linux-kernel - Re: [PATCH] [RFC] kbuild: add CLANG_TRIPLE to prevent clang from compiling with wrong --target

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Date:   Sat, 11 Sep 2021 19:55:53 +0800
From:   ashimida <ashimida@...ux.alibaba.com>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     masahiroy@...nel.org, michal.lkml@...kovi.net, nathan@...nel.org,
        linux-kbuild@...r.kernel.org, linux-kernel@...r.kernel.org,
        clang-built-linux@...glegroups.com,
        Alistair Delva <adelva@...gle.com>
Subject: Re: [PATCH] [RFC] kbuild: add CLANG_TRIPLE to prevent clang from
 compiling with wrong --target

Hi Desaulniers,

I now understand the meaning here, the command works fine for me.

Thank you very much for the reply!

On 9/10/21 1:19 AM, Nick Desaulniers wrote:
 > On Thu, Sep 9, 2021 at 12:42 AM ashimida <ashimida@...ux.alibaba.com> 
wrote:
 >>
 >> Kernel compiled with tool chain CROSS_COMPILE=aarch64-linux-android-
 >> will panic during the startup phase.
 >>
 >> Clang's --target option comes from $(CROSS_COMPILE). At the time
 >> -fstack-protector-strong is enabled, and compiled with command:
 >> make CC=clang HOSTCC=clang ARCH=arm64 
CROSS_COMPILE=aarch64-linux-android-
 >>
 >> clang will insert code like:
 >>     mrs     x8, TPIDR_EL0        //default value is zero
 >>     str     x8, [sp]
 >>     ldr     x8, [x8, #40]        //access addr 0x40
 >>
 >> instead of the code that accesses __stack_chk_guard to get the
 >> canary, which will cause the kernel to crash due to 0x40
 >> address access.
 >>
 >> This patch (from android) is used to remind the user that current
 >> tool chain cannot be used as the "--target" of clang, the user
 >> should specify an additional "--target" through CLANG_TRIPLE.
 >
 > Hi Ashimida,
 > Thanks for sending this patch; I recognize it from Android, which we
 > had to carry for years due to:
 > 1. reliance on GNU `as` ie. "GAS"
 > 2. not distributing binary prefixes of GNU binutils with a target
 > triple that clang recognized. (ie. Android's binutils were prefixed
 > aarch64-linux-android- while Clang expected something more like
 > aarch64-linux-gnu for --target=)
 >
 > We solved this by working out the issues in clang's assembler.  With
 > LLVM=1 LLVM_IAS=1, we no longer rely on GNU binutils, and no longer
 > need such patch.  You'll find it's been dropped from Android Common
 > Kernels now.  With mainline, LLVM_IAS=1 is now the default when
 > building with LLVM=1, and CROSS_COMPILE is now inferred from ARCH for
 > LLVM=1 as well.
 >
 > So all you should need is:
 > $ ARCH=arm64 make LLVM=1 -j$(nproc)
 >
 > Is there a reason why the above doesn't work for you?  I do not wish
 > to see this patch upstream (or downstream; it should be unnecessary).
 >
 >>
 >> Signed-off-by: ashimida <ashimida@...ux.alibaba.com>
 >> ---
 >>   Makefile                 | 6 +++++-
 >>   scripts/clang-android.sh | 4 ++++
 >>   2 files changed, 9 insertions(+), 1 deletion(-)
 >>   create mode 100755 scripts/clang-android.sh
 >>
 >> diff --git a/Makefile b/Makefile
 >> index 61741e9..09bb314 100644
 >> --- a/Makefile
 >> +++ b/Makefile
 >> @@ -586,7 +586,11 @@ CC_VERSION_TEXT = $(subst $(pound),,$(shell 
$(CC) --version 2>/dev/null | head -
 >>
 >>   ifneq ($(findstring clang,$(CC_VERSION_TEXT)),)
 >>   ifneq ($(CROSS_COMPILE),)
 >> -CLANG_FLAGS    += --target=$(notdir $(CROSS_COMPILE:%-=%))
 >> +CLANG_TRIPLE    ?= $(CROSS_COMPILE)
 >> +CLANG_FLAGS     += --target=$(notdir $(CLANG_TRIPLE:%-=%))
 >> +ifeq ($(shell $(srctree)/scripts/clang-android.sh $(CC) 
$(CLANG_FLAGS)), y)
 >> +$(error "Clang with Android --target detected. Did you specify 
CLANG_TRIPLE?")
 >> +endif
 >>   endif
 >>   ifeq ($(LLVM_IAS),1)
 >>   CLANG_FLAGS    += -integrated-as
 >> diff --git a/scripts/clang-android.sh b/scripts/clang-android.sh
 >> new file mode 100755
 >> index 0000000..9186c4f
 >> --- /dev/null
 >> +++ b/scripts/clang-android.sh
 >> @@ -0,0 +1,4 @@
 >> +#!/bin/sh
 >> +# SPDX-License-Identifier: GPL-2.0
 >> +
 >> +$* -dM -E - </dev/null 2>&1 | grep -q __ANDROID__ && echo "y"
 >> --
 >> 2.7.4
 >>
 >
 >