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Message-ID: <alpine.DEB.2.21.1908071517120.24014@nanos.tec.linutronix.de>
Date: Wed, 7 Aug 2019 15:17:21 +0200 (CEST)
From: Thomas Gleixner <tglx@...utronix.de>
To: Nick Desaulniers <ndesaulniers@...gle.com>
cc: mingo@...hat.com, bp@...en8.de, peterz@...radead.org,
clang-built-linux@...glegroups.com, linux-kernel@...r.kernel.org,
yamada.masahiro@...ionext.com,
Vaibhav Rustagi <vaibhavrustagi@...gle.com>,
"H. Peter Anvin" <hpa@...or.com>, x86@...nel.org
Subject: Re: [PATCH v4 2/2] x86/purgatory: use CFLAGS_REMOVE rather than
reset KBUILD_CFLAGS
On Fri, 26 Jul 2019, Thomas Gleixner wrote:
Ping...
> On Thu, 25 Jul 2019, Nick Desaulniers 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 use `CFLAGS_REMOVE_file.o = $(CC_FLAGS_FTRACE)` 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. Seems
> > also that __stack_chk_fail references are generated when using
> > CONFIG_STACKPROTECTOR or CONFIG_STACKPROTECTOR_STRONG.
>
> Looking at the resulting build flags. Most stuff looks correct but there
> are a few which need to be looked at twice.
>
> removes:
>
> -ffreestanding
> -fno-builtin
> -fno-zero-initialized-in-bss
>
> changes:
>
> -mcmodel=large to -mcmodel=kernel
>
> adds:
>
> -mindirect-branch-register
> -mindirect-branch=thunk-extern
>
> The latter makes me nervous. That probably wants to have retpoline disabled
> as well. It's not having an instance right now, but ...
>
> Thanks,
>
> tglx
>
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