compilation of minimal failing because kernel header path not in includes

[andreimatei]

On Ubuntu 20.10 this happens to me:

$ V=1 make minimal
clang -g -O2 -target bpf -D__TARGET_ARCH_x86 -I.output -c minimal.bpf.c -o .output/minimal.bpf.o
In file included from minimal.bpf.c:3:
In file included from /usr/include/linux/bpf.h:11:
/usr/include/linux/types.h:5:10: fatal error: 'asm/types.h' file not found
#include <asm/types.h>

The rub seems to be that /usr/include/linux/types.h wants asm/types.h which, on my system, I believe lives in /usr/include/x86_64-linux-gnu/asm/types.h (which file just reads #include <asm-generic/types.h>), and /usr/include/x86_64-linux-gnu does not seem to be among clang's include paths, as demonstrated by the clang -v output below.

clang -v -g -O2 -target bpf -D__TARGET_ARCH_x86 -I.output -c minimal.bpf.c -o .output/minimal.bpf.o
Ubuntu clang version 11.0.0-2
...
#include "..." search starts here:
#include <...> search starts here:
 .output
 /usr/local/include`
 /usr/lib/llvm-11/lib/clang/11.0.0/include
 /usr/include
End of search list.

FWIW, on the gcc side, the preprocessor includes /usr/include/x86_64-linux-gnu in the cpp -v output.

I'm not sure who's at fault here, or what the correct fix is, but adding -I/usr/include/x86_64-linux-gnu makes it work. Changing the source to use vmlinux.h instead of <linux/bpf.h> also makes it work.

Although I don't believe it to be the case, it is possible that my system got screwed up by me manually installing kernel headers through the Debian package created by the kernel's deb-pkg make target. I don't think it's the case because I've verified that /usr/include/linux/types.h corresponds to the file provided by the Ubuntu's linux-libc-dev package.

[andreimatei]

Another observation: clang does seem to want to search that arch-specific dir when not using -target bpf:

$ clang -v -g -O2  -D__TARGET_ARCH_x86 -I.output -c minimal.bpf.c -o .output/minimal.bpf.o
Ubuntu clang version 11.0.0-2
...
#include "..." search starts here:
#include <...> search starts here:
 .output
 /usr/local/include
 /usr/lib/llvm-11/lib/clang/11.0.0/include
 /usr/include/x86_64-linux-gnu
 /usr/include

[anakryiko]

Oh, I think this is how we deal with it in kernel selftests: https://github.com/torvalds/linux/blob/master/tools/testing/selftests/bpf/Makefile#L215-L224

This seems to be due to using -target bpf, so for some systems not all the necessary include paths used... You are right that just using "vmlinux.h" would be enough, but that puts a requirement for kernel BTF on the system, and my goal was to have something to use even without kernel built with BTF...

Do you mind trying the get_sys_includes trick and confirming if that works? We can "backport" it here. Not that it's simple and obvious, but should just work.

[andreimatei]

It works indeed. Would you like a patch?

[anakryiko]

that would be great, thanks. Please also include a comment explaining why we need that.