netdev - Re: [PATCHv2 bpf-next 1/3] bpf, tests: tweak endianness selection

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Date:   Wed, 20 Mar 2019 10:13:32 -0700
From:   Stanislav Fomichev <sdf@...ichev.me>
To:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Cc:     Shuah Khan <shuah@...nel.org>, Alexei Starovoitov <ast@...nel.org>,
        Stanislav Fomichev <sdf@...gle.com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        linux-kselftest@...r.kernel.org, netdev@...r.kernel.org,
        bpf@...r.kernel.org
Subject: Re: [PATCHv2 bpf-next 1/3] bpf, tests: tweak endianness selection

On 03/20, Sergey Senozhatsky wrote:
> Not all compilers have __builtin_bswap16() and __builtin_bswap32(),
> thus not all compilers are able to compile the following code:
> 
>         (__builtin_constant_p(x) ? \
>                 ___constant_swab16(x) : __builtin_bswap16(x))
> 
> That's the reason why bpf_ntohl() doesn't work on GCC < 4.8, for
> instance:
> 
>         error: implicit declaration of function '__builtin_bswap16'
> 
> We can use __builtin_bswap16() only if compiler has this built-in,
> that is, only if __HAVE_BUILTIN_BSWAP16__ is defined. Standard UAPI
> __swab16()/__swab32() take care of that, and, additionally, handle
> __builtin_constant_p() cases as well:
> 
>  #ifdef __HAVE_BUILTIN_BSWAP16__
>  #define __swab16(x) (__u16)__builtin_bswap16((__u16)(x))
>  #else
>  #define __swab16(x)                             \
>          (__builtin_constant_p((__u16)(x)) ?     \
>          ___constant_swab16(x) :                 \
>          __fswab16(x))
>  #endif
> 
> So we can tweak selftests/bpf/bpf_endian.h and use UAPI
> __swab16()/__swab32().
> 
> Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@...il.com>
> ---
> 
> v2: fixed build error, reshuffled patches (Stanislav Fomichev)
Tested them locally with the compiler I saw the initial issues with - all
fine, I don't see any errors with the older gcc.

One last question I have is: what happens in the llvm+bpf case? Have
you tested that? I think LLVM has all the builtins required, but since
we are relying on the swab.h now (and it relies on
__HAVE_BUILTIN_BSWAP16__), I wonder whether this detection works
correctly on the llvm when targeting bpf. (sidenote: bpf_endian.h can be
used from both userspace and bpf programs).

> 
>  tools/testing/selftests/bpf/bpf_endian.h | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/tools/testing/selftests/bpf/bpf_endian.h b/tools/testing/selftests/bpf/bpf_endian.h
> index b25595ea4a78..1ed268b2002b 100644
> --- a/tools/testing/selftests/bpf/bpf_endian.h
> +++ b/tools/testing/selftests/bpf/bpf_endian.h
> @@ -20,12 +20,12 @@
>   * use different targets.
>   */
>  #if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
> -# define __bpf_ntohs(x)			__builtin_bswap16(x)
> -# define __bpf_htons(x)			__builtin_bswap16(x)
> +# define __bpf_ntohs(x)			__swab16(x)
> +# define __bpf_htons(x)			__swab16(x)
>  # define __bpf_constant_ntohs(x)	___constant_swab16(x)
>  # define __bpf_constant_htons(x)	___constant_swab16(x)
> -# define __bpf_ntohl(x)			__builtin_bswap32(x)
> -# define __bpf_htonl(x)			__builtin_bswap32(x)
> +# define __bpf_ntohl(x)			__swab32(x)
> +# define __bpf_htonl(x)			__swab32(x)
>  # define __bpf_constant_ntohl(x)	___constant_swab32(x)
>  # define __bpf_constant_htonl(x)	___constant_swab32(x)
>  #elif __BYTE_ORDER__ == __ORDER_BIG_ENDIAN__
> -- 
> 2.21.0
>