Re: [PATCH v3] net/sched: cake: reject overhead values that underflow length

[Toke Høiland-Jørgensen]

Samuel Moelius <sam.moelius@xxxxxxxxxxxxxxx> writes:

> CAKE accepts signed overhead values and stores them in an s16, but the
> adjusted packet length calculation uses unsigned arithmetic.  A negative
> effective length can therefore wrap to a large value.
>
> Such configurations make rate accounting depend on integer wraparound
> rather than on the packet size userspace intended to model.  A static
> netlink lower bound is not enough because packets reaching CAKE can be
> smaller than any reasonable manual-overhead allowance.
>
> Fold the signed overhead adjustment into the existing datapath MPU clamp
> so negative adjusted lengths are clamped before link-layer framing
> adjustments.
>
> Fixes: a729b7f0bd5b ("sch_cake: Add overhead compensation support to the rate shaper")
> Assisted-by: Codex:gpt-5.5-cyber-preview
> Signed-off-by: Samuel Moelius <sam.moelius@xxxxxxxxxxxxxxx>

Acked-by: Toke Høiland-Jørgensen <toke@xxxxxxx>