linux-kernel - Re: [PATCH 1/1] can: skb: add and set local

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Date:   Thu, 12 May 2022 08:23:26 +0200
From:   Oliver Hartkopp <socketcan@...tkopp.net>
To:     Marc Kleine-Budde <mkl@...gutronix.de>
Cc:     Oleksij Rempel <o.rempel@...gutronix.de>,
        Wolfgang Grandegger <wg@...ndegger.com>,
        Devid Antonio Filoni <devid.filoni@...uetechnologies.com>,
        kernel@...gutronix.de, linux-can@...r.kernel.org,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        David Jander <david@...tonic.nl>
Subject: Re: [PATCH 1/1] can: skb: add and set local_origin flag

Hi Marc,

On 11.05.22 16:57, Oliver Hartkopp wrote:
> 
> 
> On 5/11/22 16:54, Marc Kleine-Budde wrote:
>> On 11.05.2022 16:50:06, Oliver Hartkopp wrote:
>>>
>>>
>>> On 5/11/22 16:36, Marc Kleine-Budde wrote:
>>>> On 11.05.2022 15:24:21, Marc Kleine-Budde wrote:
>>>>> On 11.05.2022 14:38:32, Oliver Hartkopp wrote:
>>>>>> IMO this patch does not work as intended.
>>>>>>
>>>>>> You probably need to revisit every place where can_skb_reserve() 
>>>>>> is used,
>>>>>> e.g. in raw_sendmsg().
>>>>>
>>>>> And the loopback for devices that don't support IFF_ECHO:
>>>>>
>>>>> | https://elixir.bootlin.com/linux/latest/source/net/can/af_can.c#L257
>>>>
>>>> BTW: There is a bug with interfaces that don't support IFF_ECHO.
>>>>
>>>> Assume an invalid CAN frame is passed to can_send() on an interface 
>>>> that
>>>> doesn't support IFF_ECHO. The above mentioned code does happily 
>>>> generate
>>>> an echo frame and it's send, even if the driver drops it, due to
>>>> can_dropped_invalid_skb(dev, skb).
>>>>
>>>> The echoed back CAN frame is treated in raw_rcv() as if the headroom 
>>>> is valid:
I double checked that code and when I didn't miss anything all the 
callers of can_send() (e.g. raw_sendmsg()) are creating valid skbs.

https://elixir.bootlin.com/linux/v5.17.6/A/ident/can_send

>>>>
>>>> | https://elixir.bootlin.com/linux/v5.17.6/source/net/can/raw.c#L138
>>>>
>>>> But as far as I can see the can_skb_headroom_valid() check never has
>>>> been done. What about this patch?
>>>>
>>>> index 1fb49d51b25d..fda4807ad165 100644
>>>> --- a/net/can/af_can.c
>>>> +++ b/net/can/af_can.c
>>>> @@ -255,6 +255,9 @@ int can_send(struct sk_buff *skb, int loop)
>>>>                    */
>>>>                   if (!(skb->dev->flags & IFF_ECHO)) {
>>>> +                       if (can_dropped_invalid_skb(dev, skb))
>>>> +                               return -EINVAL;
>>>> +

That would make this change unnecessary, right?

IIRC the reason for can_dropped_invalid_skb() is to prove valid skbs for 
CAN interface drivers when CAN frame skbs are created e.g. with 
PF_PACKET sockets.

Best,
Oliver


>>>
>>> Good point!
>>>
>>> But please check the rest of the code.
>>> You need 'goto inval_skb;' instead of the return ;-)
>>
>> Why? To free the skb? That's what can_dropped_invalid_skb() does, too:
>>
>> | 
>> https://elixir.bootlin.com/linux/v5.17.6/source/include/linux/can/skb.h#L130 
>>
>>
> 
> My bad!
> 
> Pointing you not reading the code ... should better have looked myself :-D
>