netdev - Re: Self-XORing BPF registers is undefined behavior

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Date:   Fri, 29 May 2020 09:46:24 +0100
From:   Edward Cree <ecree@...arflare.com>
To:     Dmitry Vyukov <dvyukov@...gle.com>
CC:     Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Alexander Potapenko <glider@...gle.com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Michal Kubecek <mkubecek@...e.cz>,
        Alexei Starovoitov <ast@...nel.org>,
        Networking <netdev@...r.kernel.org>
Subject: Re: Self-XORing BPF registers is undefined behavior

On 29/05/2020 07:14, Dmitry Vyukov wrote:
>> (In C99 it gets subtler because an 'indeterminate value' is
>>  defined to be 'either a valid value or a trap representation',
>>  so arguably the compiler can only do this stuff if it _has_
>>  trap representations for the type in question.)
> Interesting. Are you sure that's the meaning of 'indeterminate value'?
> My latest copy of the standard says:
>
> 3.19.2
> 1 indeterminate value
> either an unspecified value or a trap representation
Yes, but (from N1256):
| 3.17.3
| unspecified value
| valid value of the relevant type where this International Standard
| imposes no requirements on which value is chosen in any instance
| NOTE An unspecified value cannot be a trap representation

> My reading of this would be that this only prevents things from
> exploding in all possible random ways (like formatting drive). The
> effects are only reduced to either getting a random value, or a trap
> on access to the value. Both of these do not seem to be acceptable for
> a bpf program.
A random value, XORed with itself, produces 0, which is what we want.
(XORing a trap representation with itself, of course, produces a trap.)

So it'd be fine *unless* the 'in any instance' language can be read as
 allowing the uninitialised object to have *different* random values on
 separate accesses.

-ed