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

Eduard Zingerman
2023-10-31T15:37:18Z

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.