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Date:   Thu, 7 Jul 2022 10:17:04 -0700
From:   Nathan Chancellor <nathan@kernel.org>
To:     Dave Hansen <dave.hansen@intel.com>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Dave Hansen <dave.hansen@linux.intel.com>, x86@kernel.org,
        "H. Peter Anvin" <hpa@zytor.com>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Tom Rix <trix@redhat.com>,
        Daniel Kolesa <daniel@octaforge.org>,
        linux-kernel@vger.kernel.org, patches@lists.linux.dev,
        llvm@lists.linux.dev, stable@vger.kernel.org
Subject: Re: [PATCH] x86/Kconfig: Fix CONFIG_CC_HAS_SANE_STACKPROTECTOR when
 cross compiling with clang
Message-ID: <YscVEIBtyoX2jrBV@dev-arch.thelio-3990X>
References: <20220617180845.2788442-1-nathan@kernel.org>
 <842b718f-8207-1565-3373-61098a4c2d33@intel.com>
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On Wed, Jul 06, 2022 at 03:25:51PM -0700, Dave Hansen wrote:
> On 6/17/22 11:08, Nathan Chancellor wrote:
> > When clang is invoked without a '--target' flag, code is generated for
> > the default target, which is usually the host (it is configurable via
> > cmake). As a result, the has-stack-protector scripts will generate code
> > for the default target but check for x86 specific segment registers,
> > which cannot succeed if the default target is not x86.
> 
> I guess the real root cause here is the direct use of '$(CC)' without
> any other flags.  Adding '$(CLANG_FLAGS)' seems like a pretty normal
> fix, like in scripts/Kconfig.include.

Right, also see the following commits for other areas where this was
addressed.

58d746c119df ("efi/libstub: Add $(CLANG_FLAGS) to x86 flags")
d5cbd80e302d ("x86/boot: Add $(CLANG_FLAGS) to compressed KBUILD_CFLAGS")
8abe7fc26ad8 ("x86/build: Propagate $(CLANG_FLAGS) to $(REALMODE_FLAGS)")

> I suspect there's another one of these here:
> 
> 	arch/x86/um/vdso/Makefile:      cmd_vdso = $(CC) -nostdlib -o $@
> 
> but I wouldn't be surprised if UML doesn't work with clang in the first
> place.

We have started testing UML with clang and it does work but I suspect
there is little value to cross compiling a UML kernel, as it has to run
in an x86 userland anyways, rather than through QEMU or other
virtualization solutions. That is not something I plan to do anyways.
If someone does and a fix similar to this one is needed, it can be done
at that time.

Thank you for picking up this change!

Cheers,
Nathan