Re: [PATCH v2 3/5] ALSA: xen-front: Implement Xen event channel handling

From: Oleksandr Andrushchenko
Date: Tue Apr 24 2018 - 10:58:54 EST

On 04/24/2018 05:35 PM, Takashi Iwai wrote:
On Tue, 24 Apr 2018 16:29:15 +0200,
Oleksandr Andrushchenko wrote:
On 04/24/2018 05:20 PM, Takashi Iwai wrote:
On Mon, 16 Apr 2018 08:24:51 +0200,
Oleksandr Andrushchenko wrote:
+static irqreturn_t evtchnl_interrupt_req(int irq, void *dev_id)
+ struct xen_snd_front_evtchnl *channel = dev_id;
+ struct xen_snd_front_info *front_info = channel->front_info;
+ struct xensnd_resp *resp;
+ RING_IDX i, rp;
+ unsigned long flags;
+ if (unlikely(channel->state != EVTCHNL_STATE_CONNECTED))
+ return IRQ_HANDLED;
+ spin_lock_irqsave(&front_info->io_lock, flags);
+ rp = channel->u.req.ring.sring->rsp_prod;
+ /* ensure we see queued responses up to rp */
+ rmb();
+ for (i = channel->u.req.ring.rsp_cons; i != rp; i++) {
I'm not familiar with Xen stuff in general, but through a quick
glance, this kind of code worries me a bit.

If channel->u.req.ring.rsp_cons has a bogus number, this may lead to a
very long loop, no? Better to have a sanity check of the ring buffer
In this loop I have:
resp = RING_GET_RESPONSE(&channel->u.req.ring, i);
and the RING_GET_RESPONSE macro is designed in the way that
it wraps around when *i* in the question gets bigger than
the ring size:

ÂÂÂ (&((_r)->sring->ring[((_idx) & (RING_SIZE(_r) - 1))].req))

So, even if the counter has a bogus number it will not last long
Hm, this prevents from accessing outside the ring buffer, but does it
change the loop behavior?
no, it doesn't
Suppose channel->u.req.ring_rsp_cons = 1, and rp = 0, the loop below
would still consume the whole 32bit counts, no?

for (i = channel->u.req.ring.rsp_cons; i != rp; i++) {
resp = RING_GET_RESPONSE(&channel->u.req.ring, i);
You are right here and the comment is totally valid.
I'll put an additional check like here [1] and here [2]
Will this address your comment?

Thank you,