Subject: Re: [PATCH net-next 3/4] net, neigh: Extend neigh->flags to 32 bit to allow for extensions

From: David Ahern <[email-masked]>
Date: Tue, 12 Oct 2021 08:31:33 -0600

On 10/11/21 6:12 AM, Daniel Borkmann wrote:
> diff --git a/include/net/neighbour.h b/include/net/neighbour.h
> index eb2a7c03a5b0..26d4ada0aea9 100644
> --- a/include/net/neighbour.h
> +++ b/include/net/neighbour.h
> @@ -144,11 +144,11 @@ struct neighbour {
>  	struct timer_list	timer;
>  	unsigned long		used;
>  	atomic_t		probes;
> -	__u8			flags;
> -	__u8			nud_state;
> -	__u8			type;
> -	__u8			dead;
> +	u8			nud_state;
> +	u8			type;
> +	u8			dead;
>  	u8			protocol;
> +	u32			flags;
>  	seqlock_t		ha_lock;
>  	unsigned char		ha[ALIGN(MAX_ADDR_LEN, sizeof(unsigned long))] __aligned(8);
>  	struct hh_cache		hh;
> @@ -172,7 +172,7 @@ struct pneigh_entry {
>  	struct pneigh_entry	*next;
>  	possible_net_t		net;
>  	struct net_device	*dev;
> -	u8			flags;
> +	u32			flags;
>  	u8			protocol;
>  	u8			key[];
>  };
> @@ -258,6 +258,10 @@ static inline void *neighbour_priv(const struct neighbour *n)
>  #define NEIGH_UPDATE_F_ISROUTER			0x40000000
>  #define NEIGH_UPDATE_F_ADMIN			0x80000000
>  
> +/* In-kernel representation for NDA_FLAGS_EXT flags: */
> +#define NTF_OLD_MASK		0xff
> +#define NTF_EXT_SHIFT		8

so only 24 EXT flags can be added. That should be documented; far off
today, but that's an easy overflow to miss.

Reviewed-by: David Ahern <[email-masked]>