 13 insertions(+), 8 deletions(-)
---
base-commit: e8ee794a1f7705edd64b9eb9837e7baa4421c084
change-id: 20250801-kbuild-werror-081f72fee1de

Best regards,
-- 
Thomas Weißschuh <thomas.weissschuh@linutronix.de>

[PATCH v2 1/6] kbuild: align W=e with CONFIG_WERROR

[Thomas Weißschuh]

CONFIG_WERROR sets KBUILD_CPPFLAGS while W=e would only set KBUILD_CFLAGS.

As a preparation to unify the two mechanism, align their effects.

While at it, add some alignment whitespace to prepare for later additions
to the list of changed variables.

Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
---
 scripts/Makefile.extrawarn | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/scripts/Makefile.extrawarn b/scripts/Makefile.extrawarn
index dca175fffcabeb2d942814f0a096a7efbf8bc819..4b4e8e136ce6b45f43dc7658eda78ae2b37bd792 100644
--- a/scripts/Makefile.extrawarn
+++ b/scripts/Makefile.extrawarn
@@ -218,6 +218,6 @@ endif
 #
 ifneq ($(findstring e, $(KBUILD_EXTRA_WARN)),)
 
-KBUILD_CFLAGS += -Werror
+KBUILD_CPPFLAGS		+= -Werror
 
 endif

-- 
2.50.1

[PATCH v2 2/6] kbuild: unify W=e and CONFIG_WERROR

[Thomas Weißschuh]

The two mechanisms have the same effect, unify their implementation.

Also avoid spurious rebuilds when switching between the two.

Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
---
 scripts/Makefile.extrawarn | 6 ++----
 1 file changed, 2 insertions(+), 4 deletions(-)

diff --git a/scripts/Makefile.extrawarn b/scripts/Makefile.extrawarn
index 4b4e8e136ce6b45f43dc7658eda78ae2b37bd792..1ffc7beca43bc9e373064194dc8975182815a872 100644
--- a/scripts/Makefile.extrawarn
+++ b/scripts/Makefile.extrawarn
@@ -25,8 +25,6 @@ ifneq ($(CONFIG_FRAME_WARN),0)
 KBUILD_CFLAGS += -Wframe-larger-than=$(CONFIG_FRAME_WARN)
 endif
 
-KBUILD_CPPFLAGS-$(CONFIG_WERROR) += -Werror
-KBUILD_CPPFLAGS += $(KBUILD_CPPFLAGS-y)
 KBUILD_CFLAGS-$(CONFIG_CC_NO_ARRAY_BOUNDS) += -Wno-array-bounds
 
 ifdef CONFIG_CC_IS_CLANG
@@ -214,9 +212,9 @@ KBUILD_CFLAGS += -Wno-unused-parameter
 endif
 
 #
-# W=e - error out on warnings
+# W=e and CONFIG_WERROR - error out on warnings
 #
-ifneq ($(findstring e, $(KBUILD_EXTRA_WARN)),)
+ifneq ($(findstring e, $(KBUILD_EXTRA_WARN))$(CONFIG_WERROR),)
 
 KBUILD_CPPFLAGS		+= -Werror
 

-- 
2.50.1

[PATCH v2 3/6] kbuild: rust: move `-Dwarnings` handling to `Makefile.extrawarn`

[Thomas Weißschuh]

From: Miguel Ojeda <ojeda@kernel.org>

Following commit e88ca24319e4 ("kbuild: consolidate warning flags
in scripts/Makefile.extrawarn"), move `-Dwarnings` handling into
`Makefile.extrawarn` like C's `-Werror`.

No functional change intended.

Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
---
 Makefile                   | 3 ---
 scripts/Makefile.extrawarn | 1 +
 2 files changed, 1 insertion(+), 3 deletions(-)

diff --git a/Makefile b/Makefile
index 6bfe776bf3c5ff0cf187dc6719dd5817cd4af2ca..6630a66659c6d1ca7e72448b04d16c3fdd1fbe24 100644
--- a/Makefile
+++ b/Makefile
@@ -901,9 +901,6 @@ stackp-flags-$(CONFIG_STACKPROTECTOR_STRONG)      := -fstack-protector-strong
 
 KBUILD_CFLAGS += $(stackp-flags-y)
 
-KBUILD_RUSTFLAGS-$(CONFIG_WERROR) += -Dwarnings
-KBUILD_RUSTFLAGS += $(KBUILD_RUSTFLAGS-y)
-
 ifdef CONFIG_FRAME_POINTER
 KBUILD_CFLAGS	+= -fno-omit-frame-pointer -fno-optimize-sibling-calls
 KBUILD_RUSTFLAGS += -Cforce-frame-pointers=y
diff --git a/scripts/Makefile.extrawarn b/scripts/Makefile.extrawarn
index 1ffc7beca43bc9e373064194dc8975182815a872..b04b3062e0e49c3ae7471973251017e8738cd244 100644
--- a/scripts/Makefile.extrawarn
+++ b/scripts/Makefile.extrawarn
@@ -217,5 +217,6 @@ endif
 ifneq ($(findstring e, $(KBUILD_EXTRA_WARN))$(CONFIG_WERROR),)
 
 KBUILD_CPPFLAGS		+= -Werror
+KBUILD_RUSTFLAGS	+= -Dwarnings
 
 endif

-- 
2.50.1

[PATCH v2 4/6] kbuild: respect CONFIG_WERROR for linker and assembler

[Thomas Weißschuh]

The linker and assembler do not share the compiler flags.

Make sure they also fail on warnings with CONFIG_WERROR and W=e.

Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
---
 scripts/Makefile.extrawarn | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/scripts/Makefile.extrawarn b/scripts/Makefile.extrawarn
index b04b3062e0e49c3ae7471973251017e8738cd244..6efb9e5eeeed0696a061a17e029a92caba50826f 100644
--- a/scripts/Makefile.extrawarn
+++ b/scripts/Makefile.extrawarn
@@ -217,6 +217,8 @@ endif
 ifneq ($(findstring e, $(KBUILD_EXTRA_WARN))$(CONFIG_WERROR),)
 
 KBUILD_CPPFLAGS		+= -Werror
+KBUILD_AFLAGS		+= -Wa,--fatal-warnings
+KBUILD_LDFLAGS		+= --fatal-warnings
 KBUILD_RUSTFLAGS	+= -Dwarnings
 
 endif

-- 
2.50.1

[PATCH v2 5/6] kbuild: respect CONFIG_WERROR for userprogs

[Thomas Weißschuh]

The userprogs compiler and linker do not share the regular compiler flags.

Make sure they also fail on warnings with CONFIG_WERROR and W=e.

Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
---
 scripts/Makefile.extrawarn | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/scripts/Makefile.extrawarn b/scripts/Makefile.extrawarn
index 6efb9e5eeeed0696a061a17e029a92caba50826f..96ff3f5582d651f0016c8ccd49d46022cc6fe070 100644
--- a/scripts/Makefile.extrawarn
+++ b/scripts/Makefile.extrawarn
@@ -219,6 +219,8 @@ ifneq ($(findstring e, $(KBUILD_EXTRA_WARN))$(CONFIG_WERROR),)
 KBUILD_CPPFLAGS		+= -Werror
 KBUILD_AFLAGS		+= -Wa,--fatal-warnings
 KBUILD_LDFLAGS		+= --fatal-warnings
+KBUILD_USERCFLAGS	+= -Werror
+KBUILD_USERLDFLAGS	+= -Wl,--fatal-warnings
 KBUILD_RUSTFLAGS	+= -Dwarnings
 
 endif

-- 
2.50.1

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

[Thomas Weißschuh]

The hostprog compilers and linkers do not share the regular compiler flags,
so they are not affected by CONFIG_WERROR or W=e. As hostprogs are used
during the bootstrap of the build, they can't depend on kconfig options.

Enable -Werror unconditionally.

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

---
For testing in -next, as discussed in
https://lore.kernel.org/lkml/20250812-kbuild-werror-v1-2-36c9ff653700@linutronix.de/
---
 scripts/Makefile.extrawarn | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/scripts/Makefile.extrawarn b/scripts/Makefile.extrawarn
index 96ff3f5582d651f0016c8ccd49d46022cc6fe070..1434cb6208cb82f20aeb29cc8c059220d1a1f6d2 100644
--- a/scripts/Makefile.extrawarn
+++ b/scripts/Makefile.extrawarn
@@ -224,3 +224,8 @@ KBUILD_USERLDFLAGS	+= -Wl,--fatal-warnings
 KBUILD_RUSTFLAGS	+= -Dwarnings
 
 endif
+
+# Hostprog flags are used during build bootstrapping and can not rely on CONFIG_ symbols.
+KBUILD_HOSTCFLAGS	+= -Werror
+KBUILD_HOSTLDFLAGS	+= -Wl,--fatal-warnings
+KBUILD_HOSTRUSTFLAGS	+= -Dwarnings

-- 
2.50.1

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

[Nicolas Schier]

On Thu, Aug 14, 2025 at 12:14:46PM +0200, Thomas Weißschuh wrote:
> The hostprog compilers and linkers do not share the regular compiler flags,
> so they are not affected by CONFIG_WERROR or W=e. As hostprogs are used
> during the bootstrap of the build, they can't depend on kconfig options.
> 
> Enable -Werror unconditionally.
> 
> Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
> 
> ---
> For testing in -next, as discussed in
> https://lore.kernel.org/lkml/20250812-kbuild-werror-v1-2-36c9ff653700@linutronix.de/

yeah, I am not sure if this is a good change, but I am ok with testing
it in next, as Nathan suggested.

For the rest of the series:

Reviewed-by: Nicolas Schier <nsc@kernel.org>

Thanks.

> ---
>  scripts/Makefile.extrawarn | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/scripts/Makefile.extrawarn b/scripts/Makefile.extrawarn
> index 96ff3f5582d651f0016c8ccd49d46022cc6fe070..1434cb6208cb82f20aeb29cc8c059220d1a1f6d2 100644
> --- a/scripts/Makefile.extrawarn
> +++ b/scripts/Makefile.extrawarn
> @@ -224,3 +224,8 @@ KBUILD_USERLDFLAGS	+= -Wl,--fatal-warnings
>  KBUILD_RUSTFLAGS	+= -Dwarnings
>  
>  endif
> +
> +# Hostprog flags are used during build bootstrapping and can not rely on CONFIG_ symbols.
> +KBUILD_HOSTCFLAGS	+= -Werror
> +KBUILD_HOSTLDFLAGS	+= -Wl,--fatal-warnings
> +KBUILD_HOSTRUSTFLAGS	+= -Dwarnings
> 
> -- 
> 2.50.1
> 

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

[Nathan Chancellor]

On Thu, Aug 14, 2025 at 02:10:13PM +0200, Nicolas Schier wrote:
> On Thu, Aug 14, 2025 at 12:14:46PM +0200, Thomas Weißschuh wrote:
> > The hostprog compilers and linkers do not share the regular compiler flags,
> > so they are not affected by CONFIG_WERROR or W=e. As hostprogs are used
> > during the bootstrap of the build, they can't depend on kconfig options.
> > 
> > Enable -Werror unconditionally.
> > 
> > Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
> > 
> > ---
> > For testing in -next, as discussed in
> > https://lore.kernel.org/lkml/20250812-kbuild-werror-v1-2-36c9ff653700@linutronix.de/
> 
> yeah, I am not sure if this is a good change, but I am ok with testing
> it in next, as Nathan suggested.

Yeah, I will leave this as the top patch in kbuild-next for now so that
we can easily back out if it is disruptive.

> For the rest of the series:
> 
> Reviewed-by: Nicolas Schier <nsc@kernel.org>

I applied this tag to patches 1 through 5 and left it off of this one,
based on "for the rest of the series". Let me know if that was
unintentional.

Cheers,
Nathan

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

[Miguel Ojeda]

On Thu, Aug 14, 2025 at 12:15 PM Thomas Weißschuh
<thomas.weissschuh@linutronix.de> wrote:
>
> The hostprog compilers and linkers do not share the regular compiler flags,
> so they are not affected by CONFIG_WERROR or W=e. As hostprogs are used
> during the bootstrap of the build, they can't depend on kconfig options.
>
> Enable -Werror unconditionally.
>
> Signed-off-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>
>
> ---
> For testing in -next, as discussed in
> https://lore.kernel.org/lkml/20250812-kbuild-werror-v1-2-36c9ff653700@linutronix.de/

The discussion was on patch 0, i.e. the next message in Lore's tree
view -- I would suggest adding it to the commit message for reference:

Link: https://lore.kernel.org/rust-for-linux/CANiq72k-PdSH2BNgbq=X+FhpyEErifSCKfO5ObXz6bu9_J8+fA@mail.gmail.com/

Anyway, since Nathan already applied it, and I see a couple patches on
top, no big deal if it cannot be added.

Thanks!

Cheers,
Miguel

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

[Nathan Chancellor]

On Sun, Aug 17, 2025 at 06:31:06PM +0200, Miguel Ojeda wrote:
> The discussion was on patch 0, i.e. the next message in Lore's tree
> view -- I would suggest adding it to the commit message for reference:
> 
> Link: https://lore.kernel.org/rust-for-linux/CANiq72k-PdSH2BNgbq=X+FhpyEErifSCKfO5ObXz6bu9_J8+fA@mail.gmail.com/
> 
> Anyway, since Nathan already applied it, and I see a couple patches on
> top, no big deal if it cannot be added.

I dropped the top patch due to a build failure:

https://lore.kernel.org/20250818140143.61b8c466@canb.auug.org.au/

So I was okay with just rebasing the top commit to amend this one,
especially when giving more context to the patch. Thanks for the
suggestion!

Cheers,
Nathan

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

[Askar Safin]

"Thomas Weißschuh" <thomas.weissschuh@linutronix.de>:
> Enable -Werror unconditionally

Please, drop this patch.

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.

-- 
Askar Safin

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

[Sam James]

I tried to make this argument recently for libbpf but was unsuccessful:
https://lore.kernel.org/bpf/7e6c41e47c6a8ab73945e6aac319e0dd53337e1b.1751712192.git.sam@gentoo.org/

I agree, though.

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

[Nathan Chancellor]

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

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

[Thomas Weißschuh]

On Sun, Oct 05, 2025 at 01:53:35PM -0700, Nathan Chancellor wrote:

(...)

> 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?

Yeah, fixdep and script/kconfig seem to be specifically handled in the
'ifdef config-build' block in Makefile. And this happens explicitly
before 'include .../auto.conf'. So your diff below looks like the
correct solution. Thanks for taking a look!


Thomas

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

[Miguel Ojeda]

On Sun, Oct 5, 2025 at 10:53 PM Nathan Chancellor <nathan@kernel.org> wrote:
>
> Thomas and Nicolas, thoughts?

I think it would be good to eventually split the hostprog flags into 2
sets: the ones that are used for bootstrapping, and the ones that go
after that.

That way, we can get the best of both worlds, if I understand correctly.

Cheers,
Miguel

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

[Nathan Chancellor]

On Mon, Oct 06, 2025 at 01:27:33PM +0200, Miguel Ojeda wrote:
> On Sun, Oct 5, 2025 at 10:53 PM Nathan Chancellor <nathan@kernel.org> wrote:
> >
> > Thomas and Nicolas, thoughts?
> 
> I think it would be good to eventually split the hostprog flags into 2
> sets: the ones that are used for bootstrapping, and the ones that go
> after that.
> 
> That way, we can get the best of both worlds, if I understand correctly.

Does that buy us much? We would not want -Werror applied to
scripts/basic or scripts/kconfig without consent from the user. W=e
could provide that at that stage of the build (with an adjustment of
where scripts/Makefile.extrawarn was included) but CONFIG_WERROR would
not be.

Given how frequently fixdep and kconfig are built, I am fairly confident
that new warnings wtihin them would be reported quickly, even without
-Werror, so I do not see it as too much of a loss. All other host
programs should be covered by that diff.

It may still be worth doing for other reasons but I would have to see
what such a change would actually look like in practice before
committing to it.

Cheers,
Nathan

Re: [PATCH v2 0/6] kbuild: enable CONFIG_WERROR for more build steps

[Nathan Chancellor]

On Thu, 14 Aug 2025 12:14:40 +0200, Thomas Weißschuh wrote:
> CONFIG_WERROR is useful for all build steps, not only compilation of C and
> Rust sources linked into the kernel.
> 
> Also enable it for assembler and linker invocations and userprogs.
> 
> In addition unify the implementations of CONFIG_WERROR and W=e.
> 
> [...]

Applied, thanks!

[1/6] kbuild: align W=e with CONFIG_WERROR
      https://git.kernel.org/kbuild/c/f852ce052a8b7
[2/6] kbuild: unify W=e and CONFIG_WERROR
      https://git.kernel.org/kbuild/c/e7a10929c574c
[3/6] kbuild: rust: move `-Dwarnings` handling to `Makefile.extrawarn`
      https://git.kernel.org/kbuild/c/592b571f20c5b
[4/6] kbuild: respect CONFIG_WERROR for linker and assembler
      https://git.kernel.org/kbuild/c/ec4a3992bc0b5
[5/6] kbuild: respect CONFIG_WERROR for userprogs
      https://git.kernel.org/kbuild/c/3f0ff4cc6ffb4
[6/6] kbuild: enable -Werror for hostprogs
      https://git.kernel.org/kbuild/c/a646e96080cd5

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