Re: [PATCH net v2 1/1] ipv4: avoid divide by zero in fib_rebalance

[Eric Dumazet]

On Thu, Aug 27, 2026 at 8:27 PM Zihan Xi <zihanx@xxxxxxxxxx> wrote:
>
> fib_rebalance() computes the total eligible nexthop weight in one pass
> and programs upper bounds in a second pass. A concurrent change to
> ignore_routes_with_linkdown can make the first pass return zero while
> the second pass sees an eligible nexthop, resulting in division by zero.
>
> If the first pass reports a zero total, set each nexthop upper bound to
> -1 and skip the division. This matches the IPv6 fix in commit
> d2c26c2911dd ("ipv6: avoid divide by zero in rt6_multipath_rebalance")
> and preserves the lock-free rebalance path.
>
> Fixes: 0e884c78ee19 ("ipv4: L3 hash-based multipath")
> Cc: stable@xxxxxxxxxxxxxxx
> Reported-by: Vega <vega@xxxxxxxxxx>
> Assisted-by: Codex:gpt-5.4
> Signed-off-by: Zihan Xi <zihanx@xxxxxxxxxx>

Suggested-by: Ido Schimmel <idosch@xxxxxxxxxx>
Reviewed-by: Eric Dumazet <edumazet@xxxxxxxxxx>