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Subject: Re: [PATCH bpf 1/2] bpf, x32: Fix invalid instruction in BPF_LDX
 zero-extension
To:     Luke Nelson <lukenels@cs.washington.edu>, bpf@vger.kernel.org
Cc:     Luke Nelson <luke.r.nels@gmail.com>, Xi Wang <xi.wang@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
References: <20200421171552.28393-1-luke.r.nels@gmail.com>
From:   "H. Peter Anvin" <hpa@zytor.com>
Message-ID: <6f1130b3-eaea-cc5e-716f-5d6be77101b9@zytor.com>
Date:   Tue, 21 Apr 2020 10:39:00 -0700
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On 2020-04-21 10:15, Luke Nelson wrote:
> The current JIT uses the following sequence to zero-extend into the
> upper 32 bits of the destination register for BPF_LDX BPF_{B,H,W},
> when the destination register is not on the stack:
> 
>   EMIT3(0xC7, add_1reg(0xC0, dst_hi), 0);
> 
> However, this is not a valid instruction on x86.
> 
> This patch fixes the problem by instead emitting "xor dst_hi,dst_hi"
> to clear the upper 32 bits.

x32 is not x86-32.  In Linux we generally call the latter "i386".

C7 /0 imm32 is a valid instruction on i386. 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.

The real error is using EMIT3() instead of EMIT2_off32(), but XOR is
more efficient. However, let's make the bug statement *correct*, or it
is going to confuse the Hades out of people in the future.

	-hpa