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Subject: Re: [PATCH bpf-next] bpf, x86: remove unnecessary handling of BPF_SUB
 atomic op
To:     Brendan Jackman <jackmanb@google.com>, Hou Tao <houtao1@huawei.com>
Cc:     Alexei Starovoitov <ast@kernel.org>,
        John Fastabend <john.fastabend@gmail.com>,
        Martin KaFai Lau <kafai@fb.com>, Yonghong Song <yhs@fb.com>,
        Andrii Nakryiko <andrii@kernel.org>,
        "David S . Miller" <davem@davemloft.net>,
        Jakub Kicinski <kuba@kernel.org>, netdev@vger.kernel.org,
        bpf <bpf@vger.kernel.org>
References: <20220127083240.1425481-1-houtao1@huawei.com>
 <CA+i-1C2HBja-8Am4gHkcrYdkruw0+sOaGDejc9DS-HfYVXVfyQ@mail.gmail.com>
From:   Daniel Borkmann <daniel@iogearbox.net>
Message-ID: <c0831a45-3d39-891b-b89c-36167421d28b@iogearbox.net>
Date:   Thu, 27 Jan 2022 22:51:07 +0100
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On 1/27/22 11:48 AM, Brendan Jackman wrote:
> Yep - BPF_SUB is also excluded in Documentation/networking/filter.rst,
> plus the interpreter and verifier don't support it.
> 
> Thanks,
> 
> Acked-by: Brendan Jackman <jackmanb@google.com>

I was wondering about verifier specifically as well. Added a note to the
commit log that verifier rejects BPF_SUB while applying, thanks!

> On Thu, 27 Jan 2022 at 09:17, Hou Tao <houtao1@huawei.com> wrote:
>>
>> According to the LLVM commit (https://reviews.llvm.org/D72184),
>> sync_fetch_and_sub() is implemented as a negation followed by
>> sync_fetch_and_add(), so there will be no BPF_SUB op and just
>> remove it.
>>
>> Signed-off-by: Hou Tao <houtao1@huawei.com>
>> ---
>>   arch/x86/net/bpf_jit_comp.c | 1 -
>>   1 file changed, 1 deletion(-)
>>
>> diff --git a/arch/x86/net/bpf_jit_comp.c b/arch/x86/net/bpf_jit_comp.c
>> index ce1f86f245c9..5d643ebb1e56 100644
>> --- a/arch/x86/net/bpf_jit_comp.c
>> +++ b/arch/x86/net/bpf_jit_comp.c
>> @@ -787,7 +787,6 @@ static int emit_atomic(u8 **pprog, u8 atomic_op,
>>          /* emit opcode */
>>          switch (atomic_op) {
>>          case BPF_ADD:
>> -       case BPF_SUB:
>>          case BPF_AND:
>>          case BPF_OR:
>>          case BPF_XOR:
>> --
>> 2.29.2
>>