On Wed, Mar 2, 2022 at 4:49 AM Nick Desaulniers <ndesaulniers@xxxxxxxxxx> wrote: > > Hello Masahiro, > > We'd like to be able to better test UAPI headers via > CONFIG_UAPI_HEAD_TEST=y with clang builds during cross compiling. I missed to add the target triple to usr/include/Makefile. You might have thought you were cross-compiling the headers, but actually not. Let me fix this first. https://patchwork.kernel.org/project/linux-kbuild/patch/20220305125605.149913-1-masahiroy@xxxxxxxxxx/ > We're finding that CC_CAN_LINK=y is not enabled because clang (unlike > gcc) isn't configured with a cross libc in hand, and one may not be > available in the environment. This is a similar constraint to the > prebuilt GCC images distributed on kernel.org. > > Looking at > commit d6fc9fcbaa65 ("kbuild: compile-test exported headers to ensure > they are self-contained") > > Simply removing the dependency on CC_CAN_LINK=y allows us to run the > UAPI header tests, which pass on ARCH=arm64 and native x86_64 builds. > https://android-review.googlesource.com/c/kernel/common/+/1991156 Is it the right thing to cross-compile "#include <stdio.h>" by using /usr/include/stdio.h ? [with libc6-dev-arm64-cross package] masahiro@grover:~/workspace$ clang --target=aarch64-linux-gnu -c -o test.o -Wp,-MD,\tmp.mk test.c && grep stdio.h tmp.mk /usr/bin/../lib/gcc-cross/aarch64-linux-gnu/11/../../../../aarch64-linux-gnu/include/stdio.h \ [without libc6-dev-arm64-cross package] masahiro@grover:~/workspace$ clang --target=aarch64-linux-gnu -c -o test.o -Wp,-MD,\tmp.mk test.c && grep stdio.h tmp.mk test.o: test.c /usr/include/stdio.h /usr/include/bits/libc-header-start.h \ The header search path is like this: /usr/lib/llvm-13/lib/clang/13.0.0/include /usr/local/include /usr/bin/../lib/gcc-cross/aarch64-linux-gnu/11/../../../../aarch64-linux-gnu/include /usr/include Even if the aarch64 libc is not installed, you can cross-compile it because clang finds stdio.h in /usr/include/. Using the host's system headers for cross-compilation looks wrong to me. > I assume there may be some additional constraints perhaps for other > architectures or certain configs? No, I do not think so. This is common for all architectures. > Another suggestion from the bionic maintainer to ensure the tests are > more hermetic/self-contained would be to add -ffreestanding to the > compiler invocations of these tests. I do not think so. UAPI headers are part of hosted environments. -ffreestanding does not make sense. > Do you have thoughts on how we might be able to eliminate the > dependency on CC_CAN_LINK for UAPI_HEADER_TEST? Otherwise, I suspect > we will have to use `USERCFLAGS=--sysroot=/path/to/sysroot` to make > CC_CAN_LINK work for clang cross compilation. See also: > commit f67695c9962e ("kbuild: Add environment variables for userprogs flags") If you want to use libc installed in a non-standard path, yes, you can use --sysroot. On debian or ubuntu for example, it is unneeded. With "sudo apt install gcc-aarch64-linux-gnu", you can enable CC_CAN_LINIK for clang without using --sysroot. -- Best Regards Masahiro Yamada
