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

Andrii Nakryiko
2023-10-31T17:39:32Z

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 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.

So the logic here follows the (visual) intuition how s64 and u64 (and
also u32 and s32) correlate. That's how I saw it. TBH, the above
mathematical way seems scary and not so straightforward to follow, so
I'm hesitant to add it to comments to not scare anyone away :)

I did try to visually represent it, but I'm not creative enough ASCII
artist to pull this off, apparently. I'll just leave it as it is for
now.

>
> > ---
> > 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
> > */
>
>
>