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

Fernando Fernandez Mancera
2026-04-02T07:26:33Z

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>
---
v2: adjust nh_nlmsg_size_grp() to handle size for stats and add symbols
to the trace in commit message
v3: include NHA_FDB size into nh_nlmsg_size_grp() calculation
---
net/ipv4/nexthop.c | 38 +++++++++++++++++++++++++++-----------
1 file changed, 27 insertions(+), 11 deletions(-)

diff --git a/net/ipv4/nexthop.c b/net/ipv4/nexthop.c
index a0c694583299..2c9036c719b6 100644
--- a/net/ipv4/nexthop.c
+++ b/net/ipv4/nexthop.c
@@ -1003,16 +1003,32 @@ static size_t nh_nlmsg_size_grp_res(struct nh_group *nhg)
nla_total_size_64bit(8);/* NHA_RES_GROUP_UNBALANCED_TIME */
}

-static size_t nh_nlmsg_size_grp(struct nexthop *nh)
+static size_t nh_nlmsg_size_grp(struct nexthop *nh, u32 op_flags)
{
struct nh_group *nhg = rtnl_dereference(nh->nh_grp);
size_t sz = sizeof(struct nexthop_grp) * nhg->num_nh;
size_t tot = nla_total_size(sz) +
- nla_total_size(2); /* NHA_GROUP_TYPE */
+ nla_total_size(2) + /* NHA_GROUP_TYPE */
+ nla_total_size(0); /* NHA_FDB */

if (nhg->resilient)
tot += nh_nlmsg_size_grp_res(nhg);

+ if (op_flags & NHA_OP_FLAG_DUMP_STATS) {
+ tot += nla_total_size(0) + /* NHA_GROUP_STATS */
+ nla_total_size(4); /* NHA_HW_STATS_ENABLE */
+ tot += nhg->num_nh *
+ (nla_total_size(0) + /* NHA_GROUP_STATS_ENTRY */
+ nla_total_size(4) + /* NHA_GROUP_STATS_ENTRY_ID */
+ nla_total_size_64bit(8)); /* NHA_GROUP_STATS_ENTRY_PACKETS */
+
+ if (op_flags & NHA_OP_FLAG_DUMP_HW_STATS) {
+ tot += nhg->num_nh *
+ nla_total_size_64bit(8); /* NHA_GROUP_STATS_ENTRY_PACKETS_HW */
+ tot += nla_total_size(4); /* NHA_HW_STATS_USED */
+ }
+ }
+
return tot;
}

@@ -1047,14 +1063,14 @@ static size_t nh_nlmsg_size_single(struct nexthop *nh)
return sz;
}

-static size_t nh_nlmsg_size(struct nexthop *nh)
+static size_t nh_nlmsg_size(struct nexthop *nh, u32 op_flags)
{
size_t sz = NLMSG_ALIGN(sizeof(struct nhmsg));

sz += nla_total_size(4); /* NHA_ID */

if (nh->is_group)
- sz += nh_nlmsg_size_grp(nh) +
+ sz += nh_nlmsg_size_grp(nh, op_flags) +
nla_total_size(4) + /* NHA_OP_FLAGS */
0;
else
@@ -1070,7 +1086,7 @@ static void nexthop_notify(int event, struct nexthop *nh, struct nl_info *info)
struct sk_buff *skb;
int err = -ENOBUFS;

- skb = nlmsg_new(nh_nlmsg_size(nh), gfp_any());
+ skb = nlmsg_new(nh_nlmsg_size(nh, 0), gfp_any());
if (!skb)
goto errout;

@@ -3376,15 +3392,15 @@ static int rtm_get_nexthop(struct sk_buff *in_skb, struct nlmsghdr *nlh,
if (err)
return err;

- err = -ENOBUFS;
- skb = alloc_skb(NLMSG_GOODSIZE, GFP_KERNEL);
- if (!skb)
- goto out;
-
err = -ENOENT;
nh = nexthop_find_by_id(net, id);
if (!nh)
- goto errout_free;
+ goto out;
+
+ err = -ENOBUFS;
+ skb = nlmsg_new(nh_nlmsg_size(nh, op_flags), GFP_KERNEL);
+ if (!skb)
+ goto out;

err = nh_fill_node(skb, nh, RTM_NEWNEXTHOP, NETLINK_CB(in_skb).portid,
nlh->nlmsg_seq, 0, op_flags);
--
2.53.0