linux-kernel - Re: [PATCH v2] LoongArch: Replace -ffreestanding with finer-grained -fno-builtin's

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Message-ID: <CAAhV-H4ypd-+z-KbeGM6C_mNk0aggk2yhdDik-XfPNWqsO=j=A@mail.gmail.com>
Date:   Sun, 6 Aug 2023 17:16:28 +0800
From:   Huacai Chen <chenhuacai@...nel.org>
To:     WANG Xuerui <kernel@...0n.name>
Cc:     Nathan Chancellor <nathan@...nel.org>,
        Xi Ruoyao <xry111@...111.site>, loongarch@...ts.linux.dev,
        llvm@...ts.linux.dev, linux-kernel@...r.kernel.org,
        WANG Xuerui <git@...0n.name>,
        Nick Desaulniers <ndesaulniers@...gle.com>
Subject: Re: [PATCH v2] LoongArch: Replace -ffreestanding with finer-grained -fno-builtin's

Hi, Xuerui,

On Sun, Aug 6, 2023 at 4:30 PM WANG Xuerui <kernel@...0n.name> wrote:
>
> From: WANG Xuerui <git@...0n.name>
>
> As explained by Nick in the original issue: the kernel usually does a
> good job of providing library helpers that have similar semantics as
> their ordinary userspace libc equivalents, but -ffreestanding disables
> such libcall optimization and other related features in the compiler,
> which can lead to unexpected things such as CONFIG_FORTIFY_SOURCE not
> working (!).
>
> As it turns out to be the case, only the memory operations really need
> to be prevented from expansion by the compiler, and this is doable with
> finer-grained -fno-builtin-* toggles. So only disable memcpy, memmove
> and memset, while leaving other builtins enabled, to fix source
> fortification among others.
>
> Closes: https://github.com/ClangBuiltLinux/linux/issues/1897
> Reported-by: Nathan Chancellor <nathan@...nel.org>
> Suggested-by: Nick Desaulniers <ndesaulniers@...gle.com>
> Signed-off-by: WANG Xuerui <git@...0n.name>
> ---
>
> Changes in v2:
>
> - Keep the memory operation builtins disabled, add comments, and tweak the
>   commit message along the way.
>
>  arch/loongarch/Makefile | 9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
>
> diff --git a/arch/loongarch/Makefile b/arch/loongarch/Makefile
> index b1e5db51b61c..34fc48df87f2 100644
> --- a/arch/loongarch/Makefile
> +++ b/arch/loongarch/Makefile
> @@ -83,7 +83,14 @@ KBUILD_CFLAGS_KERNEL         += -fPIE
>  LDFLAGS_vmlinux                        += -static -pie --no-dynamic-linker -z notext
>  endif
>
> -cflags-y += -ffreestanding
> +# Make sure the memory libcalls are not expanded by the compiler, for better
> +# control over unaligned accesses with respect to CONFIG_ARCH_STRICT_ALIGN,
> +# and also for avoiding https://gcc.gnu.org/PR109465.
> +#
> +# The overly broad -ffreestanding is undesirable as it disables *all* libcall
> +# handling, that unfortunately includes proper FORTIFY_SOURCE instrumentation.
I think these comments should go to commit message rather than here,
because after this patch there is no -ffreestanding in Makefile.

Huacai

> +cflags-y += -fno-builtin-memcpy -fno-builtin-memmove -fno-builtin-memset
> +
>  cflags-y += $(call cc-option, -mno-check-zero-division)
>
>  load-y         = 0x9000000000200000
> --
> 2.40.0
>
>