Re: bpf: undefined shift in __bpf_prog_run

[David Miller]

From: Alexei Starovoitov <alexei.starovoitov@xxxxxxxxx>
Date: Fri, 4 Dec 2015 12:35:23 -0800

> On Fri, Dec 04, 2015 at 08:48:57PM +0100, Dmitry Vyukov wrote:
>> 
>> For example, a compiler can assume that result of left shift is larger
>> or equal to first operand, which in turn can allow it to elide some
>> bounds check in code, which in turn can lead to an exploit. I am not
>> saying that this particular pattern is present in the code, what I
>> want to say is that such undefined behaviors can lead to very
>> unpredictable and unexpected consequences.
> 
> Within bpf it cannot.
> shift is not used in any memory or bounds operations.
> so reg <<= 1234 cannot be exploited.

+1
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