Re: [PATCH bpf 1/6] xdp: produce a warning when calculated tailroom is negative

[Martin KaFai Lau <martin.lau@linux.dev>]

On 2/3/26 2:53 AM, Larysa Zaremba wrote:
> The issue can be fixed in all in-tree drivers, but we cannot just trust OOT
> drivers to not do this. Therefore, make tailroom a signed int and produce a
> warning when it is negative to prevent such mistakes in the future.
> 
> Fixes: bf25146a5595 ("bpf: add frags support to the bpf_xdp_adjust_tail() API")
> Reviewed-by: Aleksandr Loktionov<aleksandr.loktionov@intel.com>
> Signed-off-by: Larysa Zaremba<larysa.zaremba@intel.com>
> ---
>   net/core/filter.c | 3 ++-
>   1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/net/core/filter.c b/net/core/filter.c
> index 616e0520a0bb..9715d957e3c5 100644
> --- a/net/core/filter.c
> +++ b/net/core/filter.c
> @@ -4149,12 +4149,13 @@ static int bpf_xdp_frags_increase_tail(struct xdp_buff *xdp, int offset)
>   	struct skb_shared_info *sinfo = xdp_get_shared_info_from_buff(xdp);
>   	skb_frag_t *frag = &sinfo->frags[sinfo->nr_frags - 1];
>   	struct xdp_rxq_info *rxq = xdp->rxq;
> -	unsigned int tailroom;
> +	int tailroom;
>   
>   	if (!rxq->frag_size || rxq->frag_size > xdp->frame_sz)
>   		return -EOPNOTSUPP;
>   
>   	tailroom = rxq->frag_size - skb_frag_size(frag) - skb_frag_off(frag);
> +	WARN_ON_ONCE(tailroom < 0);
>   	if (unlikely(offset > tailroom))
>   		return -EINVAL;

Acked-by: Martin KaFai Lau <martin.lau@kernel.org>