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. > >> >> 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 >>
