On Tue, 2025-10-28 at 15:45 +0000, [email protected] wrote: > ``` > commit 16cbc4520eb13d2065d145c76b97fb93035d81e8 > Author: KaFai Wan <[email protected]> > > 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: [email protected] > > > 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
