Re: suspicious rcu_dereference_check in sctp_v6_get_dst

[Dave Jones <davej@codemonkey.org.uk>]

On Sat, Dec 05, 2015 at 05:13:06PM -0800, Eric Dumazet wrote:
 
 > > diff --git a/net/sctp/ipv6.c b/net/sctp/ipv6.c
 > > index acb45b8c2a9d..7081183f4d9f 100644
 > > --- a/net/sctp/ipv6.c
 > > +++ b/net/sctp/ipv6.c
 > > @@ -328,7 +328,9 @@ static void sctp_v6_get_dst(struct sctp_transport *t, union sctp_addr *saddr,
 > >  	if (baddr) {
 > >  		fl6->saddr = baddr->v6.sin6_addr;
 > >  		fl6->fl6_sport = baddr->v6.sin6_port;
 > > +		rcu_read_lock();
 > >  		final_p = fl6_update_dst(fl6, rcu_dereference(np->opt), &final);
 > > +		rcu_read_unlock();
 > >  		dst = ip6_dst_lookup_flow(sk, fl6, final_p);
 > >  	}
 > >  
 > 
 > Hmm, better use :
 > 
 > diff --git a/net/sctp/ipv6.c b/net/sctp/ipv6.c
 > index acb45b8c2a9d..d28c0b4c9128 100644
 > --- a/net/sctp/ipv6.c
 > +++ b/net/sctp/ipv6.c
 > @@ -323,14 +323,13 @@ static void sctp_v6_get_dst(struct sctp_transport *t, union sctp_addr *saddr,
 >  			}
 >  		}
 >  	}
 > -	rcu_read_unlock();
 > -
 >  	if (baddr) {
 >  		fl6->saddr = baddr->v6.sin6_addr;
 >  		fl6->fl6_sport = baddr->v6.sin6_port;
 >  		final_p = fl6_update_dst(fl6, rcu_dereference(np->opt), &final);
 >  		dst = ip6_dst_lookup_flow(sk, fl6, final_p);
 >  	}
 > +	rcu_read_unlock();
 >  
 >  out:
 >  	if (!IS_ERR_OR_NULL(dst)) {

I looked at that option first, but decided to mirror the other use of fl6_update_dst.

It looks like your solution would work too, so I'm not against it, but..
For my own understanding, why is this better? Just to cut down on the
number of repeated lock/unlocks in the same function?  Or is there some
semantic I'm missing in the earlier lock/unlock section that's somehow
related to the np->opt ?

thanks,

	Dave