Re: [PATCH 0/8] can: usb: remove all usb_set_intfdata(intf, NULL) in drivers' disconnect()

[Vincent MAILHOL]

On Mon. 5 Dec. 2022 at 17:39, Oliver Neukum <oneukum@xxxxxxxx> wrote:
> On 03.12.22 14:31, Vincent Mailhol wrote:
> > The core sets the usb_interface to NULL in [1]. Also setting it to
> > NULL in usb_driver::disconnects() is at best useless, at worse risky.
>
> Hi,
>
> I am afraid there is a major issue with your series of patches.
> The drivers you are removing this from often have a subsequent check
> for the data they got from usb_get_intfdata() being NULL.

ACK, but I do not see the connection.

> That pattern is taken from drivers like btusb or CDC-ACM

Where does CDC-ACM set *his* interface to NULL? Looking at:

  https://elixir.bootlin.com/linux/v6.0/source/drivers/usb/class/cdc-acm.c#L1531

I can see that cdc-acm sets acm->control and acm->data to NULL in his
disconnect(), but it doesn't set its own usb_interface to NULL.

> which claim secondary interfaces disconnect() will be called a second time
> for.

Are you saying that the disconnect() of those CAN USB drivers is being
called twice? I do not see this in the source code. The only caller of
usb_driver::disconnect() I can see is:

  https://elixir.bootlin.com/linux/v6.0/source/drivers/usb/core/driver.c#L458

> In addition, a driver can use setting intfdata to NULL as a flag
> for disconnect() having proceeded to a point where certain things
> can no longer be safely done.

Any reference that a driver can do that? This pattern seems racy.

By the way, I did check all the drivers:

  * ems_usb: intf is only used in ems_usb_probe() and
ems_usb_disconnect() functions.

  * esd_usb: intf is only used in the esd_usb_probe(),
    esd_usb_probe_one_net() (which is part of probing),
    esd_usb_disconnect() and a couple of sysfs functions (which only
    use intf to get a pointer to struct esd_usb).

  * gs_usb: intf is used several time but only to retrive struct
    usb_device. This seems useless, I will sent this patch to remove
    it:
    https://lore.kernel.org/linux-can/20221208081142.16936-3-mailhol.vincent@xxxxxxxxxx/
    Aside of that, intf is only used in gs_usb_probe(),
    gs_make_candev() (which is part of probing) and
    gs_usb_disconnect() functions.

  * kvaser_usb: intf is only used in kvaser_usb_probe() and
    kvaser_usb_disconnect() functions.

  * mcba_usb: intf is only used in mcba_usb_probe() and
    mcba_usb_disconnect() functions.

  * ucan: intf is only used in ucan_probe() and
    ucan_disconnect(). struct ucan_priv also has a pointer to intf but
    it is never used. I sent this patch to remove it:
    https://lore.kernel.org/linux-can/20221208081142.16936-2-mailhol.vincent@xxxxxxxxxx/

  * usb_8dev: intf is only used in usb_8dev_probe() and
    usb_8dev_disconnect().

With no significant use of intf outside of the probe() and
disconnect(), there is definitely no such "use intf as a flag" in any
of these drivers.

> You need to check for that in every driver
> you remove this code from and if you decide that it can safely be removed,

What makes you assume that I didn't check this in the first place? Or
do you see something I missed?

> which is likely, then please also remove checks like this:
>
>         struct ems_usb *dev = usb_get_intfdata(intf);
>
>         usb_set_intfdata(intf, NULL);
>
>         if (dev) {
>                 unregister_netdev(dev->netdev);

How is the if (dev) check related? There is no correlation between
setting intf to NULL and dev not being NULL.

I think dev is never NULL, but I did not assess that dev could not be NULL.

> Either it can be called a second time, then you need to leave it
> as is,

Really?! The first thing disconnect() does is calling
usb_get_intfdata(intf) which dereferences intf without checking if it
is NULL, c.f.:

  https://elixir.bootlin.com/linux/v6.0/source/include/linux/usb.h#L265

Then it sets intf to NULL.

The second time you call disconnect(), the usb_get_intfdata(intf)
would be a NULL pointer dereference.

> or the check for NULL is superfluous. But only removing setting
> the pointer to NULL never makes sense.


Yours sincerely,
Vincent Mailhol