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From:   "Matwey V. Kornilov" <matwey.kornilov@gmail.com>
Date:   Sat, 26 Jan 2019 12:45:45 +0300
Message-ID: <CAJs94Eb-ERrfAEONK=_14f=kMsqVBT-XkcwfWZXwUgspraD9QA@mail.gmail.com>
Subject: Re: [PATCH] usb: musb: Fix potential NULL dereference
To:     Alan Stern <stern@rowland.harvard.edu>
Cc:     Bin Liu <b-liu@ti.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        "open list:MUSB MULTIPOINT HIGH SPEED DUAL-ROLE CONTROLLER" 
        <linux-usb@vger.kernel.org>,
        open list <linux-kernel@vger.kernel.org>
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=D1=81=D0=B1, 26 =D1=8F=D0=BD=D0=B2. 2019 =D0=B3. =D0=B2 00:37, Alan Stern =
<stern@rowland.harvard.edu>:
>
> On Fri, 25 Jan 2019, Bin Liu wrote:
>
> > On Thu, Jan 24, 2019 at 09:47:02PM +0300, Matwey V. Kornilov wrote:
> > > By the way, why do we need to store the qh in urb->hcpriv?
> > > qh can always be accessible through urb->ep->hcpriv
> > > Wouldn't it be better to drop entire urb->hcpriv usage?
> >
> > I am not sure why. The code is there since the first commit in a decade
> > ago. But I tend to agree with you.
> >
> > In a quick search for urb->hcpriv and urb->ep->hcpriv, based on the
> > usage in core/hcd.c, it seems to me that urb->hcpriv should not be
> > changed in each controller driver, but I see both have been used in mos=
t
> > controller drivers. I will leave this to others to educate me.
>
> In some of the older HCDs, urb->hcpriv !=3D NULL is used to indicate that
> urb is on an endpoint queue.  Perhaps that usage was copied.
>
> Alan Stern
>

Hi,

Thank you for the explanation. I think that It would be great to
document it somewhere. Such a purpose for variable named `hcpriv' is
not entirely obvious.
Now it is clear to me, why __usb_hcd_giveback_urb() sets urb->hcpriv to NUL=
L.

Returning to my initial patch. I think that it is still valid, though
I admit that the commit message must be rewritten.
In this code path we successfully queued the new urb, so urb->hcpriv
should be NOT NULL indicating that the urb is queued according to Alan
explanation.

musb_urb_enqueue(), musb_host.c line 2345:

        if (ret =3D=3D 0) {
                urb->hcpriv =3D qh;
                /* FIXME set urb->start_frame for iso/intr, it's tested in
                 * musb_start_urb(), but otherwise only konicawc cares ...
                 */
        }

--=20
With best regards,
Matwey V. Kornilov