Re: [PATCH v5 bpf-next 03/23] bpf: derive smin/smax from umin/max bounds

[Eduard Zingerman <eddyz87@gmail.com>]

On Fri, 2023-10-27 at 11:13 -0700, Andrii Nakryiko wrote:
> Add smin/smax derivation from appropriate umin/umax values. Previously the
> logic was surprisingly asymmetric, trying to derive umin/umax from smin/smax
> (if possible), but not trying to do the same in the other direction. A simple
> addition to __reg64_deduce_bounds() fixes this.
> 
> Added also generic comment about u64/s64 ranges and their relationship.
> Hopefully that helps readers to understand all the bounds deductions
> a bit better.
> 
> 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>

Nice comment, thank you. I noticed two typos, see below.

> ---
>  kernel/bpf/verifier.c | 70 +++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 70 insertions(+)
> 
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 857d76694517..bf4193706744 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -2358,6 +2358,76 @@ static void __reg32_deduce_bounds(struct bpf_reg_state *reg)
>  
>  static void __reg64_deduce_bounds(struct bpf_reg_state *reg)
>  {
> +	/* If u64 range forms a valid s64 range (due to matching sign bit),
> +	 * try to learn from that. Let's do a bit of ASCII art to see when
> +	 * this is happening. Let's take u64 range first:
> +	 *
> +	 * 0             0x7fffffffffffffff 0x8000000000000000        U64_MAX
> +	 * |-------------------------------|--------------------------------|
> +	 *
> +	 * Valid u64 range is formed when umin and umax are anywhere in this
> +	 * range [0, U64_MAX] and umin <= umax. u64 is simple and
> +	 * straightforward. Let's where s64 range maps to this simple [0,
> +	 * U64_MAX] range, annotated below the line for comparison:

Nit: this sentence sounds a bit weird, probably some word is missing
     between "let's" and "where".

> +	 *
> +	 * 0             0x7fffffffffffffff 0x8000000000000000        U64_MAX
> +	 * |-------------------------------|--------------------------------|
> +	 * 0                        S64_MAX S64_MIN                        -1
> +	 *
> +	 * So s64 values basically start in the middle and then are contiguous
> +	 * to the right of it, wrapping around from -1 to 0, and then
> +	 * finishing as S64_MAX (0x7fffffffffffffff) right before S64_MIN.
> +	 * We can try drawing more visually continuity of u64 vs s64 values as
> +	 * mapped to just actual hex valued range of values.
> +	 *
> +	 *  u64 start                                               u64 end
> +	 *  _______________________________________________________________
> +	 * /                                                               \
> +	 * 0             0x7fffffffffffffff 0x8000000000000000        U64_MAX
> +	 * |-------------------------------|--------------------------------|
> +	 * 0                        S64_MAX S64_MIN                        -1
> +	 *                                / \
> +	 * >------------------------------   ------------------------------->
> +	 * s64 continues...        s64 end   s64 start          s64 "midpoint"
> +	 *
> +	 * What this means is that in general, we can't always derive
> +	 * something new about u64 from any random s64 range, and vice versa.
> +	 * But we can do that in two particular cases. One is when entire
> +	 * u64/s64 range is *entirely* contained within left half of the above
> +	 * diagram or when it is *entirely* contained in the right half. I.e.:
> +	 *
> +	 * |-------------------------------|--------------------------------|
> +	 *     ^                   ^            ^                 ^
> +	 *     A                   B            C                 D
> +	 *
> +	 * [A, B] and [C, D] are contained entirely in their respective halves
> +	 * and form valid contiguous ranges as both u64 and s64 values. [A, B]
> +	 * will be non-negative both as u64 and s64 (and in fact it will be
> +	 * identical ranges no matter the signedness). [C, D] treated as s64
> +	 * will be a range of negative values, while in u64 it will be
> +	 * non-negative range of values larger than 0x8000000000000000.
> +	 *
> +	 * Now, any other range here can't be represented in both u64 and s64
> +	 * simultaneously. E.g., [A, C], [A, D], [B, C], [B, D] are valid
> +	 * contiguous u64 ranges, but they are discontinuous in s64. [B, C]
> +	 * in s64 would be properly presented as [S64_MIN, C] and [B, S64_MAX],
> +	 * for example. Similarly, valid s64 range [D, A] (going from negative
> +	 * to positive values), would be two separate [D, U64_MAX] and [0, A]
> +	 * ranges as u64. Currently reg_state can't represent two segments per
> +	 * numeric domain, so in such situations we can only derive maximal
> +	 * possible range ([0, U64_MAX] for u64, and [S64_MIN, S64_MAX) for s64).
                                                                  ^
Nit:                                                      missing bracket

> +	 *
> +	 * So we use these facts to derive umin/umax from smin/smax and vice
> +	 * versa only if they stay within the same "half". This is equivalent
> +	 * to checking sign bit: lower half will have sign bit as zero, upper
> +	 * half have sign bit 1. Below in code we simplify this by just
> +	 * casting umin/umax as smin/smax and checking if they form valid
> +	 * range, and vice versa. Those are equivalent checks.
> +	 */
> +	if ((s64)reg->umin_value <= (s64)reg->umax_value) {
> +		reg->smin_value = max_t(s64, reg->smin_value, reg->umin_value);
> +		reg->smax_value = min_t(s64, reg->smax_value, reg->umax_value);
> +	}
>  	/* Learn sign from signed bounds.
>  	 * If we cannot cross the sign boundary, then signed and unsigned bounds
>  	 * are the same, so combine.  This works even in the negative case, e.g.