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Subject: Re: [PATCH bpf 1/3] bpf, x32: Fix bug with ALU64 {LSH,RSH,ARSH} BPF_X
 shift by 0
To:     Luke Nelson <lukenels@cs.washington.edu>,
        linux-kernel@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>,
        "H. Peter Anvin" <hpa@zytor.com>, x86@kernel.org,
        Alexei Starovoitov <ast@kernel.org>,
        Martin KaFai Lau <kafai@fb.com>,
        Song Liu <songliubraving@fb.com>, Yonghong Song <yhs@fb.com>,
        Shuah Khan <shuah@kernel.org>,
        Jakub Kicinski <jakub.kicinski@netronome.com>,
        Jiong Wang <jiong.wang@netronome.com>,
        =?UTF-8?B?QmrDtnJuIFTDtnBlbA==?= <bjorn.topel@gmail.com>,
        netdev@vger.kernel.org, bpf@vger.kernel.org,
        linux-kselftest@vger.kernel.org
References: <20190629055759.28365-1-luke.r.nels@gmail.com>
From:   Daniel Borkmann <daniel@iogearbox.net>
Message-ID: <5c2080f4-532e-d239-13b1-4a5a620f6c33@iogearbox.net>
Date:   Wed, 3 Jul 2019 11:49:47 +0200
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On 06/29/2019 07:57 AM, Luke Nelson wrote:
> The current x32 BPF JIT for shift operations is not correct when the
> shift amount in a register is 0. The expected behavior is a no-op, whereas
> the current implementation changes bits in the destination register.
> 
> The following example demonstrates the bug. The expected result of this
> program is 1, but the current JITed code returns 2.
> 
>   r0 = 1
>   r1 = 1
>   r2 = 0
>   r1 <<= r2
>   if r1 == 1 goto end
>   r0 = 2
> end:
>   exit
> 
> The bug is caused by an incorrect assumption by the JIT that a shift by
> 32 clear the register. On x32 however, shifts use the lower 5 bits of
> the source, making a shift by 32 equivalent to a shift by 0.
> 
> This patch fixes the bug using double-precision shifts, which also
> simplifies the code.
> 
> Fixes: 03f5781be2c7 ("bpf, x86_32: add eBPF JIT compiler for ia32")
> Co-developed-by: Xi Wang <xi.wang@gmail.com>
> Signed-off-by: Xi Wang <xi.wang@gmail.com>
> Signed-off-by: Luke Nelson <luke.r.nels@gmail.com>

Series applied, thanks!