Re: [PATCH v4 net] tcp: avoid the lookup process failing to get sk in ehash table

[Eric Dumazet]

On Mon, Jan 16, 2023 at 8:38 AM Jason Xing <kerneljasonxing@xxxxxxxxx> wrote:
>
> From: Jason Xing <kernelxing@xxxxxxxxxxx>
>
> While one cpu is working on looking up the right socket from ehash
> table, another cpu is done deleting the request socket and is about
> to add (or is adding) the big socket from the table. It means that
> we could miss both of them, even though it has little chance.
>
>
> 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/
> ---
> v4:
> 1) adjust the code style and make it easier to read.
>
> v3:
> 1) get rid of else-if statement.
>
> 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    | 18 ++++++++++++++++--
>  net/ipv4/inet_timewait_sock.c |  6 +++---
>  2 files changed, 19 insertions(+), 5 deletions(-)
>
> diff --git a/net/ipv4/inet_hashtables.c b/net/ipv4/inet_hashtables.c
> index 24a38b56fab9..c64eec874b31 100644
> --- a/net/ipv4/inet_hashtables.c
> +++ b/net/ipv4/inet_hashtables.c
> @@ -650,8 +650,21 @@ 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);
> -               ret = sk_nulls_del_node_init_rcu(osk);
> -       } else if (found_dup_sk) {
> +               if (sk_hashed(osk)) {
> +                       /* 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);
> +                       sk_nulls_del_node_init_rcu(osk);
> +               } else {
> +                       ret = false;


Well, you added another 'else' statement...

What about the following ?

diff --git a/net/ipv4/inet_hashtables.c b/net/ipv4/inet_hashtables.c
index 24a38b56fab9e9d7d893e23b30d26e275359ec70..1bcf5ce8dd1317b2144bcb47a2ad238532b9accf
100644
--- a/net/ipv4/inet_hashtables.c
+++ b/net/ipv4/inet_hashtables.c
@@ -650,8 +650,14 @@ 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);
-               ret = sk_nulls_del_node_init_rcu(osk);
-       } else if (found_dup_sk) {
+               ret = sk_hashed(osk);
+               if (ret) {
+                       __sk_nulls_add_node_tail_rcu(sk, list);
+                       sk_nulls_del_node_init_rcu(osk);
+               }
+               goto unlock;
+       }
+       if (found_dup_sk) {
                *found_dup_sk = inet_ehash_lookup_by_sk(sk, list);
                if (*found_dup_sk)
                        ret = false;
@@ -659,7 +665,7 @@ 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);

        return ret;