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From:   Paul Burton <paul.burton@mips.com>
To:     Paul Burton <pburton@wavecomp.com>
CC:     "linux-mips@vger.kernel.org" <linux-mips@vger.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>,
        "netdev@vger.kernel.org" <netdev@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Jiong Wang <jiong.wang@netronome.com>,
        Paul Burton <pburton@wavecomp.com>,
        "stable@vger.kernel.org" <stable@vger.kernel.org>,
        "linux-mips@vger.kernel.org" <linux-mips@vger.kernel.org>
Subject: Re: [PATCH 1/2] MIPS: eBPF: Always return sign extended 32b values
Thread-Topic: [PATCH 1/2] MIPS: eBPF: Always return sign extended 32b values
Thread-Index: AQHUxWsFAaDSuQE4g0KueZPNLuu8CqXnnf+A
Date:   Tue, 19 Feb 2019 20:48:08 +0000
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References: <20190215201321.15725-1-paul.burton@mips.com>
In-Reply-To: <20190215201321.15725-1-paul.burton@mips.com>
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Hello,

Paul Burton wrote:
> The function prototype used to call JITed eBPF code (ie. the type of the
> struct bpf_prog bpf_func field) returns an unsigned int. The MIPS n64
> ABI that MIPS64 kernels target defines that 32 bit integers should
> always be sign extended when passed in registers as either arguments or
> return values.
>=20
> This means that when returning any value which may not already be sign
> extended (ie. of type REG_64BIT or REG_32BIT_ZERO_EX) we need to perform
> that sign extension in order to comply with the n64 ABI. Without this we
> see strange looking test failures from test_bpf.ko, such as:
>=20
> test_bpf: #65 ALU64_MOV_X:
> dst =3D 4294967295 jited:1 ret -1 !=3D -1 FAIL (1 times)
>=20
> Although the return value printed matches the expected value, this is
> only because printf is only examining the least significant 32 bits of
> the 64 bit register value we returned. The register holding the expected
> value is sign extended whilst the v0 register was set to a zero extended
> value by our JITed code, so when compared by a conditional branch
> instruction the values are not equal.
>=20
> We already handle this when the return value register is of type
> REG_32BIT_ZERO_EX, so simply extend this to also cover REG_64BIT.
>=20
> Signed-off-by: Paul Burton <paul.burton@mips.com>
> Fixes: b6bd53f9c4e8 ("MIPS: Add missing file for eBPF JIT.")
> Cc: stable@vger.kernel.org # v4.13+

Series applied to mips-next.

Thanks,
    Paul

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