Re: [PATCH net] net: skbuff: reject zero MPLS TTL before decrementing

[Xia Yan]

On 10/31/26 11:41, Avi Weiss wrote:
> skb_mpls_dec_ttl() decrements the u8 TTL before checking whether it
> expired. An incoming TTL of zero wraps to 255, causing the TC MPLS
> dec_ttl action to pass an expired packet instead of dropping it.
>
> Reject a zero TTL before the decrement while preserving the existing
> behavior for TTL one and valid TTL values.

Nice catch. I checked the wraparound against the C promotion and
conversion rules: with ttl being a u8, a zero TTL on the old path
decrements to 255 and the packet passes with a revived TTL. The new
check catches exactly that case, TTL one still returns -EINVAL, and
values of two and above are unchanged bit for bit.

The only in-tree caller I see is the TC dec_ttl action in
tcf_mpls_act(), which maps the error to TC_ACT_SHOT, so the rejected
packet is dropped. mpls_forward() already rejects TTL zero and one
(`dec.ttl <= 1`), so the two MPLS decrement paths now agree.

One minor observation, no change requested: `!ttl || !--ttl` is
equivalent to `ttl <= 1`, the form mpls_forward() uses. Stylistic
only; the patch is correct as posted.

Reviewed-by: Xia Yan <yanxia@xxxxxxxxxxxxxxxxxxxx>