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Clang target bpf compile issue/fail on Ubuntu and Debian


Jesper Dangaard Brouer
 

The BPF UAPI header file <linux/bpf.h> includes <linux/types.h>, which gives
BPF-programs access to types e.g. __u32, __u64, __u8, etc.

On Ubuntu/Debian when compiling with clang option[1] "-target bpf" the
compile fails because it cannot find the file <asm/types.h>, which is
included from <linux/types.h>. This is because Ubuntu/Debian tries to
support multiple architectures on a single system[2]. On x86_64 the file
<asm/types.h> is located in /usr/include/x86_64-linux-gnu/, which the distro
compiler will add to it's search path (/usr/include/<triplet> [3]). Note, it
works if not specifying target bpf, but as explained in kernel doc[1] the
clang target bpf should really be used (to avoid other issues).

There are two workarounds: (1) To have an extra include dir on Ubuntu (which
seems too x86 specific) like: CFLAGS += -I/usr/include/x86_64-linux-gnu .
Or (2) install the package gcc-multilib on Ubuntu.

The question is: Should Ubuntu/Debian have a /usr/include/<triplet>
directory for BPF? (as part of their multi-arch approach)

Or should clang use the compile-host's triplet for the /usr/include/triplet
path even when giving clang -target bpf option?

p.s. GCC choose 'bpf-unknown-none' target triplet for BPF.


Links:
[1] https://www.kernel.org/doc/html/latest/bpf/bpf_devel_QA.html#q-clang-flag-for-target-bpf
[2] https://wiki.ubuntu.com/MultiarchSpec
[3] https://wiki.osdev.org/Target_Triplet
--
Best regards,
Jesper Dangaard Brouer
MSc.CS, Principal Kernel Engineer at Red Hat
LinkedIn: http://www.linkedin.com/in/brouer