Re: [PATCH bpf-next v3 07/11] bpf: Fix a false rejection caused by AND operation

[Eduard Zingerman <eddyz87@gmail.com>]

On Sun, 2024-04-28 at 23:15 +0800, Xu Kuohai wrote:

[...]

> diff --git a/kernel/bpf/tnum.c b/kernel/bpf/tnum.c
> index 9dbc31b25e3d..9d4480a683ca 100644
> --- a/kernel/bpf/tnum.c
> +++ b/kernel/bpf/tnum.c
> @@ -150,6 +150,29 @@ struct tnum tnum_intersect(struct tnum a, struct tnum b)
>          return TNUM(v & ~mu, mu);
>   }
> 
> +/*
> + * Each bit has 3 states: unkown, known 0, known 1. If using x to represent
> + * unknown state, the result of the union of two bits is as follows:
> + *
> + *         | x    0    1
> + *    -----+------------
> + *     x   | x    x    x
> + *     0   | x    0    x
> + *     1   | x    x    1
> + *
> + * For tnum a and b, only the bits that are both known 0 or known 1 in a
> + * and b are known in the result of union a and b.
> + */
> +struct tnum tnum_union(struct tnum a, struct tnum b)
> +{
> +       u64 v0, v1, mu;
> +
> +       mu = a.mask | b.mask; // unkown bits either in a or b
> +       v1 = (a.value & b.value) & ~mu; // "known 1" bits in both a and b
> +       v0 = (~a.value & ~b.value) & ~mu; // "known 0" bits in both a and b
> +       return TNUM(v1, mu | ~(v0 | v1));
> +}
> +

Zero would be represented as {.value=0,.mask=0}, suppose 'b' is zero:

1. mu = a.mask | 0;                     2. mu = a.mask;
   v1 = (a.value & 0) & ~mu;               v1 = 0;
   v0 = (~a.value & ~0) & ~mu;             v0 = ~a.value & ~mu;
   return TNUM(v1, mu | ~(v0 | v1));       return TNUM(v1, mu | ~(v0 | v1));

3. v1 = 0;                              4. v1 = 0;
   v0 = ~a.value & ~a.mask;                v0 = ~a.value & ~a.mask;
   return TNUM(v1, a.mask | ~(v0 | v1));   return TNUM(0, a.mask | ~(~a.value & ~a.mask));

5. return TNUM(0, a.mask | a.value)

So ultimately this says that for 1's that we knew
we no longer know if those are 1's.
Which seems to make sense.