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b7f97cae7e
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: a729b7f0bd ("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@trailofbits.com>
Acked-by: Toke Høiland-Jørgensen <toke@toke.dk>
Link: https://patch.msgid.link/20260702000758.297407.e5c888d9d99d.cake-overhead-underflow@trailofbits.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>