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

[Paul Burton]

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@xxxxxxxx>
> Fixes: b6bd53f9c4e8 ("MIPS: Add missing file for eBPF JIT.")
> Cc: stable@xxxxxxxxxxxxxxx # v4.13+

Series applied to mips-next.

Thanks,
    Paul

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