netdev - Re: [PATCH net] net: sched: act_api: size RTM

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Message-ID: <CAM0EoMkS2Uoarr+551wNe7zvmPTGFZxdb-otKYLBPF5+2s+FEg@mail.gmail.com>
Date: Fri, 30 Jan 2026 11:05:43 -0500
From: Jamal Hadi Salim <jhs@...atatu.com>
To: Paul Moses <p@....org>
Cc: netdev@...r.kernel.org, xiyou.wangcong@...il.com, jiri@...nulli.us, 
	davem@...emloft.net, edumazet@...gle.com, kuba@...nel.org, pabeni@...hat.com, 
	horms@...nel.org, linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH net] net: sched: act_api: size RTM_GETACTION reply by fill size

On Fri, Jan 30, 2026 at 8:43 AM Paul Moses <p@....org> wrote:
>
> tcf_action_fill_size() already computes the required dump size, but
> RTM_GETACTION replies always allocate NLMSG_GOODSIZE. Large action
> state can overrun that skb and make dumps fail.
>
> Use the computed reply size for RTM_GETACTION replies so large actions
> can be dumped, while still keeping NLMSG_GOODSIZE as a floor.
>
> Fixes: 4e76e75d6aba ("net sched actions: calculate add/delete event message size")
> Cc: stable@...r.kernel.org
> Signed-off-by: Paul Moses <p@....org>
> ---
>  net/sched/act_api.c | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
>
> diff --git a/net/sched/act_api.c b/net/sched/act_api.c
> index e1ab0faeb8113..8ab016d352850 100644
> --- a/net/sched/act_api.c
> +++ b/net/sched/act_api.c
> @@ -1685,12 +1685,12 @@ static int tca_get_fill(struct sk_buff *skb, struct tc_action *actions[],
>
>  static int
>  tcf_get_notify(struct net *net, u32 portid, struct nlmsghdr *n,
> -              struct tc_action *actions[], int event,
> +              struct tc_action *actions[], int event, size_t attr_size,
>                struct netlink_ext_ack *extack)
>  {
>         struct sk_buff *skb;
>
> -       skb = alloc_skb(NLMSG_GOODSIZE, GFP_KERNEL);
> +       skb = alloc_skb(max_t(size_t, attr_size, NLMSG_GOODSIZE), GFP_KERNEL);
>         if (!skb)
>                 return -ENOBUFS;
>         if (tca_get_fill(skb, actions, portid, n->nlmsg_seq, 0, event,
> @@ -2041,7 +2041,8 @@ tca_action_gd(struct net *net, struct nlattr *nla, struct nlmsghdr *n,
>         attr_size = tcf_action_full_attrs_size(attr_size);
>
>         if (event == RTM_GETACTION)
> -               ret = tcf_get_notify(net, portid, n, actions, event, extack);
> +               ret = tcf_get_notify(net, portid, n, actions, event,
> +                                    attr_size, extack);
>         else { /* delete */
>                 ret = tcf_del_notify(net, n, actions, portid, attr_size, extack);
>                 if (ret)

dunno. Is this based on some issue you found? This is a common pattern
in a lot of places in the stack and has not caused any issues (afaik).

cheers,
jamal