Re: [PATCH net-next v24 09/23] ovpn: implement packet processing

[Antonio Quartulli]

On 26/03/2025 11:03, Qingfang Deng wrote:
> Hi Antonio,
>
> On Wed, Mar 26, 2025 at 5:41 PM Antonio Quartulli <antonio@xxxxxxxxxxx> wrote:
> > > > +/* Get the next packet ID for xmit */
> > > > +static inline int ovpn_pktid_xmit_next(struct ovpn_pktid_xmit *pid, u32 *pktid)
> > > > +{
> > > > +    const s64 seq_num = atomic64_fetch_add_unless(&pid->seq_num, 1,
> > > > +                                                  0x100000000LL);
> > > > +    /* when the 32bit space is over, we return an error because the packet
> > > > +     * ID is used to create the cipher IV and we do not want to reuse the
> > > > +     * same value more than once
> > > > +     */
> > > > +    if (unlikely(seq_num == 0x100000000LL))
> > > > +            return -ERANGE;
> > >
> > > You may use a 32-bit atomic_t, instead of checking if it equals
> > > 0x1_00000000, check if it wraparounds to zero.
> > > Additionally, you don't need full memory ordering as you just want an
> > > incrementing value:
> > >
> > > int seq_num = atomic_fetch_inc_relaxed(&pid->seq_num);
> > >
> > > if (unlikely(!seq_num))
> > >        ...
> >
> > But then if we have concurrent invocations of ovpn_pktid_xmit_next()
> > only the first one will error out on wrap-around, while the others will
> > return no error (seq_num becomes > 0) and will allow the packets to go
> > through.
> >
> > This is not what we want.
>
> Got it. You could replace it with
> atomic_fetch_add_unless(&pid->seq_num, 1, 0) and check if it wraps
> around to zero.

What about the first time when seq_num is 0? It will already stop, no?

> However, what about the opposite scenario? If multiple threads
> concurrently invoke ovpn_pktid_xmit_next() and all detect the
> wraparound condition, could this lead to simultaneous calls to
> ovpn_crypto_kill_key() and ovpn_nl_key_swap_notify()?

Calling ovpn_crypto_kill_key() multiple times is not an issue, as only 
the first time it'll do something. Subsequent calls are no-op.

But you're right about ovpn_nl_key_swap_notify(): each call will produce 
a notification which we don't want.
I'll make it conditional so that we'll send a notification only if 
ovpn_crypto_kill_key() did the killing.

Thanks for asking this!


Regards,

--
Antonio Quartulli
OpenVPN Inc.