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

Andrii Nakryiko
2023-10-31T17:30:53Z

On Tue, Oct 31, 2023 at 8:37 AM Eduard Zingerman <eddyz87@gmail.com> wrote:
>
> 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".
>

I don't know what's going on here, I wasn't drunk when I wrote this
and I don't remember it being so incoherent :) Will re-read and try to
make it clearer.

> > + *
> > + * 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
>

it's actually a typo, ) -> ], which is now fixed as well, thanks

> > + *
> > + * 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.
>
>
>