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

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:

#define RING_GET_REQUEST(_r, _idx)                    \
     (&((_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,

[1] https://elixir.bootlin.com/linux/v4.17-rc2/source/drivers/block/xen-blkback/blkback.c#L1127 [2] https://elixir.bootlin.com/linux/v4.17-rc2/source/drivers/block/xen-blkback/blkback.c#L1135

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