Re: [PATCH 1/3] rtnetlink: return EMSGSIZE when first dump entry does not fit

[Hangbin Liu]

On Sun, Nov 08, 2026 at 05:13:12PM -0800, Stephen Hemminger wrote:
> If a route entry is too large for the dump skb, the dump is silently
> truncated and no error is returned.
> 
> For handlers with RTNL_FLAG_DUMP_SPLIT_NLM_DONE, rtnl_dumpit() drops
> -EMSGSIZE and returns skb->len. With an empty skb this is 0, so the
> dump ends with NLMSG_DONE and status 0, and the remaining entries are
> lost.
> 
> The first dump skb is allocated from __netlink_dump_start() while
> max_recvmsg_len is still 0, so it is only NLMSG_GOODSIZE. On a 4K page
> kernel that is under 4000 bytes, and a 500 way multipath route does not
> fit:
> 
>   ip link add dummy0 type dummy
>   ip link set dummy0 up
>   ip route add 198.51.100.0/24 table 100 \
> 	$(for i in $(seq 500); do echo -n "nexthop dev dummy0 "; done)
>   ip route show table 100
> 
> The dump prints nothing and exits 0, although "ip route get
> 198.51.100.1 table 100" still shows the route.
> 
> Only ignore -EMSGSIZE when the skb has data, so that an entry which
> does not fit in an empty skb reports the error to user space.
> 
> Fixes: 5b4b62a169e1 ("rtnetlink: make the "split" NLM_DONE handling generic")
> Assisted-by: Claude:claude-opus-5-5
> Signed-off-by: Stephen Hemminger <stephen@xxxxxxxxxxxxxxxxxx>
> ---
>  net/core/rtnetlink.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/net/core/rtnetlink.c b/net/core/rtnetlink.c
> index e141340172ce..2777796a4ed2 100644
> --- a/net/core/rtnetlink.c
> +++ b/net/core/rtnetlink.c
> @@ -7043,7 +7043,10 @@ static int rtnl_dumpit(struct sk_buff *skb, struct netlink_callback *cb)
>  	 * Some applications which parse netlink manually depend on this.
>  	 */
>  	if (cb->flags & RTNL_FLAG_DUMP_SPLIT_NLM_DONE) {
> -		if (err < 0 && err != -EMSGSIZE)
> +		/* -EMSGSIZE with nothing in the skb is an error, as returning
> +		 * a length of 0 would end the dump as if it was complete.
> +		 */
> +		if (err < 0 && (err != -EMSGSIZE || !skb->len))
>  			return err;
>  		if (!err)
>  			cb->data = NULL;
> -- 
> 2.53.0
> 

Reviewed-by: Hangbin Liu <liuhangbin@xxxxxxxxxx>