| lists.openwall.net | lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC | |
|
Open Source and information security mailing list archives
| ||
|
Message-ID: <8ab21b4e-885d-e91e-1614-f1b15660ce4c@iogearbox.net>
Date: Thu, 21 Mar 2019 00:08:15 +0100
From: Daniel Borkmann <daniel@...earbox.net>
To: Yonghong Song <yhs@...com>, Stanislav Fomichev <sdf@...ichev.me>
Cc: Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
Shuah Khan <shuah@...nel.org>,
Alexei Starovoitov <ast@...nel.org>,
Stanislav Fomichev <sdf@...gle.com>, Martin Lau <kafai@...com>,
Song Liu <songliubraving@...com>,
"linux-kselftest@...r.kernel.org" <linux-kselftest@...r.kernel.org>,
"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
"bpf@...r.kernel.org" <bpf@...r.kernel.org>
Subject: Re: [PATCHv2 bpf-next 1/3] bpf, tests: tweak endianness selection
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@...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).
>>>
>>> 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.
>>>>> 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
>>>>>
Powered by blists - more mailing lists