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In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 26/03/2025 11:03, Qingfang Deng wrote: > Hi Antonio, > > On Wed, Mar 26, 2025 at 5:41 PM Antonio Quartulli 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.