Re: [PATCH bpf-next v2 1/2] bpf: Fix tnum_overlap to check for zero mask intersection

On Tue, 2025-10-28 at 15:45 +0000, bot+bpf-ci@kernel.org wrote:

commit 16cbc4520eb13d2065d145c76b97fb93035d81e8
Author: KaFai Wan <kafai.wan@linux.dev>

bpf: Fix tnum_overlap to check for zero mask intersection

This patch adds an early return check in tnum_overlap() to detect
non-overlapping tnums when their masks have no overlapping bits. The
fix addresses a syzbot report where the verifier failed to recognize
that two tnums couldn't represent the same value.

Reported-by: syzbot+c950cc277150935cc0b5@syzkaller.appspotmail.com

> diff --git a/kernel/bpf/tnum.c b/kernel/bpf/tnum.c
> index f8e70e9c3..1a75b7c9a 100644
> --- a/kernel/bpf/tnum.c
> +++ b/kernel/bpf/tnum.c
> @@ -163,6 +163,8 @@ bool tnum_overlap(struct tnum a, struct tnum b)
>  {
>  	u64 mu;
> 
> +	if (a.mask && b.mask && !(a.mask & b.mask))
> +		return false;
                              ^^^^^

Does this logic correctly detect non-overlapping tnums? Consider this
counter-example:

  a = (value=0x5, mask=0xA)  // binary: value=0101, mask=1010
  b = (value=0xA, mask=0x5)  // binary: value=1010, mask=0101

The masks are disjoint: (0xA & 0x5) == 0, so the new check returns
false (no overlap).

However, these tnums actually represent overlapping sets:
  a represents: {5, 7, 13, 15}  // 0b0101, 0b0111, 0b1101, 0b1111
  b represents: {10, 11, 14, 15} // 0b1010, 0b1011, 0b1110, 0b1111

Both sets contain 15, so they do overlap. Can this cause incorrect
verifier behavior when is_branch_taken() calls tnum_overlap() to
determine if JEQ/JNE branches are reachable?

This is a legit bug, please address.

...

mu = ~a.mask & ~b.mask; return (a.value & mu) == (b.value & mu); }

---
AI reviewed your patch. Please fix the bug or email reply why it's not a bug.
See: https://github.com/kernel-patches/vmtest/blob/master/ci/claude/README.md

In-Reply-To-Subject: `bpf: Fix tnum_overlap to check for zero mask intersection`
CI run summary: https://github.com/kernel-patches/bpf/actions/runs/18880108453