linux-kernel - Re: [PATCH 1/2] MIPS: eBPF: Always return sign extended 32b values

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Message-ID: <MWHPR2201MB1277F46755F2546E50A5EA9EC17C0@MWHPR2201MB1277.namprd22.prod.outlook.com>
Date:   Tue, 19 Feb 2019 20:48:08 +0000
From:   Paul Burton <paul.burton@...s.com>
To:     Paul Burton <pburton@...ecomp.com>
CC:     "linux-mips@...r.kernel.org" <linux-mips@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Jiong Wang <jiong.wang@...ronome.com>,
        Paul Burton <pburton@...ecomp.com>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>,
        "linux-mips@...r.kernel.org" <linux-mips@...r.kernel.org>
Subject: Re: [PATCH 1/2] MIPS: eBPF: Always return sign extended 32b values

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.
> 
> 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:
> 
> test_bpf: #65 ALU64_MOV_X:
> dst = 4294967295 jited:1 ret -1 != -1 FAIL (1 times)
> 
> 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.
> 
> 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.
> 
> Signed-off-by: Paul Burton <paul.burton@...s.com>
> Fixes: b6bd53f9c4e8 ("MIPS: Add missing file for eBPF JIT.")
> Cc: stable@...r.kernel.org # v4.13+

Series applied to mips-next.

Thanks,
    Paul

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