Re: [PATCH 01/13] kdbus: add documentation

[David Herrmann]

Hi Michael

On Thu, Jan 22, 2015 at 11:18 AM, Michael Kerrisk (man-pages)
<mtk.manpages@xxxxxxxxx> wrote:
> On 01/21/2015 05:58 PM, Daniel Mack wrote:
>>>> Also, the context the kdbus commands operate on originate from a
>>>> mountable special-purpose file system.
>>>
>>> It's not clear to me how this point implies any particular API design
>>> choice.
>>
>> It emphasizes the fact that our ioctl API can only be used with the
>> nodes exposed by kdbusfs, and vice versa. I think operations on
>> driver-specific files do not justify a new 'generic' syscall API.
>
> I see your (repeated) use of words like "driver" as just a distraction,
> I'm sorry. "Driver-specific files" is an implementation detail. The
> point is that kdbus provides a complex, general-purpose feature for all
> of userland. It is core infrastructure that will be used by key pieces
> of the plumbing layer, and likely by many other applications as well.
> *Please* stop suggesting that it is not core infrastructure: kdbus
> has the potential to be a great IPC that will be very useful to many,
> many user-space developers.

We called it an 'ipc driver' so far. It is in no way meant as
distraction. Feel free to call it 'core infrastructure'. I think we
can agree that we want it to be generically useful, like other ipc
mechanisms, including UDS and netlink.

> (By the way, we have precedents for device/filesystem-specific system
> calls. Even a recent one, in memfd_create().)

memfd_create() is in no way file-system specific. Internally, it uses
shmem, but that's an implementation detail. The API does not expose
this in any way. If you were referring to sealing, it's implemented as
fcntl(), not as a separate syscall. Furthermore, sealing is only
limited to shmem as it's the only volatile storage. It's not an API
restriction. Other volatile file-systems are free to implement
sealing.

>>> ioctl() is a get-out-of-jail free card when it comes to API design.
>>
>> And how are syscalls different in that regard when they would both
>> transport the same data structures?
>
> My general impression is that when it comes to system calls,
> there's usually a lot more up front thought about API design.

This is no technical reason why to use syscalls over ioctls. Imho,
it's rather a reason to improve the kernel's ioctl-review process.

>> Also note that all kdbus ioctls
>> necessarily operate on a file descriptor context, which an ioctl passes
>> along by default.
>
> I fail to see your point here. The same statement applies to multiple
> special-purpose system calls (epoll, inotify, various shared memory APIs,
> and so on).

epoll and inotify don't have a 'parent' object living in the
file-system. They *need* an entry-point. We can use open() for that.

You're right, from a technical viewpoint, there's no restriction.
There're examples for both (eg., see Solaris /dev/poll, as
ioctl()-based 'epoll').

>>> Rather
>>> than thinking carefully and long about a set of coherent, stable APIs that
>>> provide some degree of type-safety to user-space, one just adds/changes/removes
>>> an ioctl.
>>
>> Adding another ioctl in the future for cases we didn't think about
>> earlier would of course be considered a workaround; and even though such
>> things happen all the time, it's certainly something we'd like to avoid.
>>
>> However, we would also need to add another syscall in such cases, which
>> is even worse IMO. So that's really not an argument against ioctls after
>> all. What fundamental difference between a raw syscall and a ioctl,
>> especially with regards to type safety, is there which would help us here?
>
> Type safety helps user-space application developers. System calls have
> it, ioctl() does not.

This is simply not true. There is no type-safety in:
    syscall(__NR_xyz, some, random, arguments)

The only way a syscall gets 'type-safe', is to provide a wrapper
function. Same applies to ioctls. But people tend to not do that for
ioctls, which is, again, not a technical argument against ioctls. It's
a matter of psychology, though.

I still don't see a technical reason to use syscalls. API proposals welcome!

We're now working on a small kdbus helper library, which provides
type-safe ioctl wrappers, item-iterators and documented examples. But,
like syscalls, nobody is forced to use the wrappers. The API design is
not affected by this.

>>> And that process seems to be frequent and ongoing even now. (And
>>> it's to your great credit that the API/ABI breaks are clearly and honestly
>>> marked in the kdbus.h changelog.) All of this lightens the burden of API
>>> design for kernel developers, but I'm concerned that the long-term pain
>>> for user-space developers who use an API which (in my estimation) may
>>> come to be widely used will be enormous.
>>
>> Yes, we've jointly reviewed the API details again until just recently to
>> unify structs and enums etc, and added fields to make the ioctls structs
>> more versatile for possible future additions. By that, we effectively
>> broke the ABI, but we did that because we know we can't do such things
>> again in the future.
>>
>> But again - I don't see how this would be different when using syscalls
>> rather than ioctls to transport information between the driver and
>> userspace. Could you elaborate?
>
> My suspicion is that not nearly enough thinking has yet been done about
> the design of the API. That's based on these observations:
>
> * Documentation that is, considering the size of the API, *way* too thin.

Ok, working on that.

> * Some parts of the API not documented at all (various kdbus_item blobs)

All public structures have documentation in kdbus.h. It may need
improvements, though.

> * ABI changes happening even quite recently

Please elaborate why 'recent ABI-changes' are a sign of a premature API.

I seriously doubt any API can be called 'perfect'. On the contrary, I
believe that all APIs could be improved continuously. The fact that
we, at one point, settle on an API is an admission of
backwards-compatibility. I in no way think it's a sign of
'perfection'.
With kdbus our plan is to give API-guarantees starting with upstream
inclusion. We know, that our API will not be perfect, none is. But we
will try hard to fix anything that comes up, as long as we can. And
this effort will manifest in ABI-breaks.

> * API oddities such as the 'kernel_flags' fields. Why do I need to
>   be told what flags the kernel supports on *every* operation?

If we only returned EINVAL on invalid arguments, user-space had to
probe for each flag to see whether it's supported. By returning the
set of supported flags, user-space can cache those and _reliably_ know
which flags are supported.
We decided the overhead of a single u64 copy on each ioctl is
preferred over a separate syscall/ioctl to query kernel flags. If you
disagree, please elaborate (preferably with a suggestion how to do it
better).

> The above is just after a day of looking hard at kdbus.txt. I strongly
> suspect I'd find a lot of other issues if I spent more time on kdbus.

If you find the time, please do! Any hints how a specific part of the
API could be done better, is highly appreciated. A lot of the more or
less recent changes were done due to reviews from glib developers.
More help is always welcome!

Thanks
David
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