Re: [PATCH 2/2 net v3] ipv4: nexthop: allocate skb dynamically in rtm_get_nexthop()

[Ido Schimmel <idosch@nvidia.com>]

On Thu, Apr 02, 2026 at 09:26:13AM +0200, Fernando Fernandez Mancera wrote:
> When querying a nexthop object via RTM_GETNEXTHOP, the kernel currently
> allocates a fixed-size skb using NLMSG_GOODSIZE. While sufficient for
> single nexthops and small Equal-Cost Multi-Path groups, this fixed
> allocation fails for large nexthop groups like 512 nexthops.
> 
> This results in the following warning splat:
> 
>  WARNING: net/ipv4/nexthop.c:3395 at rtm_get_nexthop+0x176/0x1c0, CPU#20: rep/4608
>  [...]
>  RIP: 0010:rtm_get_nexthop (net/ipv4/nexthop.c:3395)
>  [...]
>  Call Trace:
>   <TASK>
>   rtnetlink_rcv_msg (net/core/rtnetlink.c:6989)
>   netlink_rcv_skb (net/netlink/af_netlink.c:2550)
>   netlink_unicast (net/netlink/af_netlink.c:1319 net/netlink/af_netlink.c:1344)
>   netlink_sendmsg (net/netlink/af_netlink.c:1894)
>   ____sys_sendmsg (net/socket.c:721 net/socket.c:736 net/socket.c:2585)
>   ___sys_sendmsg (net/socket.c:2641)
>   __sys_sendmsg (net/socket.c:2671)
>   do_syscall_64 (arch/x86/entry/syscall_64.c:63 arch/x86/entry/syscall_64.c:94)
>   entry_SYSCALL_64_after_hwframe (arch/x86/entry/entry_64.S:130)
>   </TASK>
> 
> Fix this by allocating the size dynamically using nh_nlmsg_size() and
> using nlmsg_new(), this is consistent with nexthop_notify() behavior. In
> addition, adjust nh_nlmsg_size_grp() so it calculates the size needed
> based on flags passed. While at it, also add the size of NHA_FDB for
> nexthop group size calculation as it was missing too.
> 
> This cannot be reproduced via iproute2 as the group size is currently
> limited and the command fails as follows:
> 
> addattr_l ERROR: message exceeded bound of 1048
> 
> Fixes: 430a049190de ("nexthop: Add support for nexthop groups")
> Reported-by: Yiming Qian <yimingqian591@gmail.com>
> Closes: https://lore.kernel.org/netdev/CAL_bE8Li2h4KO+AQFXW4S6Yb_u5X4oSKnkywW+LPFjuErhqELA@mail.gmail.com/
> Signed-off-by: Fernando Fernandez Mancera <fmancera@suse.de>
> Reviewed-by: Eric Dumazet <edumazet@google.com>

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