Subject: Re: [PATCH v2 net-next] net: reduce rtnetlink_rcv_msg() stack usage

From: Simon Horman <[email-masked]>
Date: Thu, 11 Jul 2024 13:46:45 +0100

On Wed, Jul 10, 2024 at 03:16:53PM +0000, Eric Dumazet wrote:
> IFLA_MAX is increasing slowly but surely.
> 
> Some compilers use more than 512 bytes of stack in rtnetlink_rcv_msg()
> because it calls rtnl_calcit() for RTM_GETLINK message.
> 
> Use noinline_for_stack attribute to not inline rtnl_calcit(),
> and directly use nla_for_each_attr_type() (Jakub suggestion)
> because we only care about IFLA_EXT_MASK at this stage.
> 
> Signed-off-by: Eric Dumazet <[email-masked]>

Reviewed-by: Simon Horman <[email-masked]>

> ---
>  net/core/rtnetlink.c | 18 ++++++++++++------
>  1 file changed, 12 insertions(+), 6 deletions(-)
> 
> diff --git a/net/core/rtnetlink.c b/net/core/rtnetlink.c
> index eabfc8290f5e29f2ef3e5c1481715ae9056ea689..87e67194f24046a8420bbb51c19fb0a686b9b06b 100644
> --- a/net/core/rtnetlink.c
> +++ b/net/core/rtnetlink.c
> @@ -3969,22 +3969,28 @@ static int rtnl_dellinkprop(struct sk_buff *skb, struct nlmsghdr *nlh,
>  	return rtnl_linkprop(RTM_DELLINKPROP, skb, nlh, extack);
>  }
>  
> -static u32 rtnl_calcit(struct sk_buff *skb, struct nlmsghdr *nlh)
> +static noinline_for_stack u32 rtnl_calcit(struct sk_buff *skb,
> +					  struct nlmsghdr *nlh)
>  {
>  	struct net *net = sock_net(skb->sk);
>  	size_t min_ifinfo_dump_size = 0;
> -	struct nlattr *tb[IFLA_MAX+1];
>  	u32 ext_filter_mask = 0;
>  	struct net_device *dev;
> -	int hdrlen;
> +	struct nlattr *nla;
> +	int hdrlen, rem;
>  
>  	/* Same kernel<->userspace interface hack as in rtnl_dump_ifinfo. */
>  	hdrlen = nlmsg_len(nlh) < sizeof(struct ifinfomsg) ?
>  		 sizeof(struct rtgenmsg) : sizeof(struct ifinfomsg);
>  
> -	if (nlmsg_parse_deprecated(nlh, hdrlen, tb, IFLA_MAX, ifla_policy, NULL) >= 0) {
> -		if (tb[IFLA_EXT_MASK])
> -			ext_filter_mask = nla_get_u32(tb[IFLA_EXT_MASK]);
> +	if (nlh->nlmsg_len < nlmsg_msg_size(hdrlen))
> +		return NLMSG_GOODSIZE;
> +
> +	nla_for_each_attr_type(nla, IFLA_EXT_MASK,
> +			       nlmsg_attrdata(nlh, hdrlen),
> +			       nlmsg_attrlen(nlh, hdrlen), rem) {
> +		if (nla_len(nla) == sizeof(u32))
> +			ext_filter_mask = nla_get_u32(nla);

I guess that in practice we can break here.
But perhaps there is some case where there is more than one
IFLA_EXT_MASK attribute and it changes behaviour.

>  	}
>  
>  	if (!ext_filter_mask)
> -- 
> 2.45.2.993.g49e7a77208-goog
> 
>