On Wed, Jul 22, 2020 at 9:14 AM Fangrui Song maskray@google.com wrote:
On 2020-07-22, Masahiro Yamada wrote:
On Wed, Jul 22, 2020 at 2:31 AM 'Fangrui Song' via Clang Built Linux clang-built-linux@googlegroups.com wrote:
When CROSS_COMPILE is set (e.g. aarch64-linux-gnu-), if $(CROSS_COMPILE)elfedit is found at /usr/bin/aarch64-linux-gnu-elfedit, GCC_TOOLCHAIN_DIR will be set to /usr/bin/. --prefix= will be set to /usr/bin/ and Clang as of 11 will search for both $(prefix)aarch64-linux-gnu-$needle and $(prefix)$needle.
GCC searchs for $(prefix)aarch64-linux-gnu/$version/$needle, $(prefix)aarch64-linux-gnu/$needle and $(prefix)$needle. In practice, $(prefix)aarch64-linux-gnu/$needle rarely contains executables.
To better model how GCC's -B/--prefix takes in effect in practice, newer Clang (since https://github.com/llvm/llvm-project/commit/3452a0d8c17f7166f479706b293caf6a...) only searches for $(prefix)$needle. Currently it will find /usr/bin/as instead of /usr/bin/aarch64-linux-gnu-as.
Set --prefix= to $(GCC_TOOLCHAIN_DIR)$(CROSS_COMPILE) (/usr/bin/aarch64-linux-gnu-) so that newer Clang can find the appropriate cross compiling GNU as (when -no-integrated-as is in effect).
Cc: stable@vger.kernel.org Reported-by: Nathan Chancellor natechancellor@gmail.com Signed-off-by: Fangrui Song maskray@google.com Reviewed-by: Nathan Chancellor natechancellor@gmail.com Tested-by: Nathan Chancellor natechancellor@gmail.com Tested-by: Nick Desaulniers ndesaulniers@google.com Link: https://github.com/ClangBuiltLinux/linux/issues/1099
Changes in v2:
- Updated description to add tags and the llvm-project commit link.
- Fixed a typo.
Changes in v3:
- Add Cc: stable@vger.kernel.org
Makefile | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/Makefile b/Makefile index 0b5f8538bde5..3ac83e375b61 100644 --- a/Makefile +++ b/Makefile @@ -567,7 +567,7 @@ ifneq ($(shell $(CC) --version 2>&1 | head -n 1 | grep clang),) ifneq ($(CROSS_COMPILE),) CLANG_FLAGS += --target=$(notdir $(CROSS_COMPILE:%-=%)) GCC_TOOLCHAIN_DIR := $(dir $(shell which $(CROSS_COMPILE)elfedit)) -CLANG_FLAGS += --prefix=$(GCC_TOOLCHAIN_DIR) +CLANG_FLAGS += --prefix=$(GCC_TOOLCHAIN_DIR)$(CROSS_COMPILE)
CROSS_COMPILE may contain the directory path to the cross toolchains.
For example, I use aarch64-linux-gnu-* installed in /home/masahiro/tools/aarch64-linaro-7.5/bin
Basically, there are two ways to use it.
[1] PATH=$PATH:/home/masahiro/tools/aarch64-linaro-7.5/bin CROSS_COMPILE=aarch64-linux-gnu-
[2] Without setting PATH, CROSS_COMPILE=~/tools/aarch64-linaro-7.5/bin/aarch64-linux-gnu-
I usually do [2] (and so does intel's 0day bot).
This patch works for the use-case [1] but if I do [2], --prefix is set to a strange path:
--prefix=/home/masahiro/tools/aarch64-linaro-7.5/bin//home/masahiro/tools/aarch64-linaro-7.5/bin/aarch64-linux-gnu-
Thanks. I did not know the use-case [2]. This explains why there is a `$(notdir ...)` in `CLANG_FLAGS += --target=$(notdir $(CROSS_COMPILE:%-=%))`
Interestingly, the build is still successful. Presumably Clang searches for more paths when $(prefix)$needle is not found ?
The priority order is:
-B(--prefix), COMPILER_PATH, detected gcc-cross paths
(In GCC, -B paths get prepended to the COMPILER_PATH list. Clang<=11 incorrectly adds -B to the COMPILER_PATH list. I have fixed it for 12.0.0)
If -B fails, the detected gcc-cross paths may still be able to find /usr/bin/aarch64-linux-gnu-
For example, on my machine (a variant of Debian testing), Clang finds $(realpath /usr/lib/gcc-cross/aarch64-linux-gnu/9/../../../../aarch64-linux-gnu/bin/as), which is /usr/bin/aarch64-linux-gnu-as
I applied your patch and added -v option to see which assembler was internally invoked:
"/home/masahiro/tools/aarch64-linaro-7.5/lib/gcc/aarch64-linux-gnu/7.5.0/../../../../aarch64-linux-gnu/bin/as" -EL -I ./arch/arm64/include -I ./arch/arm64/include/generated -I ./include -I ./arch/arm64/include/uapi -I ./arch/arm64/include/generated/uapi -I ./include/uapi -I ./include/generated/uapi -o kernel/smp.o /tmp/smp-2ec2c7.s
Ok, it looks like Clang found an alternative path to the correct 'as'.
But, to keep the original behavior for both [1] and [2], how about this?
CLANG_FLAGS += --prefix=$(GCC_TOOLCHAIN_DIR)$(notdir $(CROSS_COMPILE))
Then, I can get this:
"/home/masahiro/tools/aarch64-linaro-7.5/bin/aarch64-linux-gnu-as" -EL -I ./arch/arm64/include -I ./arch/arm64/include/generated -I ./include -I ./arch/arm64/include/uapi -I ./arch/arm64/include/generated/uapi -I ./include/uapi -I ./include/generated/uapi -o kernel/smp.o /tmp/smp-16d76f.s
This looks good.
Agreed that `--prefix=$(GCC_TOOLCHAIN_DIR)$(notdir $(CROSS_COMPILE))` should work for both [1] and [2].
Shall I send a v4? Or you are kind enough that you'll just add your Signed-off-by: tag and fix that for me? :)
I fixed it up and applied to linux-kbuild/fixes. Thanks.
While I am here, could you teach me a bit more?
The top Makefile sets the following option as well:
CLANG_GCC_TC := --gcc-toolchain=$(GCC_TOOLCHAIN)
I checked the manual: https://clang.llvm.org/docs/ClangCommandLineReference.html
-B<dir>, --prefix <arg>, --prefix=<arg> Add <dir> to search path for binaries and object files used implicitly
--gcc-toolchain=<arg>, -gcc-toolchain <arg> Use the gcc toolchain at the given directory
It is not clear to me how they work differently when clang searches for toolchains.
If I delete --gcc-toolchain from the top Makefile, clang fails to link standalone programs because it wrongly invokes /usr/bin/ld instead of aarch64-linux-gnu-ld.
Does Clang search for gnu assembler based on --prefix option?
And, searches for a linker based on --gcc-toolchain ?