[PATCH] kbuild: enable unused-function warnings for W= build with Clang

[PATCH] kbuild: enable unused-function warnings for W= build with Clang

[Masahiro Yamada]

GCC and Clang have different policy for -Wunused-function; GCC does
not report unused-function warnings at all for the functions marked
as 'static inline'. Clang does report unused-function warnings if they
are defined in source files instead of headers.

We could use Clang for detecting unused functions, but it has been
suppressed since commit abb2ea7dfd82 ("compiler, clang: suppress
warning for unused static inline functions").

So, we never notice left-over code if functions in .c files are
marked as inline.

Let's remove __maybe_unused from the inline macro. As always, it is
not a good idea to sprinkle warnings for the normal build. So, these
warnings will be shown for the W= build.

If you contribute to code clean-up, please run "make CC=clang W=1"
and check -Wunused-function warnings. You will find lots of unused
functions.

Some of them are false-positives because the call-sites are disabled
by #ifdef. I do not like to abuse the inline keyword for suppressing
unused-function warnings because it might affect the compiler's
optimization. When I need to fix unused-functions warnings, I prefer
adding #ifdef or __maybe_unused to function definitions.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
---

 include/linux/compiler_types.h | 10 ++--------
 scripts/Makefile.extrawarn     |  1 +
 2 files changed, 3 insertions(+), 8 deletions(-)

diff --git a/include/linux/compiler_types.h b/include/linux/compiler_types.h
index 599c27b56c29..14de8d0162fb 100644
--- a/include/linux/compiler_types.h
+++ b/include/linux/compiler_types.h
@@ -130,10 +130,6 @@ struct ftrace_likely_data {
 
 /*
  * Force always-inline if the user requests it so via the .config.
- * GCC does not warn about unused static inline functions for
- * -Wunused-function.  This turns out to avoid the need for complex #ifdef
- * directives.  Suppress the warning in clang as well by using "unused"
- * function attribute, which is redundant but not harmful for gcc.
  * Prefer gnu_inline, so that extern inline functions do not emit an
  * externally visible function. This makes extern inline behave as per gnu89
  * semantics rather than c99. This prevents multiple symbol definition errors
@@ -143,11 +139,9 @@ struct ftrace_likely_data {
  * (which would break users of __always_inline).
  */
 #if !defined(CONFIG_OPTIMIZE_INLINING)
-#define inline inline __attribute__((__always_inline__)) __gnu_inline \
-	__maybe_unused notrace
+#define inline inline __attribute__((__always_inline__)) __gnu_inline notrace
 #else
-#define inline inline                                    __gnu_inline \
-	__maybe_unused notrace
+#define inline inline                                    __gnu_inline notrace
 #endif
 
 #define __inline__ inline
diff --git a/scripts/Makefile.extrawarn b/scripts/Makefile.extrawarn
index a74ce2e3c33e..92f542797e03 100644
--- a/scripts/Makefile.extrawarn
+++ b/scripts/Makefile.extrawarn
@@ -70,5 +70,6 @@ KBUILD_CFLAGS += -Wno-initializer-overrides
 KBUILD_CFLAGS += -Wno-format
 KBUILD_CFLAGS += -Wno-sign-compare
 KBUILD_CFLAGS += -Wno-format-zero-length
+KBUILD_CFLAGS += -Wno-unused-function
 endif
 endif
-- 
2.17.1

Re: [PATCH] kbuild: enable unused-function warnings for W= build with Clang

[Nathan Chancellor]

On Mon, Aug 19, 2019 at 07:51:38PM +0900, Masahiro Yamada wrote:
> GCC and Clang have different policy for -Wunused-function; GCC does
> not report unused-function warnings at all for the functions marked
> as 'static inline'. Clang does report unused-function warnings if they
> are defined in source files instead of headers.
> 
> We could use Clang for detecting unused functions, but it has been
> suppressed since commit abb2ea7dfd82 ("compiler, clang: suppress
> warning for unused static inline functions").
> 
> So, we never notice left-over code if functions in .c files are
> marked as inline.
> 
> Let's remove __maybe_unused from the inline macro. As always, it is
> not a good idea to sprinkle warnings for the normal build. So, these
> warnings will be shown for the W= build.
> 
> If you contribute to code clean-up, please run "make CC=clang W=1"
> and check -Wunused-function warnings. You will find lots of unused
> functions.
> 
> Some of them are false-positives because the call-sites are disabled
> by #ifdef. I do not like to abuse the inline keyword for suppressing
> unused-function warnings because it might affect the compiler's
> optimization. When I need to fix unused-functions warnings, I prefer
> adding #ifdef or __maybe_unused to function definitions.
> 
> Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

So if I understand everything correctly, this change allows us to start
finding unused static inline functions with clang at W=1 but disables
-Wunused-function by default... I am not sure that is a good tradeoff
as I am pretty sure that W=1 is fairly noisy for clang although I
haven't checked lately. I'd argue most regular developers do not build
with W=1 meaning -Wunused-function generally will not be run with clang
at all, missing stuff like this:

https://lore.kernel.org/lkml/20190523010235.GA105588@archlinux-epyc/

https://lore.kernel.org/lkml/1558574945-19275-1-git-send-email-skomatineni@nvidia.com/

Furthermore, per the documemtation [1], -Wno-unused-function will also
disable -Wunneeded-internal-declaration, which can help find bugs like
commit 8289c4b6f2e5 ("platform/x86: mlx-platform: Properly use
mlxplat_mlxcpld_msn201x_items").

[1]: https://clang.llvm.org/docs/DiagnosticsReference.html#wunused-function

Is there a way to conditionally remove __maybe_unused from the inline
defintion so that we keep the current behavior but we can still
selectively find potentially unused functions?

Cheers,
Nathan

Re: [PATCH] kbuild: enable unused-function warnings for W= build with Clang

[Masahiro Yamada]

Hi Nathan,

On Tue, Aug 20, 2019 at 1:09 AM Nathan Chancellor
<natechancellor@gmail.com> wrote:
>
> On Mon, Aug 19, 2019 at 07:51:38PM +0900, Masahiro Yamada wrote:
> > GCC and Clang have different policy for -Wunused-function; GCC does
> > not report unused-function warnings at all for the functions marked
> > as 'static inline'. Clang does report unused-function warnings if they
> > are defined in source files instead of headers.
> >
> > We could use Clang for detecting unused functions, but it has been
> > suppressed since commit abb2ea7dfd82 ("compiler, clang: suppress
> > warning for unused static inline functions").
> >
> > So, we never notice left-over code if functions in .c files are
> > marked as inline.
> >
> > Let's remove __maybe_unused from the inline macro. As always, it is
> > not a good idea to sprinkle warnings for the normal build. So, these
> > warnings will be shown for the W= build.
> >
> > If you contribute to code clean-up, please run "make CC=clang W=1"
> > and check -Wunused-function warnings. You will find lots of unused
> > functions.
> >
> > Some of them are false-positives because the call-sites are disabled
> > by #ifdef. I do not like to abuse the inline keyword for suppressing
> > unused-function warnings because it might affect the compiler's
> > optimization. When I need to fix unused-functions warnings, I prefer
> > adding #ifdef or __maybe_unused to function definitions.
> >
> > Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
>
> So if I understand everything correctly, this change allows us to start
> finding unused static inline functions with clang at W=1 but disables
> -Wunused-function by default... I am not sure that is a good tradeoff
> as I am pretty sure that W=1 is fairly noisy for clang although I
> haven't checked lately. I'd argue most regular developers do not build
> with W=1 meaning -Wunused-function generally will not be run with clang
> at all, missing stuff like this:


Try "git log --grep=W=1"

Some people are making efforts to fix W=1 warnings.
I believe somebody will start to remove unused static inline functions.



>
> https://lore.kernel.org/lkml/20190523010235.GA105588@archlinux-epyc/
>
> https://lore.kernel.org/lkml/1558574945-19275-1-git-send-email-skomatineni@nvidia.com/
>
> Furthermore, per the documemtation [1], -Wno-unused-function will also
> disable -Wunneeded-internal-declaration, which can help find bugs like
> commit 8289c4b6f2e5 ("platform/x86: mlx-platform: Properly use
> mlxplat_mlxcpld_msn201x_items").
>
> [1]: https://clang.llvm.org/docs/DiagnosticsReference.html#wunused-function


How about this?

KBUILD_CFLAGS += -Wno-unused-function
KBUILD_CFLAGS += -Wunneeded-internal-declaration



> Is there a way to conditionally remove __maybe_unused from the inline
> defintion so that we keep the current behavior but we can still
> selectively find potentially unused functions?

It would be possible by tweaking include/linux/compiler_types.h
but I am not a big fan of uglifying the 'inline' replacement any more.


-- 
Best Regards
Masahiro Yamada

Re: [PATCH] kbuild: enable unused-function warnings for W= build with Clang

[Nathan Chancellor]

On Tue, Aug 20, 2019 at 01:58:26AM +0900, Masahiro Yamada wrote:
> Hi Nathan,
> 
> On Tue, Aug 20, 2019 at 1:09 AM Nathan Chancellor
> <natechancellor@gmail.com> wrote:
> >
> > On Mon, Aug 19, 2019 at 07:51:38PM +0900, Masahiro Yamada wrote:
> > > GCC and Clang have different policy for -Wunused-function; GCC does
> > > not report unused-function warnings at all for the functions marked
> > > as 'static inline'. Clang does report unused-function warnings if they
> > > are defined in source files instead of headers.
> > >
> > > We could use Clang for detecting unused functions, but it has been
> > > suppressed since commit abb2ea7dfd82 ("compiler, clang: suppress
> > > warning for unused static inline functions").
> > >
> > > So, we never notice left-over code if functions in .c files are
> > > marked as inline.
> > >
> > > Let's remove __maybe_unused from the inline macro. As always, it is
> > > not a good idea to sprinkle warnings for the normal build. So, these
> > > warnings will be shown for the W= build.
> > >
> > > If you contribute to code clean-up, please run "make CC=clang W=1"
> > > and check -Wunused-function warnings. You will find lots of unused
> > > functions.
> > >
> > > Some of them are false-positives because the call-sites are disabled
> > > by #ifdef. I do not like to abuse the inline keyword for suppressing
> > > unused-function warnings because it might affect the compiler's
> > > optimization. When I need to fix unused-functions warnings, I prefer
> > > adding #ifdef or __maybe_unused to function definitions.
> > >
> > > Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
> >
> > So if I understand everything correctly, this change allows us to start
> > finding unused static inline functions with clang at W=1 but disables
> > -Wunused-function by default... I am not sure that is a good tradeoff
> > as I am pretty sure that W=1 is fairly noisy for clang although I
> > haven't checked lately. I'd argue most regular developers do not build
> > with W=1 meaning -Wunused-function generally will not be run with clang
> > at all, missing stuff like this:
> 
> 
> Try "git log --grep=W=1"
> 
> Some people are making efforts to fix W=1 warnings.
> I believe somebody will start to remove unused static inline functions.

Yes, it could be a good way to get people involved with working with
clang.

> 
> 
> 
> >
> > https://lore.kernel.org/lkml/20190523010235.GA105588@archlinux-epyc/
> >
> > https://lore.kernel.org/lkml/1558574945-19275-1-git-send-email-skomatineni@nvidia.com/
> >
> > Furthermore, per the documemtation [1], -Wno-unused-function will also
> > disable -Wunneeded-internal-declaration, which can help find bugs like
> > commit 8289c4b6f2e5 ("platform/x86: mlx-platform: Properly use
> > mlxplat_mlxcpld_msn201x_items").
> >
> > [1]: https://clang.llvm.org/docs/DiagnosticsReference.html#wunused-function
> 
> 
> How about this?
> 
> KBUILD_CFLAGS += -Wno-unused-function
> KBUILD_CFLAGS += -Wunneeded-internal-declaration

Yes, that would work.

> 
> 
> 
> > Is there a way to conditionally remove __maybe_unused from the inline
> > defintion so that we keep the current behavior but we can still
> > selectively find potentially unused functions?
> 
> It would be possible by tweaking include/linux/compiler_types.h
> but I am not a big fan of uglifying the 'inline' replacement any more.

I agree that ugly is not ideal but I think it is even less ideal to
weaken the default set of warnings for clang... Merely food for thought
though.

Cheers,
Nathan

Re: [PATCH] kbuild: enable unused-function warnings for W= build with Clang

[Kees Cook]

On Mon, Aug 19, 2019 at 07:51:38PM +0900, Masahiro Yamada wrote:
> GCC and Clang have different policy for -Wunused-function; GCC does
> not report unused-function warnings at all for the functions marked
> as 'static inline'. Clang does report unused-function warnings if they
> are defined in source files instead of headers.
> 
> We could use Clang for detecting unused functions, but it has been
> suppressed since commit abb2ea7dfd82 ("compiler, clang: suppress
> warning for unused static inline functions").
> 
> So, we never notice left-over code if functions in .c files are
> marked as inline.
> 
> Let's remove __maybe_unused from the inline macro. As always, it is
> not a good idea to sprinkle warnings for the normal build. So, these
> warnings will be shown for the W= build.
> 
> If you contribute to code clean-up, please run "make CC=clang W=1"
> and check -Wunused-function warnings. You will find lots of unused
> functions.
> 
> Some of them are false-positives because the call-sites are disabled
> by #ifdef. I do not like to abuse the inline keyword for suppressing
> unused-function warnings because it might affect the compiler's
> optimization. When I need to fix unused-functions warnings, I prefer
> adding #ifdef or __maybe_unused to function definitions.
> 
> Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

Reviewed-by: Kees Cook <keescook@chromium.org>

-Kees

> ---
> 
>  include/linux/compiler_types.h | 10 ++--------
>  scripts/Makefile.extrawarn     |  1 +
>  2 files changed, 3 insertions(+), 8 deletions(-)
> 
> diff --git a/include/linux/compiler_types.h b/include/linux/compiler_types.h
> index 599c27b56c29..14de8d0162fb 100644
> --- a/include/linux/compiler_types.h
> +++ b/include/linux/compiler_types.h
> @@ -130,10 +130,6 @@ struct ftrace_likely_data {
>  
>  /*
>   * Force always-inline if the user requests it so via the .config.
> - * GCC does not warn about unused static inline functions for
> - * -Wunused-function.  This turns out to avoid the need for complex #ifdef
> - * directives.  Suppress the warning in clang as well by using "unused"
> - * function attribute, which is redundant but not harmful for gcc.
>   * Prefer gnu_inline, so that extern inline functions do not emit an
>   * externally visible function. This makes extern inline behave as per gnu89
>   * semantics rather than c99. This prevents multiple symbol definition errors
> @@ -143,11 +139,9 @@ struct ftrace_likely_data {
>   * (which would break users of __always_inline).
>   */
>  #if !defined(CONFIG_OPTIMIZE_INLINING)
> -#define inline inline __attribute__((__always_inline__)) __gnu_inline \
> -	__maybe_unused notrace
> +#define inline inline __attribute__((__always_inline__)) __gnu_inline notrace
>  #else
> -#define inline inline                                    __gnu_inline \
> -	__maybe_unused notrace
> +#define inline inline                                    __gnu_inline notrace
>  #endif
>  
>  #define __inline__ inline
> diff --git a/scripts/Makefile.extrawarn b/scripts/Makefile.extrawarn
> index a74ce2e3c33e..92f542797e03 100644
> --- a/scripts/Makefile.extrawarn
> +++ b/scripts/Makefile.extrawarn
> @@ -70,5 +70,6 @@ KBUILD_CFLAGS += -Wno-initializer-overrides
>  KBUILD_CFLAGS += -Wno-format
>  KBUILD_CFLAGS += -Wno-sign-compare
>  KBUILD_CFLAGS += -Wno-format-zero-length
> +KBUILD_CFLAGS += -Wno-unused-function
>  endif
>  endif
> -- 
> 2.17.1
> 

-- 
Kees Cook