linux-kernel - Re: [PATCH v5 2/6] net/handshake: Define handshake_sk_destruct

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Message-ID: <CAKmqyKN4SN6DkjaRMe4st23Xnc3gb6DcqUGHi72UTgaiE9EqGw@mail.gmail.com>
Date: Thu, 13 Nov 2025 20:19:45 +1000
From: Alistair Francis <alistair23@...il.com>
To: Chuck Lever <chuck.lever@...cle.com>
Cc: hare@...nel.org, kernel-tls-handshake@...ts.linux.dev, 
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org, 
	linux-doc@...r.kernel.org, linux-nvme@...ts.infradead.org, 
	linux-nfs@...r.kernel.org, kbusch@...nel.org, axboe@...nel.dk, hch@....de, 
	sagi@...mberg.me, kch@...dia.com, hare@...e.de, 
	Alistair Francis <alistair.francis@....com>
Subject: Re: [PATCH v5 2/6] net/handshake: Define handshake_sk_destruct_req

On Thu, Nov 13, 2025 at 1:47 AM Chuck Lever <chuck.lever@...cle.com> wrote:
>
> On 11/11/25 11:27 PM, alistair23@...il.com wrote:
> > From: Alistair Francis <alistair.francis@....com>
> >
> > Define a `handshake_sk_destruct_req()` function to allow the destruction
> > of the handshake req.
> >
> > This is required to avoid hash conflicts when handshake_req_hash_add()
> > is called as part of submitting the KeyUpdate request.
> >
> > Signed-off-by: Alistair Francis <alistair.francis@....com>
> > Reviewed-by: Hannes Reinecke <hare@...e.de>
> > ---
> > v5:
> >  - No change
> > v4:
> >  - No change
> > v3:
> >  - New patch
> >
> >  net/handshake/request.c | 16 ++++++++++++++++
> >  1 file changed, 16 insertions(+)
> >
> > diff --git a/net/handshake/request.c b/net/handshake/request.c
> > index 274d2c89b6b2..0d1c91c80478 100644
> > --- a/net/handshake/request.c
> > +++ b/net/handshake/request.c
> > @@ -98,6 +98,22 @@ static void handshake_sk_destruct(struct sock *sk)
> >               sk_destruct(sk);
> >  }
> >
> > +/**
> > + * handshake_sk_destruct_req - destroy an existing request
> > + * @sk: socket on which there is an existing request
>
> Generally the kdoc style is unnecessary for static helper functions,
> especially functions with only a single caller.
>
> This all looks so much like handshake_sk_destruct(). Consider
> eliminating the code duplication by splitting that function into a
> couple of helpers instead of adding this one.
>
>
> > + */
> > +static void handshake_sk_destruct_req(struct sock *sk)
>
> Because this function is static, I imagine that the compiler will
> bark about the addition of an unused function. Perhaps it would
> be better to combine 2/6 and 3/6.
>
> That would also make it easier for reviewers to check the resource
> accounting issues mentioned below.
>
>
> > +{
> > +     struct handshake_req *req;
> > +
> > +     req = handshake_req_hash_lookup(sk);
> > +     if (!req)
> > +             return;
> > +
> > +     trace_handshake_destruct(sock_net(sk), req, sk);
>
> Wondering if this function needs to preserve the socket's destructor
> callback chain like so:
>
> +       void (sk_destruct)(struct sock sk);
>
>   ...
>
> +       sk_destruct = req->hr_odestruct;
> +       sk->sk_destruct = sk_destruct;
>
> then:
>
> > +     handshake_req_destroy(req);
>
> Because of the current code organization and patch ordering, it's
> difficult to confirm that sock_put() isn't necessary here.
>
>
> > +}
> > +
> >  /**
> >   * handshake_req_alloc - Allocate a handshake request
> >   * @proto: security protocol
>
> There's no synchronization preventing concurrent handshake_req_cancel()
> calls from accessing the request after it's freed during handshake
> completion. That is one reason why handshake_complete() leaves completed
> requests in the hash.

Ah, so you are worried that free-ing the request will race with
accessing the request after a handshake_req_hash_lookup().

Ok, makes sense. It seems like one answer to that is to add synchronisation

>
> So I'm thinking that removing requests like this is not going to work
> out. Would it work better if handshake_req_hash_add() could recognize
> that a KeyUpdate is going on, and allow replacement of a hashed
> request? I haven't thought that through.

I guess the idea would be to do something like this in
handshake_req_hash_add() if the entry already exists?

    if (test_and_set_bit(HANDSHAKE_F_REQ_COMPLETED, &req->hr_flags)) {
        /* Request already completed */
        rhashtable_replace_fast(...);
    }

I'm not sure that's better. That could possibly still race with
something that hasn't yet set HANDSHAKE_F_REQ_COMPLETED and overwrite
the request unexpectedly.

What about adding synchronisation and keeping the current approach?
>From a quick look it should be enough to just edit
handshake_sk_destruct() and handshake_req_cancel()

Alistair

>
>
> As always, please double-check my questions and assumptions before
> revising this patch!
>
>
> --
> Chuck Lever