Re: [PATCH v2 net] tcp: avoid the lookup process failing to get sk in ehash table

[Jason Xing]

On Mon, Jan 16, 2023 at 8:36 AM Jason Xing <kerneljasonxing@xxxxxxxxx> wrote:
>
> On Mon, Jan 16, 2023 at 12:12 AM Eric Dumazet <edumazet@xxxxxxxxxx> wrote:
> >
> > On Sat, Jan 14, 2023 at 2:27 PM Jason Xing <kerneljasonxing@xxxxxxxxx> wrote:
> > >
> > > From: Jason Xing <kernelxing@xxxxxxxxxxx>
> > >
> > >
> > > Fixes: 5e0724d027f0 ("tcp/dccp: fix hashdance race for passive sessions")
> > > Suggested-by: Eric Dumazet <edumazet@xxxxxxxxxx>
> > > Signed-off-by: Jason Xing <kernelxing@xxxxxxxxxxx>
> > > Link: https://lore.kernel.org/lkml/20230112065336.41034-1-kerneljasonxing@xxxxxxxxx/
> > > ---
> > > v2:
> > > 1) adding the sk node into the tail of list to prevent the race.
> > > 2) fix the race condition when handling time-wait socket hashdance.
> > > ---
> > >  net/ipv4/inet_hashtables.c    | 10 ++++++++++
> > >  net/ipv4/inet_timewait_sock.c |  6 +++---
> > >  2 files changed, 13 insertions(+), 3 deletions(-)
> > >
> > > diff --git a/net/ipv4/inet_hashtables.c b/net/ipv4/inet_hashtables.c
> > > index 24a38b56fab9..b0b54ad55507 100644
> > > --- a/net/ipv4/inet_hashtables.c
> > > +++ b/net/ipv4/inet_hashtables.c
> > > @@ -650,7 +650,16 @@ bool inet_ehash_insert(struct sock *sk, struct sock *osk, bool *found_dup_sk)
> > >         spin_lock(lock);
> > >         if (osk) {
> > >                 WARN_ON_ONCE(sk->sk_hash != osk->sk_hash);
> > > +               if (sk_hashed(osk))
> >
> >
> > nit: this should be:
> >
> > if (sk_hashed(osk)) {  [1]
> >     /* multi-line ....
> >      * .... comment.
> >      */
> >    ret = sk_nulls_del_node_init_rcu(osk);
> >    goto unlock;
> > }

Well, after I dug into this part, I found something as below.
If we enter into the 'if (osk) {', we should always skip the next
if-statement which is 'if (found_dup_sk) {' and return a proper value
depending on if the osk is hashed.
However, the code as above would leave variable @ret to be true if the
sk_hashed(osk) returned false, then It would not go to unlock and then
add the node to the list and at last return true which is unexpected.

> > if (found_dup_sk) {  [2]
> >
> > 1) parentheses needed in [1]

> > 2) No else if in [2], since you added a "goto unlock;"

I think this modification is fine and makes the code clearer.

Thanks,
Jason

> >
>
> I'll do that. It looks much better.
>
> Thanks,
> Jason
>
> > > +                       /* Before deleting the node, we insert a new one to make
> > > +                        * sure that the look-up-sk process would not miss either
> > > +                        * of them and that at least one node would exist in ehash
> > > +                        * table all the time. Otherwise there's a tiny chance
> > > +                        * that lookup process could find nothing in ehash table.
> > > +                        */
> > > +                       __sk_nulls_add_node_tail_rcu(sk, list);
> > >                 ret = sk_nulls_del_node_init_rcu(osk);
> > > +               goto unlock;
> > >         } else if (found_dup_sk) {
> > >                 *found_dup_sk = inet_ehash_lookup_by_sk(sk, list);
> > >                 if (*found_dup_sk)
> > > @@ -660,6 +669,7 @@ bool inet_ehash_insert(struct sock *sk, struct sock *osk, bool *found_dup_sk)
> > >         if (ret)
> > >                 __sk_nulls_add_node_rcu(sk, list);
> > >
> > > +unlock:
> > >         spin_unlock(lock);
> >
> > Thanks.