Re: [PATCH bpf-next v2] bpf, mips: Fix build errors about __NR_bpf undeclared

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

On Thu, Nov 25, 2021 at 3:05 PM Daniel Borkmann <daniel@iogearbox.net> wrote:
>
> [ +Johan ]
>
> On 11/25/21 2:36 AM, Tiezhu Yang wrote:
> > Add the __NR_bpf definitions to fix the following build errors for mips.
> >
> >   $ cd tools/bpf/bpftool
> >   $ make
> >   [...]
> >   bpf.c:54:4: error: #error __NR_bpf not defined. libbpf does not support your arch.
> >    #  error __NR_bpf not defined. libbpf does not support your arch.
> >       ^~~~~
> >   bpf.c: In function ‘sys_bpf’:
> >   bpf.c:66:17: error: ‘__NR_bpf’ undeclared (first use in this function); did you mean ‘__NR_brk’?
> >     return syscall(__NR_bpf, cmd, attr, size);
> >                    ^~~~~~~~
> >                    __NR_brk
> >   [...]
> >   In file included from gen_loader.c:15:0:
> >   skel_internal.h: In function ‘skel_sys_bpf’:
> >   skel_internal.h:53:17: error: ‘__NR_bpf’ undeclared (first use in this function); did you mean ‘__NR_brk’?
> >     return syscall(__NR_bpf, cmd, attr, size);
> >                    ^~~~~~~~
> >                    __NR_brk
> >
> > We can see the following generated definitions:
> >
> >   $ grep -r "#define __NR_bpf" arch/mips
> >   arch/mips/include/generated/uapi/asm/unistd_o32.h:#define __NR_bpf (__NR_Linux + 355)
> >   arch/mips/include/generated/uapi/asm/unistd_n64.h:#define __NR_bpf (__NR_Linux + 315)
> >   arch/mips/include/generated/uapi/asm/unistd_n32.h:#define __NR_bpf (__NR_Linux + 319)
> >
> > The __NR_Linux is defined in arch/mips/include/uapi/asm/unistd.h:
> >
> >   $ grep -r "#define __NR_Linux" arch/mips
> >   arch/mips/include/uapi/asm/unistd.h:#define __NR_Linux      4000
> >   arch/mips/include/uapi/asm/unistd.h:#define __NR_Linux      5000
> >   arch/mips/include/uapi/asm/unistd.h:#define __NR_Linux      6000
> >
> > That is to say, __NR_bpf is
> > 4000 + 355 = 4355 for mips o32,
> > 6000 + 319 = 6319 for mips n32,
> > 5000 + 315 = 5315 for mips n64.
> >
> > So use the GCC pre-defined macro _ABIO32, _ABIN32 and _ABI64 [1] to define
> > the corresponding __NR_bpf.
> >
> > This patch is similar with commit bad1926dd2f6 ("bpf, s390: fix build for
> > libbpf and selftest suite").
> >
> > [1] https://gcc.gnu.org/git/?p=gcc.git;a=blob;f=gcc/config/mips/mips.h#l549
> >
> > Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
> > ---
> >
> > v2: use a final number without __NR_Linux to define __NR_bpf
> >      suggested by Andrii Nakryiko, thank you.
> >
> >   tools/build/feature/test-bpf.c |  6 ++++++
> >   tools/lib/bpf/bpf.c            |  6 ++++++
> >   tools/lib/bpf/skel_internal.h  | 10 ++++++++++
> >   3 files changed, 22 insertions(+)
> >
> > diff --git a/tools/build/feature/test-bpf.c b/tools/build/feature/test-bpf.c
> > index 82070ea..727d22e 100644
> > --- a/tools/build/feature/test-bpf.c
> > +++ b/tools/build/feature/test-bpf.c
> > @@ -14,6 +14,12 @@
> >   #  define __NR_bpf 349
> >   # elif defined(__s390__)
> >   #  define __NR_bpf 351
> > +# elif defined(__mips__) && defined(_ABIO32)
> > +#  define __NR_bpf 4355
> > +# elif defined(__mips__) && defined(_ABIN32)
> > +#  define __NR_bpf 6319
> > +# elif defined(__mips__) && defined(_ABI64)
> > +#  define __NR_bpf 5315
> >   # else
> >   #  error __NR_bpf not defined. libbpf does not support your arch.
> >   # endif
> > diff --git a/tools/lib/bpf/bpf.c b/tools/lib/bpf/bpf.c
> > index 94560ba..17f9fe2 100644
> > --- a/tools/lib/bpf/bpf.c
> > +++ b/tools/lib/bpf/bpf.c
> > @@ -50,6 +50,12 @@
> >   #  define __NR_bpf 351
> >   # elif defined(__arc__)
> >   #  define __NR_bpf 280
> > +# elif defined(__mips__) && defined(_ABIO32)
> > +#  define __NR_bpf 4355
> > +# elif defined(__mips__) && defined(_ABIN32)
> > +#  define __NR_bpf 6319
> > +# elif defined(__mips__) && defined(_ABI64)
> > +#  define __NR_bpf 5315
> >   # else
> >   #  error __NR_bpf not defined. libbpf does not support your arch.
> >   # endif
> > diff --git a/tools/lib/bpf/skel_internal.h b/tools/lib/bpf/skel_internal.h
> > index 9cf6670..064da66 100644
> > --- a/tools/lib/bpf/skel_internal.h
> > +++ b/tools/lib/bpf/skel_internal.h
> > @@ -7,6 +7,16 @@
> >   #include <sys/syscall.h>
> >   #include <sys/mman.h>
> >
> > +#ifndef __NR_bpf
> > +# if defined(__mips__) && defined(_ABIO32)
> > +#  define __NR_bpf 4355
> > +# elif defined(__mips__) && defined(_ABIN32)
> > +#  define __NR_bpf 6319
> > +# elif defined(__mips__) && defined(_ABI64)
> > +#  define __NR_bpf 5315
> > +# endif
> > +#endif
>
> Bit unfortunate that mips is the only arch where we run into this apparently? :/
> Given we also redefine a skel_sys_bpf(), maybe libbpf should just provide something
> like a `LIBBPF_API int libbpf_sys_bpf(enum bpf_cmd cmd, [...])` so we rely implicitly
> only on the libbpf-internal __NR_bpf instead of duplicating again?

Unfortunately the point of skel_internal.h is to not depend on libbpf
APIs. I think for now it should be fine to have this duplication (not
that those numbers are going to change ever) and we can improve this
later.

>
> >   /* This file is a base header for auto-generated *.lskel.h files.
> >    * Its contents will change and may become part of auto-generation in the future.
> >    *
> >
>