[RFC] pf ioctl changes

[Kristof Provost]

On 29 Mar 2021, at 17:16, Cy Schubert wrote:
> In message <18DC1EA9-ABFC-4A06-8710-A3068370EC52 at FreeBSD.org>, 
> "Kristof
> Provost
> " writes:
>> On 29 Mar 2021, at 16:03, Cy Schubert wrote:
>>> In message <24E09373-EBCD-4ED1-8B59-A44E687F287E at FreeBSD.org>,
>>> "Kristof
>>> Provost
>>> " writes:
>>>> Hi,
>>>>
>>>> There are several patches in the pipeline that require changes in
>>>> pf’s
>>>> interface between kernel and userspace.
>>>> In the past these have been handled in multiple ways. Either by
>>>> simply
>>>> making the change, breaking binary compatibility, or by introducing 
>>>> a
>>>> v2
>>>> ioctl (e.g. DIOCADDALTQV1).
>>>>
>>>> While one is better than the other neither is wholly satisfying. 
>>>> New
>>>> versions of calls constitute a maintenance burden after all.
>>>>
>>>> I’d like to change the ioctl interface to use nvlists, 
>>>> which
>>>> would
>>>> make such extensions much easier, because fields can be optional.
>>>> That is, if userspace doesn’t supply the
>>>> ‘shinynewfeature’ field
>>>> the kernel can assume the default value and things just work.
>>>> Similarly,
>>>> if the kernel supplies a ’shinynewfeature’ 
>>>> which userspace
>>>> doesn’t
>>>> know about it’s simply ignored.
>>>>
>>>> The rough plan is to introduce nvlist versions of the get/add rules
>>>> calls for now. Others will follow as the need presents itself.
>>>> As these are new ioctls it is safe to MFC them to stable/12 and
>>>> stable/13.
>>>> The old interface will remain supported in those branches, but
>>>> I’d
>>>> like to remove it from main (and thus FreeBSD 14).
>>>>
>>>> As part of this effort I may end up splitting off the ioctl 
>>>> interface
>>>> code from pfctl into libpfctl, which should make reuse of that code
>>>> easier.
>>>>
>>>> I hope to post preliminary patches in the coming week.
>>>>
>>>> Thoughts? Objections?
>>>
>>> Kernel and userland should be, I'd say must be, kept in sync. We 
>>> have
>>> many
>>> examples of userland and kernel not being in sync over the years. 
>>> For
>>> ipfilter, I've made incompatible changes to data structures 
>>> requiring
>>> userand and kernel be in sync. These are few and far between.
>>>
>>> I've gotten away with this because there is no third party software
>>> that
>>> relies on the ipfilter kernel interfaces. I could be wrong but I 
>>> doubt
>>> there may be third party software requiring pf ABI compatibility. 
>>> But
>>> if
>>> there is then verstioned library interfaces are required.
>>>
>>> Given that the advice is to keep kernel and userland in sync there
>>> probably
>>> is no requirement for an UPDATING entry but that would be your call.
>>>
>> There are out-of-tree users of the pf ioctl interface.
>> security/expiretable[1] for example.
>> security/snort2pfcd appears to as well.
>> sysutils/pfstat and sysutils/pftop use the ioctl interface as well,
>> although not the three specific calls of immediate interest.
>
> This complicates things. IMO you'll probably need versioned function 
> calls
> for at least 13-STABLE EOL. Or, versioning the data structures passed 
> into
> the kernel such that the new fields are at the tail of the existing
> structures.
>
That’s essentially the plan. I plan to keep the existing definitions 
(of both structure and ioctl numbers) in stable/12 and stable/13.
They’ll disappear in main (i.e. 14).

Alongside we’ll introduce new nvlist variants for those calls, which 
will have the new features.

>> I’m trying to work out how many examples there are, because one 
>> way or
>> the other they’re going to have to cope with changes.
>>
>> So, I’d prefer to not just change the definitions of structs, 
>> even if
>> we’ve done that in the past. struct pf_rule contains a few
>> peculiarities from historical mistakes that I hope to correct now.
>
> Technical debt is difficult to eliminate. We either fix it, paying it 
> off
> in one lump sum or we pay it off through aggravation and design
> limitations, with interest, over time.
>
Indeed.

To take struct pf_rule as an example: it contains counter_u64’s, which 
don’t really work for userspace, so we’ve added uint64_t versions of 
those variables. Now the struct has two version of the same field.
That can be cleaned up once the ioctls which use the struct have been 
removed (so on main only). My plan is to remove the struct definition 
from the kernel’s headers (again, once there are alternative ioctls 
and only in main), moving it into libpfctl.
Then there will be nothing to stop us from removing the counter_u64 
versions of those fields, cleaning up the struct.

> Given that pf uses ioctl, versioned function calls won't help. A new 
> ioctl
> may be the only answer. If you do choose this, add an identifier and
> version number to the head of each new struct to future proof pf.
>
The nvlist versions will be much more flexible, so embedding a version 
number seem redundant.

Best regards,
Kristof