netdev - Re: [PATCH net-next 1/5] ipv6: addrconf: cleanup locking in ipv6_add

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Date:   Sun, 15 Oct 2017 18:59:38 +0300
From:   Ido Schimmel <idosch@...sch.org>
To:     David Ahern <dsahern@...il.com>
Cc:     netdev@...r.kernel.org, jiri@...lanox.com, idosch@...lanox.com,
        kjlx@...pleofstupid.com, davem@...emloft.net,
        yoshfuji@...ux-ipv6.org
Subject: Re: [PATCH net-next 1/5] ipv6: addrconf: cleanup locking in
 ipv6_add_addr

On Sun, Oct 15, 2017 at 09:24:07AM -0600, David Ahern wrote:
> On 10/15/17 1:50 AM, Ido Schimmel wrote:
> > On Fri, Oct 13, 2017 at 04:02:09PM -0700, David Ahern wrote:
> >> ipv6_add_addr is called in process context with rtnl lock held
> >> (e.g., manual config of an address) or during softirq processing
> >> (e.g., autoconf and address from a router advertisement).
> >>
> >> Currently, ipv6_add_addr calls rcu_read_lock_bh shortly after entry
> >> and does not call unlock until exit, minus the call around the address
> >> validator notifier. Similarly, addrconf_hash_lock is taken after the
> >> validator notifier and held until exit. This forces the allocation of
> >> inet6_ifaddr to always be atomic.
> >>
> >> Refactor ipv6_add_addr as follows:
> >> 1. add an input boolean to discriminate the call path (process context
> >>    or softirq). This new flag controls whether the alloc can be done
> >>    with GFP_KERNEL or GFP_ATOMIC.
> >>
> >> 2. Move the rcu_read_lock_bh and unlock calls only around functions that
> >>    do rcu updates.
> >>
> >> 3. Remove the in6_dev_hold and put added by 3ad7d2468f79f ("Ipvlan should
> >>    return an error when an address is already in use."). This was done
> >>    presumably because rcu_read_unlock_bh needs to be called before calling
> >>    the validator. Since rcu_read_lock is not needed before the validator
> >>    runs revert the hold and put added by 3ad7d2468f79f and only do the
> >>    hold when setting ifp->idev.
> >>
> >> 4. move duplicate address check and insertion of new address in the global
> >>    address hash into a helper. The helper is called after an ifa is
> >>    allocated and filled in.
> >>
> >> This allows the ifa for manually configured addresses to be done with
> >> GFP_KERNEL and reduces the overall amount of time with rcu_read_lock held
> >> and hash table spinlock held.
> >>
> >> Signed-off-by: David Ahern <dsahern@...il.com>
> > 
> > [...]
> > 
> >> @@ -1073,21 +1085,19 @@ ipv6_add_addr(struct inet6_dev *idev, const struct in6_addr *addr,
> >>  
> >>  	in6_ifa_hold(ifa);
> >>  	write_unlock(&idev->lock);
> >> -out2:
> >> +
> >>  	rcu_read_unlock_bh();
> >>  
> >> -	if (likely(err == 0))
> >> -		inet6addr_notifier_call_chain(NETDEV_UP, ifa);
> >> -	else {
> >> +	inet6addr_notifier_call_chain(NETDEV_UP, ifa);
> >> +out:
> >> +	if (unlikely(err < 0)) {
> >> +		if (rt)
> >> +			ip6_rt_put(rt);
> > 
> > I believe 'rt' needs to be set to NULL after addrconf_dst_alloc()
> > fails.
> 
> The above frees rt and the line below frees the ifa and resets the value
> to an error, so after the line above rt is no longer referenced.

Earlier in the code we have:

rt = addrconf_dst_alloc(idev, addr, false);
if (IS_ERR(rt)) {
	err = PTR_ERR(rt);
	goto out;
}

So we end up calling ip6_rt_put() with an error value. I believe it
should be:

rt = addrconf_dst_alloc(idev, addr, false);
if (IS_ERR(rt)) {
	err = PTR_ERR(rt);
	rt = NULL;
	goto out;
}