Clang target bpf compile issue/fail on Ubuntu and Debian

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

Re: Clang target bpf compile issue/fail on Ubuntu and Debian

[Yonghong Song]

On 7/29/20 12:13 PM, Jesper Dangaard Brouer wrote:
> 
> 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 .

How about non x86 architecture? It will be something like 
/usr/include/aarch64-linux-gnu? Do you think we could have some header 
file conflict
since this directory may contain a lot other header files as well?

> Or (2) install the package gcc-multilib on Ubuntu.

I think having a /usr/include/bpf-unknown-none and the distro add this
path to the compiler has target bpf. In this way, only necessary header
files will be put there. Less potential issues.

But in either case, it will be good if this level of details are
hidden from the user.

I am not super familiar with the distro packaging of a particular 
compiler and its include directories. The above is just my 2 cents.

> 
> 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://urldefense.proofpoint.com/v2/url?u=https-3A__wiki.ubuntu.com_MultiarchSpec&d=DwICAg&c=5VD0RTtNlTh3ycd41b3MUw&r=DA8e1B5r073vIqRrFz7MRA&m=ZEguArah72427WZMt53NXZa8LuQEGJKvCOgjH96NggQ&s=5xL0VWya8ULHcj4pfw6oxqHoKWIXNfJnj2G7rdaQyR8&e=
> [3] https://urldefense.proofpoint.com/v2/url?u=https-3A__wiki.osdev.org_Target-5FTriplet&d=DwICAg&c=5VD0RTtNlTh3ycd41b3MUw&r=DA8e1B5r073vIqRrFz7MRA&m=ZEguArah72427WZMt53NXZa8LuQEGJKvCOgjH96NggQ&s=mym8eKus7o0PzfkC2-IMyPU84OwVb2_A4ky2JHcdexQ&e=
>