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

[Yonghong Song <yhs@fb.com>]

On 3/20/19 5:03 PM, Stanislav Fomichev wrote:
> On 03/21, Daniel Borkmann wrote:
>> On 03/20/2019 11:45 PM, Yonghong Song wrote:
>>> On 3/20/19 3:27 PM, Stanislav Fomichev wrote:
>>>> 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.
>>>
>>> Okay, I see. Then this patch will not achieve that.
>>> The following are two common ways to compile a bpf program:
>>>     - "clang -target bpf ...", maybe add macro __BPF__ somewhere
>>>       to indicate builtin_bswap16 always available?
>>>     - "clang <host target> ..." and then "llc -march=bpf ..."
>>>       in this case, __BPF__ macro is not available and
>>>       we will not be able to use builtin swap for bpf program.
>>>
>>> Maybe use __clang__ macro (or gcc macro) to distinguish between clang
>>> and gcc. If it is gcc we will check builtin availability, otherwise,
>>> we assume builtin always available? This not pretty though.
>>
>> I think the way this should be fixed is the following: In case
>> of LLVM (aka compiling BPF prog), we want the code to be as-is,
>> in case if gcc is compiling the hostprog, we either want to keep
>> using __builtin_bswap16() or fall-back to something else. Thus,
>> I would suggest, we add a new feature test for tooling infra under
>> tools/build/feature/ that compiles a dummy prog with __builtin_bswap16().
>> And in the bpf_endian.h we define __bpf_ntohs(x) to __bpf_swab16(x)
>> which either resolves to __builtin_bswap16() or some fallback
>> implementation if not available. I don't think there should be much
>> of an issue and it would follow the standard way to do it.
> It's not as easy as llvm vs gcc. We can compile userland tests with
> llvm/clang as well. We really need to distinguish between the target: bfp vs
> non-bpf: always use builtins in bpf case and fallback to swab.h for
> userland (or use feature detection, but swab.h should be enough in
> theory).
> 
> Can we rely on __bpf__ define?
> 
> $ cat tmp.c
> #ifdef __bpf__
> #error a
> #else
> #error b
> #endif
> $ clang -c -target bpf tmp.c
> tmp.c:2:2: error: a
> #error a
>   ^
>   1 error generated.

Yes, you can rely this, __bpf__, __bpf or __BPF__. These three
are clang predefined macros for target bpf.

> 
>>
>>>>>>>     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
>>>>>>>
>>