linux-kernel - Re: [PATCH] powerpc: Avoid clang warnings around setjmp and longjmp

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Message-ID: <20190827001237.GA117465@archlinux-threadripper>
Date:   Mon, 26 Aug 2019 17:12:37 -0700
From:   Nathan Chancellor <natechancellor@...il.com>
To:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org,
        clang-built-linux@...glegroups.com, stable@...r.kernel.org
Subject: Re: [PATCH] powerpc: Avoid clang warnings around setjmp and longjmp

On Sun, Aug 11, 2019 at 07:32:15PM -0700, Nathan Chancellor wrote:
> Commit aea447141c7e ("powerpc: Disable -Wbuiltin-requires-header when
> setjmp is used") disabled -Wbuiltin-requires-header because of a warning
> about the setjmp and longjmp declarations.
> 
> r367387 in clang added another diagnostic around this, complaining that
> there is no jmp_buf declaration.
> 
> In file included from ../arch/powerpc/xmon/xmon.c:47:
> ../arch/powerpc/include/asm/setjmp.h:10:13: error: declaration of
> built-in function 'setjmp' requires the declaration of the 'jmp_buf'
> type, commonly provided in the header <setjmp.h>.
> [-Werror,-Wincomplete-setjmp-declaration]
> extern long setjmp(long *);
>             ^
> ../arch/powerpc/include/asm/setjmp.h:11:13: error: declaration of
> built-in function 'longjmp' requires the declaration of the 'jmp_buf'
> type, commonly provided in the header <setjmp.h>.
> [-Werror,-Wincomplete-setjmp-declaration]
> extern void longjmp(long *, long);
>             ^
> 2 errors generated.
> 
> Take the same approach as the above commit by disabling the warning for
> the same reason, we provide our own longjmp/setjmp function.
> 
> Cc: stable@...r.kernel.org # 4.19+
> Link: https://github.com/ClangBuiltLinux/linux/issues/625
> Link: https://github.com/llvm/llvm-project/commit/3be25e79477db2d31ac46493d97eca8c20592b07
> Signed-off-by: Nathan Chancellor <natechancellor@...il.com>
> ---
> 
> It may be worth using -fno-builtin-setjmp and -fno-builtin-longjmp
> instead as it makes it clear to clang that we are not using the builtin
> longjmp and setjmp functions, which I think is why these warnings are
> appearing (at least according to the commit that introduced this waring).
> 
> Sample patch:
> https://github.com/ClangBuiltLinux/linux/issues/625#issuecomment-519251372
> 
> However, this is the most conservative approach, as I have already had
> someone notice this error when building LLVM with PGO on tip of tree
> LLVM.
> 
>  arch/powerpc/kernel/Makefile | 5 +++--
>  arch/powerpc/xmon/Makefile   | 5 +++--
>  2 files changed, 6 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/powerpc/kernel/Makefile b/arch/powerpc/kernel/Makefile
> index ea0c69236789..44e340ed4722 100644
> --- a/arch/powerpc/kernel/Makefile
> +++ b/arch/powerpc/kernel/Makefile
> @@ -5,8 +5,9 @@
>  
>  CFLAGS_ptrace.o		+= -DUTS_MACHINE='"$(UTS_MACHINE)"'
>  
> -# Disable clang warning for using setjmp without setjmp.h header
> -CFLAGS_crash.o		+= $(call cc-disable-warning, builtin-requires-header)
> +# Avoid clang warnings about longjmp and setjmp built-ins (inclusion of setjmp.h and declaration of jmp_buf type)
> +CFLAGS_crash.o		+= $(call cc-disable-warning, builtin-requires-header) \
> +			   $(call cc-disable-warning, incomplete-setjmp-declaration)
>  
>  ifdef CONFIG_PPC64
>  CFLAGS_prom_init.o	+= $(NO_MINIMAL_TOC)
> diff --git a/arch/powerpc/xmon/Makefile b/arch/powerpc/xmon/Makefile
> index f142570ad860..53f341391210 100644
> --- a/arch/powerpc/xmon/Makefile
> +++ b/arch/powerpc/xmon/Makefile
> @@ -1,8 +1,9 @@
>  # SPDX-License-Identifier: GPL-2.0
>  # Makefile for xmon
>  
> -# Disable clang warning for using setjmp without setjmp.h header
> -subdir-ccflags-y := $(call cc-disable-warning, builtin-requires-header)
> +# Avoid clang warnings about longjmp and setjmp built-ins (inclusion of setjmp.h and declaration of jmp_buf type)
> +subdir-ccflags-y := $(call cc-disable-warning, builtin-requires-header) \
> +		    $(call cc-disable-warning, incomplete-setjmp-declaration)
>  
>  GCOV_PROFILE := n
>  KCOV_INSTRUMENT := n
> -- 
> 2.23.0.rc2
> 

Did any of the maintainers have any comment on this patch?

Cheers,
Nathan