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Date:   Wed, 22 Apr 2020 00:13:05 -0700
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Subject: Re: [PATCH bpf 1/2] bpf, x32: Fix invalid instruction in BPF_LDX zero-extension
To:     Xi Wang <xi.wang@gmail.com>
CC:     Luke Nelson <lukenels@cs.washington.edu>, bpf@vger.kernel.org,
        Luke Nelson <luke.r.nels@gmail.com>,
        Wang YanQing <udknight@gmail.com>,
        "David S. Miller" <davem@davemloft.net>,
        Alexey Kuznetsov <kuznet@ms2.inr.ac.ru>,
        Hideaki YOSHIFUJI <yoshfuji@linux-ipv6.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        x86@kernel.org, Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Martin KaFai Lau <kafai@fb.com>,
        Song Liu <songliubraving@fb.com>, Yonghong Song <yhs@fb.com>,
        Andrii Nakryiko <andriin@fb.com>,
        John Fastabend <john.fastabend@gmail.com>,
        KP Singh <kpsingh@chromium.org>, netdev@vger.kernel.org,
        linux-kernel@vger.kernel.org
From:   hpa@zytor.com
Message-ID: <05CE7897-C58E-40C0-8E08-C8E948B70286@zytor.com>
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On April 21, 2020 12:26:12 PM PDT, Xi Wang <xi=2Ewang@gmail=2Ecom> wrote:
>On Tue, Apr 21, 2020 at 10:39 AM H=2E Peter Anvin <hpa@zytor=2Ecom> wrote=
:
>> x32 is not x86-32=2E  In Linux we generally call the latter "i386"=2E
>
>Agreed=2E  Most of the previous patches to this file use "x32" and this
>one just wanted to be consistent=2E
>
>> C7 /0 imm32 is a valid instruction on i386=2E However, it is also
>> inefficient when the destination is a register, because B8+r imm32 is
>> equivalent, and when the value is zero, XOR is indeed more efficient=2E
>>
>> The real error is using EMIT3() instead of EMIT2_off32(), but XOR is
>> more efficient=2E However, let's make the bug statement *correct*, or
>it
>> is going to confuse the Hades out of people in the future=2E
>
>I don't see how the bug statement is incorrect, which merely points
>out that "C7 C0 0" is an invalid instruction, regardless of whether
>the JIT intended to emit C7 /0 imm32, B8+r imm32, 31 /r, 33 /r, or any
>other equivalent form=2E

C7 C0 0 is *not* an invalid instruction, although it is incomplete=2E It i=
s a different, but arguably even more serious, problem=2E
--=20
Sent from my Android device with K-9 Mail=2E Please excuse my brevity=2E