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

[ashimida]

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@xxxxxxxxxxxxxxxxx> 
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@xxxxxxxxxxxxxxxxx>
>> ---
>>   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
>>
>
>