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

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Message-ID: <CANn89iJGJQeYiUYX9a_tPB00fCibuOED_mbxZwcJAQLbMYiy8w@mail.gmail.com>
Date: Thu, 25 Apr 2024 22:35:38 +0200
From: Eric Dumazet <edumazet@...gle.com>
To: Kuniyuki Iwashima <kuniyu@...zon.com>
Cc: davem@...emloft.net, dsahern@...nel.org, kuba@...nel.org, 
	kuni1840@...il.com, netdev@...r.kernel.org, pabeni@...hat.com
Subject: Re: [PATCH v2 net-next 6/6] arp: Convert ioctl(SIOCGARP) to RCU.

On Thu, Apr 25, 2024 at 7:52 PM Kuniyuki Iwashima <kuniyu@...zon.com> wrote:
>
> From: Eric Dumazet <edumazet@...gle.com>
> Date: Thu, 25 Apr 2024 19:12:56 +0200
> > 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
>
> Ah, I missed this point, thanks for catching!
>
>
> >
> > Perhaps we should add a helper and use a seqlock to safely copy
> > dev->name into a temporary variable.
>
> So it's preferable to add seqlock around memcpy() in dev_change_name()
> and use a helper in arp_req_get() rather than adding devnet_rename_sem
> locking around memcpy() in arp_req_get() ?

Under rcu_read_lock(), we can not sleep.

devnet_rename_sem is a semaphore... down_read() might sleep.

So if you plan using current netdev_get_name(), you must call it
outside of rcu_read_lock() section.