On 12/13/2018 03:00 AM, Alexander Potapenko wrote:
> Hi BPF maintainers,
>
> some time ago KMSAN found an issue in BPF code which we decided to
> suppress at that point, but now I'd like to bring it to your
> attention.
> Namely, some BPF programs may contain instructions that XOR a register
> with itself.
> This effectively results in the following C code:
> regs[BPF_REG_A] = regs[BPF_REG_A] ^ regs[BPF_REG_A];
> or
> regs[BPF_REG_X] = regs[BPF_REG_X] ^ regs[BPF_REG_X];
> being executed.
>
> According to the C11 standard this is undefined behavior, so KMSAN
> reports an error in this case.
eBPF is not C11 ;)
XOR boolean operation on a cpu is following boolean logic, which is much
stronger than
any C standard.
>
> Do you think it's feasible to explicitly initialize the register
> values like it's done here:
> https://github.com/google/kmsan/commit/813c0f3d45ebfa321d70b4b06cc054518dd1d90d
> ?
>
> Thanks,
> Alexander Potapenko
> Software Engineer
>
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