netdev - Re: [PATCH iproute2 master] bpf: provide fallback defs for __NR

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Date:   Wed, 14 Jun 2017 16:34:08 -0700
From:   Stephen Hemminger <stephen@...workplumber.org>
To:     Daniel Borkmann <daniel@...earbox.net>
Cc:     alexei.starovoitov@...il.com, netdev@...r.kernel.org
Subject: Re: [PATCH iproute2 master] bpf: provide fallback defs for __NR_bpf
 when not avail

On Thu, 15 Jun 2017 01:01:14 +0200
Daniel Borkmann <daniel@...earbox.net> wrote:

> On 06/15/2017 12:56 AM, Stephen Hemminger wrote:
> > On Thu, 15 Jun 2017 00:47:15 +0200
> > Daniel Borkmann <daniel@...earbox.net> wrote:
> >  
> >> panji reported that he wasn't able to build iproute2's bpf library
> >> due to lack of __NR_bpf in his system headers. Providing a fallback
> >> definition when __NR_bpf is not available in the system lets the
> >> loader compile just fine, so lets add them for majority of archs.
> >>
> >> Reported-by: panji <jpan@...e.nl>
> >> Signed-off-by: Daniel Borkmann <daniel@...earbox.net>
> >> ---
> >>   lib/bpf.c | 20 ++++++++++++++++++++
> >>   1 file changed, 20 insertions(+)
> >>
> >> diff --git a/lib/bpf.c b/lib/bpf.c
> >> index ae4d97d..e1e29cc 100644
> >> --- a/lib/bpf.c
> >> +++ b/lib/bpf.c
> >> @@ -128,6 +128,26 @@ static inline __u64 bpf_ptr_to_u64(const void *ptr)
> >>   	return (__u64)(unsigned long)ptr;
> >>   }
> >>
> >> +#ifndef __NR_bpf
> >> +# if defined(__i386__)
> >> +#  define __NR_bpf 357
> >> +# elif defined(__x86_64__)
> >> +#  define __NR_bpf 321
> >> +# elif defined(__aarch64__)
> >> +#  define __NR_bpf 280
> >> +# elif defined(__sparc__)
> >> +#  define __NR_bpf 349
> >> +# elif defined(__arm__)
> >> +#  define __NR_bpf 386
> >> +# elif defined(__powerpc__)
> >> +#  define __NR_bpf 361
> >> +# elif defined(__s390__)
> >> +#  define __NR_bpf 351
> >> +# else
> >> +#  error __NR_bpf not defined. Update kernel headers.
> >> +# endif
> >> +#endif
> >> +
> >>   static int bpf(int cmd, union bpf_attr *attr, unsigned int size)
> >>   {
> >>   #ifdef __NR_bpf  
> >
> > Sorry this looks like a mess. enumerating architectures in two different
> > projects is likely to break in future.  
> 
> It says ifndef __NR_bpf, so only used then. And the numbers are uabi,
> what will break here exactly? libbpf in kernel tree is having a similar
> approach by the way.

You are defining values in two places (kernel and userspace) which has caused
lots of mismatch in the past. Why isn't this in a kernel uapi header somewhere?