Re: [PATCH v2 3/3] can: gs_usb: fix DMA memory leak on close

[Vincent Mailhol <vincent.mailhol@gmail.com>]

On Fri. 10 Jun 2022 at 05:53, Rhett Aultman <rhett.aultman@samsara.com> wrote:
> The gs_usb driver allocates DMA memory with usb_alloc_coherent() in
> gs_can_open() and then keeps this memory in an URB, with the expectation
> that the memory will be freed when the URB is killed in gs_can_close().
> Memory allocated with usb_alloc_coherent() cannot be freed in this way
> and much be freed using usb_free_coherent() instead.  This means that
      ^^^^
and must

> repeated cycles of calling gs_can_open() and gs_can_close() will lead to
> a memory leak.
>
> Historically, drivers have handled this by keeping an array of pointers
> to their DMA rx buffers and explicitly freeing them.  For an example of
> this technique used in the esd_usb2 driver, see here:
> https://www.spinics.net/lists/linux-can/msg08203.html
>
> While the above method works, the conditions that cause this leak are
> not apparent to driver writers and the method for solving it at the
> driver level has been piecemeal.  This patch makes use of a new
> URB_FREE_COHERENT flag on the URB, reducing the solution of this memory
> leak down to a single line of code.
>
> Signed-off-by: Rhett Aultman <rhett.aultman@samsara.com>
> ---
>  drivers/net/can/usb/gs_usb.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/drivers/net/can/usb/gs_usb.c b/drivers/net/can/usb/gs_usb.c
> index b29ba9138866..3ded3e14c830 100644
> --- a/drivers/net/can/usb/gs_usb.c
> +++ b/drivers/net/can/usb/gs_usb.c
> @@ -768,7 +768,7 @@ static int gs_can_open(struct net_device *netdev)
>                                           buf,
>                                           dev->parent->hf_size_rx,
>                                           gs_usb_receive_bulk_callback, parent);
>
> -                       urb->transfer_flags |= URB_NO_TRANSFER_DMA_MAP;
> +                       urb->transfer_flags |= (URB_NO_TRANSFER_DMA_MAP | URB_FREE_COHERENT);

Nitpick but parenthesis are not needed here. Also, there are no
reasons to do a |=. I would prefer to see this:
        urb->transfer_flags = URB_NO_TRANSFER_DMA_MAP | URB_FREE_COHERENT;

>                         usb_anchor_urb(urb, &parent->rx_submitted);

I looked at the code of gs_usb, this driver has a lot of dust. What
strikes me the most is that it uses usb_alloc_coherent() each time in
its xmit() function instead of allocating the TX URB once in the
open() function and resubmitting then in the callback function. That
last comment is not a criticism of this patch. But if someone has the
motivation to do some cleaning, go ahead…

Aside from the nitpicks, the patch looks good to me. You can add this
to your v3:
Reviewed-by: Vincent Mailhol <mailhol.vincent@wanadoo.fr>


Yours sincerely,
Vincent Mailhol