Re: [PATCH] net: fib_rules: add FRA_GOTO and FRA_L3MDEV to fib_rule_nlmsg_size()

[Linkui Xiao]

On 2026/9/13 09:06, David Ahern wrote:
> On 9/11/26 2:12 AM, Linkui Xiao wrote:
> > From: Linkui Xiao <xiaolinkui@xxxxxxxxxx>
> >
> > fib_nl_fill_rule() emits FRA_GOTO and FRA_L3MDEV, but
> > fib_rule_nlmsg_size() reserves room for neither. Its only caller,
> > notify_rule_change(), allocates the notification skb with that size and
> > treats a failure as a bug:
> >
> > 	/* -EMSGSIZE implies BUG in fib_rule_nlmsg_size() */
>
> not really going to fail since the size has so many attributes that are
> not applicable to this rule.
You are right, the other attributes reserved in fib_rule_nlmsg_size()
already provide more than enough slack for the common case, so the
missing FRA_GOTO/FRA_L3MDEV accounting cannot actually overflow the skb.

I will drop this patch. Thanks for taking the time to review it.

Thanks,
Linkui Xiao
> > A rule that has both a goto target and l3mdev set needs 16 more bytes
> > than are reserved. nlmsg_new() rounds the allocation up through
> > SKB_DATA_ALIGN(), so the slack hides the mistake most of the time and
> > the bug stays latent, but the accounting is wrong and the notification
> > can still fail with -EMSGSIZE.
> >
> > Reserve the two missing attributes, keeping the order used by
> > fib_nl_fill_rule().
> >
> > Fixes: 0947c9fe56d9 ("[NET] fib_rules: goto rule action")
> > Fixes: 96c63fa7393d ("net: Add l3mdev rule")
> > Signed-off-by: Linkui Xiao <xiaolinkui@xxxxxxxxxx>
> > ---
> >   net/core/fib_rules.c | 2 ++
> >   1 file changed, 2 insertions(+)
> >
> > diff --git a/net/core/fib_rules.c b/net/core/fib_rules.c
> > index 7df216c17c67..5e84bc740191 100644
> > --- a/net/core/fib_rules.c
> > +++ b/net/core/fib_rules.c
> > @@ -1133,7 +1133,9 @@ static inline size_t fib_rule_nlmsg_size(struct fib_rules_ops *ops,
> >   			 + nla_total_size(4) /* FRA_SUPPRESS_IFGROUP */
> >   			 + nla_total_size(4) /* FRA_FWMARK */
> >   			 + nla_total_size(4) /* FRA_FWMASK */
> > +			 + nla_total_size(4) /* FRA_GOTO */
> >   			 + nla_total_size_64bit(8) /* FRA_TUN_ID */
> > +			 + nla_total_size(1) /* FRA_L3MDEV */
> >   			 + nla_total_size(sizeof(struct fib_kuid_range))
> >   			 + nla_total_size(1) /* FRA_PROTOCOL */
> >   			 + nla_total_size(1) /* FRA_IP_PROTO */