Re: [PATCH net v2] net: Drop the lock in skb_may_tx_timestamp()

[Willem de Bruijn <willemdebruijn.kernel@gmail.com>]

Sebastian Andrzej Siewior wrote:
> skb_may_tx_timestamp() may acquire sock::sk_callback_lock. The lock must
> not be taken in IRQ context, only softirq is okay. A few drivers receive
> the timestamp via a dedicated interrupt and complete the TX timestamp
> from that handler. This will lead to a deadlock if the lock is already
> write-locked on the same CPU.
> 
> Taking the lock can be avoided. The socket (pointed by the skb) will
> remain valid until the skb is released. The ->sk_socket and ->file
> member will be set to NULL once the user closes the socket which may
> happen before the timestamp arrives.
> If we happen to observe the pointer while the socket is closing but
> before the pointer is set to NULL then we may use it because both
> pointer (and the file's cred member) are RCU freed.
> 
> Drop the lock. Use READ_ONCE() to obtain the individual pointer. Add a
> matching WRITE_ONCE() where the pointer are cleared.
> 
> Link: https://lore.kernel.org/all/20260205145104.iWinkXHv@linutronix.de
> Fixes: b245be1f4db1a ("net-timestamp: no-payload only sysctl")
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>

Reviewed-by: Willem de Bruijn <willemb@google.com>