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

From: Stanislav Fomichev <sdf@fomichev.me>
To: Yonghong Song <yhs@fb.com>
Cc: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
	Shuah Khan <shuah@kernel.org>,
	Alexei Starovoitov <ast@kernel.org>,
	Stanislav Fomichev <sdf@google.com>,
	Daniel Borkmann <daniel@iogearbox.net>, Martin Lau <kafai@fb.com>,
	Song Liu <songliubraving@fb.com>,
	"linux-kselftest@vger.kernel.org"
	<linux-kselftest@vger.kernel.org>,
	"netdev@vger.kernel.org" <netdev@vger.kernel.org>,
	"bpf@vger.kernel.org" <bpf@vger.kernel.org>
Subject: Re: [PATCHv2 bpf-next 1/3] bpf, tests: tweak endianness selection
Date: Wed, 20 Mar 2019 15:27:59 -0700	[thread overview]
Message-ID: <20190320222759.GQ7431@mini-arch.hsd1.ca.comcast.net> (raw)
In-Reply-To: <4c6db4e5-8e77-2e01-a4ab-25808df2b857@fb.com>

On 03/20, Yonghong Song wrote:
> 
> 
> On 3/20/19 10:13 AM, Stanislav Fomichev wrote:
> > 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@gmail.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).
> 
> Inside kernel clang compiler header (linux/compiler-clang.h) does not 
> define __HAVE_BUILTIN_BSWAP16__. So it will go to the "else" branch in 
> the above. So I think it should work with clang + bpf.
Hm, isn't it the opposite of what we want then? I think for llvm+bpf we always
want to use the builtins to make it properly generate
BPF_TO_BE/BPF_TO_LE instructions.

> > 
> >>
> >>   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
> >>