Re: [PATCH v5 bpf-next 06/23] bpf: add special smin32/smax32 derivation from 64-bit bounds

[Eduard Zingerman <eddyz87@gmail.com>]

On Fri, 2023-10-27 at 11:13 -0700, Andrii Nakryiko wrote:
> Add a special case where we can derive valid s32 bounds from umin/umax
> or smin/smax by stitching together negative s32 subrange and
> non-negative s32 subrange. That requires upper 32 bits to form a [N, N+1]
> range in u32 domain (taking into account wrap around, so 0xffffffff
> to 0x00000000 is a valid [N, N+1] range in this sense). See code comment
> for concrete examples.
> 
> Acked-by: Shung-Hsi Yu <shung-hsi.yu@suse.com>
> Signed-off-by: Andrii Nakryiko <andrii@kernel.org>

Acked-by: Eduard Zingerman <eddyz87@gmail.com>

fwiw, an alternative explanation might be arithmetic based.
Suppose:
. there are numbers a, b, c
. 2**31 <= b < 2**32
. 0 <= c < 2**31
. umin = 2**32 * a + b
. umax = 2**32 * (a + 1) + c

The number of values in the range represented by [umin; umax] is:
. N = umax - umin + 1 = 2**32 + c - b + 1
. min(N) = 2**32 + 0 - (2**32-1) + 1 = 2
. max(N) = 2**32 + (2**31 - 1) - 2**31 + 1 = 2**32
Hence [(s32)b; (s32)c] form a valid range.

At-least that's how I convinced myself.

> ---
>  kernel/bpf/verifier.c | 23 +++++++++++++++++++++++
>  1 file changed, 23 insertions(+)
> 
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 5082ca1ea5dc..38d21d0e46bd 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -2369,6 +2369,29 @@ static void __reg32_deduce_bounds(struct bpf_reg_state *reg)
>  			reg->s32_max_value = min_t(s32, reg->s32_max_value, (s32)reg->smax_value);
>  		}
>  	}
> +	/* Special case where upper bits form a small sequence of two
> +	 * sequential numbers (in 32-bit unsigned space, so 0xffffffff to
> +	 * 0x00000000 is also valid), while lower bits form a proper s32 range
> +	 * going from negative numbers to positive numbers. E.g., let's say we
> +	 * have s64 range [-1, 1] ([0xffffffffffffffff, 0x0000000000000001]).
> +	 * Possible s64 values are {-1, 0, 1} ({0xffffffffffffffff,
> +	 * 0x0000000000000000, 0x00000000000001}). Ignoring upper 32 bits,
> +	 * we still get a valid s32 range [-1, 1] ([0xffffffff, 0x00000001]).
> +	 * Note that it doesn't have to be 0xffffffff going to 0x00000000 in
> +	 * upper 32 bits. As a random example, s64 range
> +	 * [0xfffffff0ffffff00; 0xfffffff100000010], forms a valid s32 range
> +	 * [-16, 16] ([0xffffff00; 0x00000010]) in its 32 bit subregister.
> +	 */
> +	if ((u32)(reg->umin_value >> 32) + 1 == (u32)(reg->umax_value >> 32) &&
> +	    (s32)reg->umin_value < 0 && (s32)reg->umax_value >= 0) {
> +		reg->s32_min_value = max_t(s32, reg->s32_min_value, (s32)reg->umin_value);
> +		reg->s32_max_value = min_t(s32, reg->s32_max_value, (s32)reg->umax_value);
> +	}
> +	if ((u32)(reg->smin_value >> 32) + 1 == (u32)(reg->smax_value >> 32) &&
> +	    (s32)reg->smin_value < 0 && (s32)reg->smax_value >= 0) {
> +		reg->s32_min_value = max_t(s32, reg->s32_min_value, (s32)reg->smin_value);
> +		reg->s32_max_value = min_t(s32, reg->s32_max_value, (s32)reg->smax_value);
> +	}
>  	/* if u32 range forms a valid s32 range (due to matching sign bit),
>  	 * try to learn from that
>  	 */