netdev - Re: [PATCH v2 1/2] selftests/bpf: Makefile: add includes to fix broken test build

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Date:   Mon, 30 Apr 2018 12:12:02 +0200
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Sirio Balmelli <sirio@...d.ch>
Cc:     netdev@...r.kernel.org
Subject: Re: [PATCH v2 1/2] selftests/bpf: Makefile: add includes to fix
 broken test build

On 04/30/2018 09:17 AM, Sirio Balmelli wrote:
> Build fails with missing 'asm/bitsperlong.h'.
> 
> Include this from tools/arch/[arch]/include/uapi
> using an architecture-specific include sourced from 'Module.symvers'
> (is this legitimate? another idea was to do a `uname -p` but what if
> caller was cross-compiling?)

To point to an arch specific include, the way this works under tools
can be seen e.g. in barrier.h, see tools/include/asm/barrier.h. This
will then pull in one of the tools/arch/*/include/asm/barrier.h. We
would need to do something similar for tools/arch/*/include/uapi/asm/bitsperlong.h

Then you also don't need the Module.symvers hack to derive the underlying
arch. See 720f228e8d31 ("bpf: fix broken BPF selftest build") for another,
similar example we had in the past.

> Once the arch-specific include is fixed, a lack of 'asm/byteorder.h'
> now rears its ugly head.
> The only sane (ie: will not pull in kernel headers) place I can find it
> is in $(ROOT)/usr/include - add this to the include.

Why it doesn't work adding a new file tools/include/uapi/asm/byteorder.h
that points to one present in tools/arch/*/include/uapi/asm/bitsperlong.h
or tools/include/uapi/asm-generic/bitsperlong.h otherwise?

> Note that $(ROOT)/usr/include is generated on kernel build, hence my
> assumption that 'Module.symvers' will also be present.

The problem with this approach is that you need to do 'make header_install'
in order to resolve this dependency, whereas tools infrastructure has no
such dependency. Here's some more background context on the tools infra
that perhaps helps why it's done this way:

  https://www.spinics.net/lists/netdev/msg487309.html

Please see to add a tools/include/uapi/asm/byteorder.h to fix the issue.

Thanks a lot,
Daniel