Re: [net-rfc 04/16] can: dev: can_get_len(): add a helper function to get the correct length of Classical frames

[Oliver Hartkopp <socketcan@hartkopp.net>]

On 22.10.20 05:30, Vincent MAILHOL wrote:
> On 21.10.21 02:52, Oliver Hartkopp wrote:
>> On 21.10.20 13:55, Vincent MAILHOL wrote:
>>>> On 21.10.20 18:48, Oliver Hartkopp wrote:
>>
>>>> To be more compatible to non raw dlc CAN frame sources the tx handling
>>>> could also be like this:
>>>>
>>>> if ((can_dlc == CAN_MAX_DLEN) &&
>>>>        (raw_dlc >= CAN_MAX_DLC && raw_dlc <= CAN_MAX_RAW_DLC))
>>>> => use raw_dlc
>>>>
> 
> In addition to that, we would have to decide what to do with malformed
> frames. If can_dlc and can_raw are set to contradictory values, do we
> drop the frame, do we just ignore the raw_dlc? For example, if can_dlc
> is 8 but raw_dlc is 2, I think that we should drop.

The pseudo code is pretty clear on this: Use can_dlc.

The nice thing about testing raw_dlc to be 9..15 when can_dlc == 8 is, 
that we always have a correct CAN frame on the wire with 8 bytes of content.

Ok, the raw_dlc might be an uninitialized value - but still everything 
remains fine for operation. So why dropping frames here?

The problem of uninitialized values could be solved with a 
CAN_RAW_RAW_DLC sockopt BUT you will be always able to send a CAN frame 
with an AF_PACKET socket which has no CAN_RAW_RAW_DLC sockopt.

And then?

The above pseudo code together with CAN_CTRLMODE_RAW_DLC seems to be a 
pretty safe option to me. Even if 'legacy' applications with 
uninitialized raw_dlc send CAN frames or AF_PACKET users enable 
CAN_CTRLMODE_RAW_DLC we always end up with a proper can_dlc == 8  and a 
fitting valid raw_dlc.

> 
> If we go this direction, I would propose below logic:
> 
>   1. if raw_dlc equals can_dlc: nominal case.

Is not tested. We would start on testing can_dlc == 8

> 
>   2. if can_dlc equals 8 and raw_dlc is greater than 8: use raw_dlc
>      case, already covered in your code.

ACK

>   3. if raw_dlc is 0, then consider it as unset: overwrite raw_dlc with
>      can_dlc (so that it becomes case 1).

Is not tested like this. We would start on can_dlc == 8 - and then check 
for valid raw_dlc 9..15

> 
>   4. Other scenarios: the frame is malformed: drop it.

No dropping.

> Logic should apply for both Tx and Rx paths.

So test for can_dlc == 8 - and then check for valid raw_dlc 9..15 in the 
user space?! Fine.

Btw. we can make sure that we do not have uninitialized raw_dlc value in 
the rx path in the enabled drivers. In fact you could always use the 
raw_dlc then.

BUT it makes sense to use this test to detect cases, where someone 
forgot to switch on CAN_CTRLMODE_RAW_DLC or the driver doesn't support 
it and the programmer did not check the ctrlmode netlink return values.

> Then, the drivers/users would only have to manage scenarios 1 and 2
> (and 2 can be ignored if CAN_CTRLMODE_RAW_DLC is not advertised).
> 
>>>
>>> If I understand well, the idea would be not to use a setsockopt() but
>>> instead rely on some logic on the can_dlc and raw_dlc to determine
>>> which one to use.
>>>
>>> Unfortunately, this approach has one issue in the TX path that could
>>> break existing applications.
>>>
>>> Consider below code (which I think is fairly realistic):
>>>
>>> void send_frame (int soc, canid_t can_id, char *data, size_t len)
>>> {
>>>       struct can_frame cf;
>>>       size_t dlc = len > sizeof(cf.data) ? sizeof(cf.data) : len;
>>>
>>>       cf.can_id = can_id;
>>>       cf.can_dlc = dlc;
>>>       memcpy(cf.data, data, dlc);
>>>
>>>       write(soc, &cf, sizeof(cf));
>>> }
>>>
>>> Here, the user did not initialize the can frame (cf) but assigned all
>>> the relevant fields manually. Because cf is not initialized, the newly
>>> introduced cf.dlc_raw field would have any of the values which was
>>> present on the stack at the moment (rationale: the C standard does not
>>> guarantee zero initialization). If 9 <= raw_dlc <= 15, the can frame
>>> will be transmitted with a bad DLC value. If raw_dlc > 15, the can
>>> frame will be discarded.
>>
>> No, this is not what I wrote. With my suggestion you need to populate
>> both dlc elements to use the new "raw dlc" feature.
>>
>> if (can_dlc == 8) && (9 <= raw_dlc <= 15)
>> => put raw_dlc value into the controller register
>> else
>> => put can_dlc value into the controller register
>>
>> When you have a test system to make security tests and you enable
>> CAN_CTRLMODE_RAW_DLC on a specific CAN interface - which applications
>> would you run to send CAN frames on this interface?
>>
>> I assume only the test application which is really aware of setting
>> can_dlc and raw_dlc correctly.
> 
> My point is that this assumption is dangerous.
> 
> I do not think I made myself clear. I am speaking about old "non-DLC
> aware" code running in CAN_CTRLMODE_RAW_DLC context.
> 
> Consider two applications: the first application is a test application
> which is "DLC aware", the second is a legacy application which
> contains the code which I presented in my previous message.
> 
> A user who wants to run both in parallel sets CAN_CTRLMODE_RAW_DLC at
> netlink level. First application works fine, second application which
> contains the legacy code gets impacted as explained previously and
> stops behaving as intended.
> 
> Newly introduced option should not break existing code regardless
> why. In opposition, we can introduce new rules if these are strictly
> tied to the new option.

Yes but it doesn't help you with AF_PACKET.

> In my mind, imposing a rule that old code should not be used in
> CAN_CTRLMODE_RAW_DLC context would create a huge pitfall and would
> violate the "don't break userland" principle.

CAN_RAW_RAW_DLC could help in the case of legacy code only when they are 
using CAN_RAW sockets. But nothing more.

Therefore we need the robustness on driver level with the checking of 
can_dlc == 8.

(..)

>>> For userland, I think that this is acceptable because the very instant
>>> the user calls setsockopt() with the CAN_RAW_RAW_DLC, he should be
>>> aware of the consequences and should resign to use can_dlc field as a
>>> plain length.
>>
>> Not filling can_dlc would cause tons of changes for sanity checks and
>> feature switches.
> 
> I never spoke about "not filling can_dlc". My point is to not use it
> as a length but use it as a DLC according to ISO definition. In my
> approach, can_dlc should always be filled with the raw DLC value.

Ok, let me rephrase: Filling can_dlc with something else than a plain 
length information 0..8 ;-)

(..)

>> The 14 year old documentation in can.h says:
>> @can_dlc: frame payload length in byte (0 .. 8)
>>
>> I will not break this established rule for a testing feature. The
>> question from Joakim clearly showed: Don't touch this!
> 
> My point is this is an expert feature: if you do not understand it, do
> not use it and you are safe to go using the 14 years old definition.

Full ACK - but you can't imagine what people do in the real world. Do 
you know these 'more helps more' guys that switch everything to 'on'? :-D

The approach with CAN_CTRLMODE_RAW_DLC and the described testing would 
be even robust against unintended miss-use.

>> At the end it would have made more sense to call it can_frame.len in the
>> first place. But this first came into our mind when CAN FD showed up.
>> The discussion about the can_dlc meaning can be closed IMO. It is a
>> plain length from 0..8 which is unfortunately named can_dlc.
> 
> I agree that it should have been named len from the beginning, but as
> a matter of fact it has been named "can_dlc". For me as a user, I
> expect can_dlc to follow the DLC definition in ISO and so I *prefer*
> to opt for the compromise of losing the plain length property rather
> than losing the semantic meaning.

I don't think you can claim to be a standard user. You are an expert for 
security testing and digged into the ISO standard and intentionally 
write DLC values >8 into controller registers. Users don't do things 
like this ;-)

Clean-up and renaming typically ends when it breaks APIs and commonly 
followed rules. There are many examples in Linux where these clean-up 
attempts have been canceled for these reasons.

I would have liked to rename can_dlc to len too.
But now that we have a compatible canfd_frame which can contain a 
can_frame this is somehow settled: can_dlc == len.

> I would like to conclude by saying that I do necessarily think that
> your approach of raw_dlc field is bad (if used in combination with the
> setsockopt()). It has its pros and cons. However, if you ask me for
> feedback, *my answer* is that *I prefer* to use can_dlc as a DLC. But
> at the end of the day, I would be happy if the feature gets
> implemented, regardless how, so that I can do my testing :-)

So let's see how we can get there best :-)

Best regards,
Oliver