Re: [PATCH net v2 1/3] rtnetlink: return EMSGSIZE when first dump entry does not fit

[Ido Schimmel <idosch@nvidia.com>]

On Tue, Nov 10, 2026 at 09:26:16AM -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
> 
> 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")
> Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>

Reviewed-by: Ido Schimmel <idosch@nvidia.com>