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Date:   Wed, 12 Jun 2019 10:25:20 +0100
From:   Dave Martin <Dave.Martin@arm.com>
To:     Nathan Chancellor <natechancellor@gmail.com>
Cc:     Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will.deacon@arm.com>,
        Nick Desaulniers <ndesaulniers@google.com>,
        linux-kernel@vger.kernel.org, clang-built-linux@googlegroups.com,
        Qian Cai <cai@lca.pw>, linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH v2] arm64: Don't unconditionally add -Wno-psabi to
 KBUILD_CFLAGS
Message-ID: <20190612092519.GP28398@e103592.cambridge.arm.com>
References: <20190607161201.73430-1-natechancellor@gmail.com>
 <20190611171931.99705-1-natechancellor@gmail.com>
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On Tue, Jun 11, 2019 at 10:19:32AM -0700, Nathan Chancellor wrote:
> This is a GCC only option, which warns about ABI changes within GCC, so
> unconditionally adding it breaks Clang with tons of:
> 
> warning: unknown warning option '-Wno-psabi' [-Wunknown-warning-option]
> 
> and link time failures:
> 
> ld.lld: error: undefined symbol: __efistub___stack_chk_guard
> >>> referenced by arm-stub.c:73
> (/home/nathan/cbl/linux/drivers/firmware/efi/libstub/arm-stub.c:73)
> >>>               arm-stub.stub.o:(__efistub_install_memreserve_table)
> in archive ./drivers/firmware/efi/libstub/lib.a
> 
> These failures come from the lack of -fno-stack-protector, which is
> added via cc-option in drivers/firmware/efi/libstub/Makefile. When an
> unknown flag is added to KBUILD_CFLAGS, clang will noisily warn that it
> is ignoring the option like above, unlike gcc, who will just error.
> 
> $ echo "int main() { return 0; }" > tmp.c
> 
> $ clang -Wno-psabi tmp.c; echo $?
> warning: unknown warning option '-Wno-psabi' [-Wunknown-warning-option]
> 1 warning generated.
> 0
> 
> $ gcc -Wsometimes-uninitialized tmp.c; echo $?
> gcc: error: unrecognized command line option
> ‘-Wsometimes-uninitialized’; did you mean ‘-Wmaybe-uninitialized’?
> 1
> 
> For cc-option to work properly with clang and behave like gcc, -Werror
> is needed, which was done in commit c3f0d0bc5b01 ("kbuild, LLVMLinux:
> Add -Werror to cc-option to support clang").
> 
> $ clang -Werror -Wno-psabi tmp.c; echo $?
> error: unknown warning option '-Wno-psabi'
> [-Werror,-Wunknown-warning-option]
> 1
> 
> As a consequence of this, when an unknown flag is unconditionally added
> to KBUILD_CFLAGS, it will cause cc-option to always fail and those flags
> will never get added:
> 
> $ clang -Werror -Wno-psabi -fno-stack-protector tmp.c; echo $?
> error: unknown warning option '-Wno-psabi'
> [-Werror,-Wunknown-warning-option]
> 1
> 
> This can be seen when compiling the whole kernel as some warnings that
> are normally disabled (see below) show up. The full list of flags
> missing from drivers/firmware/efi/libstub are the following (gathered
> from diffing .arm64-stub.o.cmd):
> 
> -fno-delete-null-pointer-checks
> -Wno-address-of-packed-member
> -Wframe-larger-than=2048
> -Wno-unused-const-variable
> -fno-strict-overflow
> -fno-merge-all-constants
> -fno-stack-check
> -Werror=date-time
> -Werror=incompatible-pointer-types
> -ffreestanding
> -fno-stack-protector
> 
> Use cc-disable-warning so that it gets disabled for GCC and does nothing
> for Clang.
> 
> Fixes: ebcc5928c5d9 ("arm64: Silence gcc warnings about arch ABI drift")
> Link: https://github.com/ClangBuiltLinux/linux/issues/511
> Reported-by: Qian Cai <cai@lca.pw>
> Acked-by: Dave Martin <Dave.Martin@arm.com>
> Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
> Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
> ---
> 
> v1 -> v2:
> 
> * Improve commit message explanation, I wasn't entirely happy with the
>   first one; let me know if there are any issues/questions.
> 
> * Carry forward Dave's ack and Nick's review (let me know if you
>   disagree with the commit messasge rewording).
> 
>  arch/arm64/Makefile | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/Makefile b/arch/arm64/Makefile
> index 8fbd583b18e1..e9d2e578cbe6 100644
> --- a/arch/arm64/Makefile
> +++ b/arch/arm64/Makefile
> @@ -51,7 +51,7 @@ endif
>  
>  KBUILD_CFLAGS	+= -mgeneral-regs-only $(lseinstr) $(brokengasinst)
>  KBUILD_CFLAGS	+= -fno-asynchronous-unwind-tables
> -KBUILD_CFLAGS	+= -Wno-psabi
> +KBUILD_CFLAGS	+= $(call cc-disable-warning, psabi)
>  KBUILD_AFLAGS	+= $(lseinstr) $(brokengasinst)
>  
>  KBUILD_CFLAGS	+= $(call cc-option,-mabi=lp64)

Looks OK to me.  Thanks for the additional explanation.

Cheers
---Dave