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@xxxxxxxxx> > >>>>> --- > >>>>> > >>>>> 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. > > >>>>> 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 > >>>>> >
