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

[Daniel Borkmann]

[ +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@xxxxxxxxxxx>
> ---
>
> 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?

>   /* 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.
>    *