linux-kernel - Re: [PATCH v2 2/2] x86/purgatory: use CFLAGS_REMOVE rather than reset KBUILD

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Message-ID: <CAKwvOdm3iyeJfuivhQJqXB9FfC0zHgrfgoN_qW4poEyfcw3C9A@mail.gmail.com>
Date:   Mon, 22 Jul 2019 15:10:04 -0700
From:   Nick Desaulniers <ndesaulniers@...gle.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Vaibhav Rustagi <vaibhavrustagi@...gle.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 2/2] x86/purgatory: use CFLAGS_REMOVE rather than reset KBUILD_CFLAGS

On Mon, Jul 22, 2019 at 2:33 PM Nick Desaulniers
<ndesaulniers@...gle.com> wrote:
>
> KBUILD_CFLAGS is very carefully built up in the top level Makefile,
> particularly when cross compiling or using different build tools.
> Resetting KBUILD_CFLAGS via := assignment is an antipattern.
>
> The comment above the reset mentions that -pg is problematic.  Other
> Makefiles like arch/x86/xen/vdso/Makefile use
> `CFLAGS_REMOVE_file.o = -pg` when CONFIG_FUNCTION_TRACER is set. Prefer
> that pattern to wiping out all of the important KBUILD_CFLAGS then
> manually having to re-add them.
>
> Fixes: 8fc5b4d4121c ("purgatory: core purgatory functionality")
> Reported-by: Vaibhav Rustagi <vaibhavrustagi@...gle.com>
> Suggested-by: Peter Zijlstra <peterz@...radead.org>
> Signed-off-by: Nick Desaulniers <ndesaulniers@...gle.com>
> ---
> Rather than manually add -mno-sse, -mno-mmx, -mno-sse2, prefer to filter
> -pg flags.
>
>  arch/x86/purgatory/Makefile | 12 +++++++-----
>  1 file changed, 7 insertions(+), 5 deletions(-)
>
> diff --git a/arch/x86/purgatory/Makefile b/arch/x86/purgatory/Makefile
> index 91ef244026d2..56bcabca283f 100644
> --- a/arch/x86/purgatory/Makefile
> +++ b/arch/x86/purgatory/Makefile
> @@ -20,11 +20,13 @@ KCOV_INSTRUMENT := n
>
>  # Default KBUILD_CFLAGS can have -pg option set when FTRACE is enabled. That
>  # in turn leaves some undefined symbols like __fentry__ in purgatory and not
> -# sure how to relocate those. Like kexec-tools, use custom flags.
> -
> -KBUILD_CFLAGS := -fno-strict-aliasing -Wall -Wstrict-prototypes -fno-zero-initialized-in-bss -fno-builtin -ffreestanding -c -Os -mcmodel=large
> -KBUILD_CFLAGS += -m$(BITS)

Is purgatory/kexec supported for CONFIG_X86_32?  Should I be keeping
`-m$(BITS)`?  arch/x86/purgatory/Makefile mentions
`setup-x86_$(BITS).o` which I assume is broken as there is no
arch/x86/purgatory/setup-x86_32.S?

> -KBUILD_CFLAGS += $(call cc-option,-fno-PIE)
> +# sure how to relocate those.
> +ifdef CONFIG_FUNCTION_TRACER
> +CFLAGS_REMOVE_sha256.o = -pg
> +CFLAGS_REMOVE_purgatory.o = -pg
> +CFLAGS_REMOVE_string.o = -pg
> +CFLAGS_REMOVE_kexec-purgatory.o = -pg
> +endif
>
>  $(obj)/purgatory.ro: $(PURGATORY_OBJS) FORCE
>                 $(call if_changed,ld)
> --
> 2.22.0.657.g960e92d24f-goog
>


-- 
Thanks,
~Nick Desaulniers