netdev - Re: [PATCH] Bluetooth: Always call advertising disable before setting params

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Date:   Wed, 24 Mar 2021 11:49:02 -0700
From:   Daniel Winkler <danielwinkler@...gle.com>
To:     Marcel Holtmann <marcel@...tmann.org>
Cc:     linux-bluetooth <linux-bluetooth@...r.kernel.org>,
        CrosBT Upstreaming <chromeos-bluetooth-upstreaming@...omium.org>,
        Miao-chen Chou <mcchou@...omium.org>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Johan Hedberg <johan.hedberg@...il.com>,
        Luiz Augusto von Dentz <luiz.dentz@...il.com>,
        LKML <linux-kernel@...r.kernel.org>,
        "open list:NETWORKING [GENERAL]" <netdev@...r.kernel.org>
Subject: Re: [PATCH] Bluetooth: Always call advertising disable before setting params

Hello Marcel,

Thank you for the feedback. I have just sent a V2 with a btmon snippet
showing the HCI Set Advertising Parameters "Command Disallowed"
failure that occurs as a result of this issue. I tried to provide some
annotation for context. Please take a look.

Thanks!
Daniel


On Wed, Mar 24, 2021 at 12:06 AM Marcel Holtmann <marcel@...tmann.org> wrote:
>
> Hi Daniel,
>
> > In __hci_req_enable_advertising, the HCI_LE_ADV hdev flag is temporarily
> > cleared to allow the random address to be set, which exposes a race
> > condition when an advertisement is configured immediately (<10ms) after
> > software rotation starts to refresh an advertisement.
> >
> > In normal operation, the HCI_LE_ADV flag is updated as follows:
> >
> > 1. adv_timeout_expire is called, HCI_LE_ADV gets cleared in
> >   __hci_req_enable_advertising, but hci_req configures an enable
> >   request
> > 2. hci_req is run, enable callback re-sets HCI_LE_ADV flag
> >
> > However, in this race condition, the following occurs:
> >
> > 1. adv_timeout_expire is called, HCI_LE_ADV gets cleared in
> >   __hci_req_enable_advertising, but hci_req configures an enable
> >   request
> > 2. add_advertising is called, which also calls
> >   __hci_req_enable_advertising. Because HCI_LE_ADV was cleared in Step
> >   1, no "disable" command is queued.
> > 3. hci_req for adv_timeout_expire is run, which enables advertising and
> >   re-sets HCI_LE_ADV
> > 4. hci_req for add_advertising is run, but because no "disable" command
> >   was queued, we try to set advertising parameters while advertising is
> >   active, causing a Command Disallowed error, failing the registration.
> >
> > To resolve the issue, this patch removes the check for the HCI_LE_ADV
> > flag, and always queues the "disable" request, since HCI_LE_ADV could be
> > very temporarily out-of-sync. According to the spec, there is no harm in
> > calling "disable" when advertising is not active.
> >
> > Reviewed-by: Miao-chen Chou <mcchou@...omium.org>
> > Signed-off-by: Daniel Winkler <danielwinkler@...gle.com>
> > ---
> >
> > net/bluetooth/hci_request.c | 6 ++++--
> > 1 file changed, 4 insertions(+), 2 deletions(-)
> >
> > diff --git a/net/bluetooth/hci_request.c b/net/bluetooth/hci_request.c
> > index 8ace5d34b01efe..2b4b99f4cedf21 100644
> > --- a/net/bluetooth/hci_request.c
> > +++ b/net/bluetooth/hci_request.c
> > @@ -1547,8 +1547,10 @@ void __hci_req_enable_advertising(struct hci_request *req)
> >       if (!is_advertising_allowed(hdev, connectable))
> >               return;
> >
> > -     if (hci_dev_test_flag(hdev, HCI_LE_ADV))
> > -             __hci_req_disable_advertising(req);
> > +     /* Request that the controller stop advertising. This can be called
> > +      * whether or not there is an active advertisement.
> > +      */
> > +     __hci_req_disable_advertising(req);
>
> can you include a btmon trace that shows that we don’t get a HCI error. Since if we get one, then the complete request will fail. And that has further side effects.
>
> Regards
>
> Marcel
>