netdev - Re: [PATCH v2 net-next 6/6] arp: Convert ioctl(SIOCGARP) to RCU.

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Message-ID: <CANn89iJEWs7AYSJqGCUABeVqOCTkErponfZdT5kV-iD=-SajnQ@mail.gmail.com>
Date: Thu, 25 Apr 2024 19:12:56 +0200
From: Eric Dumazet <edumazet@...gle.com>
To: Kuniyuki Iwashima <kuniyu@...zon.com>
Cc: "David S. Miller" <davem@...emloft.net>, David Ahern <dsahern@...nel.org>, 
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, 
	Kuniyuki Iwashima <kuni1840@...il.com>, netdev@...r.kernel.org
Subject: Re: [PATCH v2 net-next 6/6] arp: Convert ioctl(SIOCGARP) to RCU.

On Thu, Apr 25, 2024 at 7:02 PM Kuniyuki Iwashima <kuniyu@...zon.com> wrote:
>
> ioctl(SIOCGARP) holds rtnl_lock() for __dev_get_by_name() and
> later calls neigh_lookup(), which calls rcu_read_lock().
>
> Let's replace __dev_get_by_name() with dev_get_by_name_rcu() to
> avoid locking rtnl_lock().
>
> Signed-off-by: Kuniyuki Iwashima <kuniyu@...zon.com>
> ---
>  net/ipv4/arp.c | 26 +++++++++++++++++---------
>  1 file changed, 17 insertions(+), 9 deletions(-)
>
> diff --git a/net/ipv4/arp.c b/net/ipv4/arp.c
> index 5034920be85a..9430b64558cd 100644
> --- a/net/ipv4/arp.c
> +++ b/net/ipv4/arp.c
> @@ -1003,11 +1003,15 @@ static int arp_rcv(struct sk_buff *skb, struct net_device *dev,
>   *     User level interface (ioctl)
>   */
>
> -static struct net_device *arp_req_dev_by_name(struct net *net, struct arpreq *r)
> +static struct net_device *arp_req_dev_by_name(struct net *net, struct arpreq *r,
> +                                             bool getarp)
>  {
>         struct net_device *dev;
>
> -       dev = __dev_get_by_name(net, r->arp_dev);
> +       if (getarp)
> +               dev = dev_get_by_name_rcu(net, r->arp_dev);
> +       else
> +               dev = __dev_get_by_name(net, r->arp_dev);
>         if (!dev)
>                 return ERR_PTR(-ENODEV);
>
> @@ -1028,7 +1032,7 @@ static struct net_device *arp_req_dev(struct net *net, struct arpreq *r)
>         __be32 ip;
>
>         if (r->arp_dev[0])
> -               return arp_req_dev_by_name(net, r);
> +               return arp_req_dev_by_name(net, r, false);
>
>         if (r->arp_flags & ATF_PUBL)
>                 return NULL;
> @@ -1166,7 +1170,7 @@ static int arp_req_get(struct net *net, struct arpreq *r)
>         if (!r->arp_dev[0])
>                 return -ENODEV;
>
> -       dev = arp_req_dev_by_name(net, r);
> +       dev = arp_req_dev_by_name(net, r, true);
>         if (IS_ERR(dev))
>                 return PTR_ERR(dev);
>
> @@ -1287,23 +1291,27 @@ int arp_ioctl(struct net *net, unsigned int cmd, void __user *arg)
>         else if (*netmask && *netmask != htonl(0xFFFFFFFFUL))
>                 return -EINVAL;
>
> -       rtnl_lock();
> -
>         switch (cmd) {
>         case SIOCDARP:
> +               rtnl_lock();
>                 err = arp_req_delete(net, &r);
> +               rtnl_unlock();
>                 break;
>         case SIOCSARP:
> +               rtnl_lock();
>                 err = arp_req_set(net, &r);
> +               rtnl_unlock();
>                 break;
>         case SIOCGARP:
> +               rcu_read_lock();
>                 err = arp_req_get(net, &r);
> +               rcu_read_unlock();


Note that arp_req_get() uses :

strscpy(r->arp_dev, dev->name, sizeof(r->arp_dev));

This currently depends on RTNL or devnet_rename_sem

Perhaps we should add a helper and use a seqlock to safely copy
dev->name into a temporary variable.

netdev_get_name() can not be called from rcu_read_lock() at this
moment unfortunately.