linux-kernel - Re: [RFC PATCH net-next] tools/bpftool: use version from the kernel source tree

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Date:   Thu, 21 Dec 2017 09:34:21 -0800
From:   Jakub Kicinski <jakub.kicinski@...ronome.com>
To:     Roman Gushchin <guro@...com>
Cc:     <netdev@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <kernel-team@...com>, <scientist@...com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Arnaldo Carvalho de Melo <acme@...hat.com>
Subject: Re: [RFC PATCH net-next] tools/bpftool: use version from the kernel
 source tree

On Thu, 21 Dec 2017 12:07:42 +0000, Roman Gushchin wrote:
> On Wed, Dec 20, 2017 at 01:52:18PM -0800, Jakub Kicinski wrote:
> > On Wed, 20 Dec 2017 20:53:41 +0000, Roman Gushchin wrote:  
> > > On Wed, Dec 20, 2017 at 12:29:21PM -0800, Jakub Kicinski wrote:  
> > > Hm, why it's better? It's not only about the kernel version,
> > > IMO it's generally better to use includes from the source tree,
> > > rather then system-wide installed kernel headers.  
> > 
> > Right I agree the kernel headers are preferred.  I'm not entirely sure
> > why we don't use them, if it was OK to assume usr/ is there we wouldn't
> > need the tools/include/uapi/ contraption.  Maybe Arnaldo could explain?
> >   
> > > I've got about out-of-source builds, but do we support it in general?
> > > How can I build bpftool outside of the kernel tree?
> > > I've tried a bit, but failed.  
> > 
> > This is what I do:
> > 
> > make -C tools/bpf/bpftool/ W=1 O=/tmp/builds/bpftool  
> 
> This works perfectly with my patch:
> 
> $ make -C ~/linux/tools/bpf/ W=1 O=/home/guro/build/ --trace
> <...>
> echo '  CC       '/home/guro/build/main.o;gcc -O2 -W -Wall -Wextra -Wno-unused-parameter -Wshadow -D__EXPORTED_HEADERS__ -I/home/guro/linux/tools/include/uapi -I/home/guro/linux/tools/include -I/home/guro/linux/tools/lib/bpf -I/home/guro/linux/kernel/bpf/ -I/home/guro/linux/usr/include -DNEW_DISSASSEMBLER_SIGNATURE   -c -MMD -o /home/guro/build/main.o main.c
> <...>
> echo '  LINK     '/home/guro/build/bpftool;gcc -O2 -W -Wall -Wextra -Wno-unused-parameter -Wshadow -D__EXPORTED_HEADERS__ -I/home/guro/linux/tools/include/uapi -I/home/guro/linux/tools/include -I/home/guro/linux/tools/lib/bpf -I/home/guro/linux/kernel/bpf/ -I/home/guro/linux/usr/include -DNEW_DISSASSEMBLER_SIGNATURE -o /home/guro/build/bpftool /home/guro/build/common.o /home/guro/build/cgroup.o /home/guro/build/main.o /home/guro/build/json_writer.o /home/guro/build/prog.o /home/guro/build/map.o /home/guro/build/jit_disasm.o /home/guro/build/disasm.o /home/guro/build/libbpf.a -lelf -lbfd -lopcodes /home/guro/build/libbpf.a
>   LINK     /home/guro/build/bpftool
> make[1]: Leaving directory '/home/guro/linux/tools/bpf/bpftool'
> make: Leaving directory '/home/guro/linux/tools/bpf'
> 
> $ ./build/bpftool version
> ./build/bpftool v4.15.0

Argh, sorry for the confusion you need to build the kernel out-of-source
as well.  In my case I build the kernel and bpftool out of source, and
then the usr/ doesn't actually contain the auto-generated headers:

$ ls ~/devel/linux/usr/
gen_init_cpio.c  initramfs_data.S  Kconfig  Makefile

Only build directory does:

$ ls /tmp/builds/usr/
built-in.o  gen_init_cpio  include  initramfs_data.cpio  initramfs_data.o  modules.builtin  modules.order

Let me reiterate, the user space headers we need should all be already
included in -I$(srctree)/tools/include/uapi, and make kernelversion is
nice because it also adds the -rc tags.