Date:   Fri, 24 Feb 2023 15:14:11 +0000
From:   Robin Murphy <robin.murphy@....com>
To:     Asahi Lina <lina@...hilina.net>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Miguel Ojeda <ojeda@...nel.org>,
        Alex Gaynor <alex.gaynor@...il.com>,
        Wedson Almeida Filho <wedsonaf@...il.com>,
        Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>,
        Björn Roy Baron <bjorn3_gh@...tonmail.com>,
        Will Deacon <will@...nel.org>, Joerg Roedel <joro@...tes.org>,
        Hector Martin <marcan@...can.st>,
        Sven Peter <sven@...npeter.dev>, Arnd Bergmann <arnd@...db.de>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Alyssa Rosenzweig <alyssa@...enzweig.io>,
        Neal Gompa <neal@...pa.dev>, rust-for-linux@...r.kernel.org,
        linux-kernel@...r.kernel.org, asahi@...ts.linux.dev
Subject: Re: [PATCH 2/5] rust: device: Add a minimal RawDevice trait

On 2023-02-24 14:48, Asahi Lina wrote:
> 
> 
> On 2023/02/24 23:32, Robin Murphy wrote:
>> On 2023-02-24 14:11, Greg Kroah-Hartman wrote:
>>> Thanks for the detailed rust explainations, I'd like to just highlight
>>> one thing:
>>>
>>> On Fri, Feb 24, 2023 at 10:15:12PM +0900, Asahi Lina wrote:
>>>> On 24/02/2023 20.23, Greg Kroah-Hartman wrote:
>>>>> And again, why are bindings needed for a "raw" struct device at all?
>>>>> Shouldn't the bus-specific wrappings work better?
>>>>
>>>> Because lots of kernel subsystems need to be able to accept "any" device
>>>> and don't care about the bus! That's what this is for.
>>>
>>> That's great, but:
>>>
>>>> All the bus
>>>> wrappers would implement this so they can be used as an argument for all
>>>> those subsystems (plus a generic one when you just need to pass around
>>>> an actual owned generic reference and no longer need bus-specific
>>>> operations - you can materialize that out of a RawDevice impl, which is
>>>> when get_device() would be called). That's why I'm introducing this now,
>>>> because both io_pgtable and rtkit need to take `struct device` pointers
>>>> on the C side so we need some "generic struct device" view on the
>>>> Rust side.
>>>
>>> In looking at both ftkit and io_pgtable, those seem to be good examples
>>> of how "not to use a struct device", so trying to make safe bindings
>>> from Rust to these frameworks is very ironic :)
>>>
>>> rtkit takes a struct device pointer and then never increments it,
>>> despite saving it off, which is unsafe.  It then only uses it to print
>>> out messages if things go wrong (or right in some cases), which is odd.
>>> So it can get away from using a device pointer entirely, except for the
>>> devm_apple_rtkit_init() call, which I doubt you want to call from rust
>>> code, right?
>>>
>>> for io_pgtable, that's a bit messier, you want to pass in a device that
>>> io_pgtable treats as a "device" but again, it is NEVER properly
>>> reference counted, AND, it is only needed to try to figure out the bus
>>> operations that dma memory should be allocated from for this device.  So
>>> what would be better to save off there would be a pointer to the bus,
>>> which is constant and soon will be read-only so there are no lifetime
>>> rules needed at all (see the major struct bus_type changes going into
>>> 6.3-rc1 that will enable that to happen).
>>
>> FWIW the DMA API *has* to know which specific device it's operating
>> with, since the relevant properties can and do vary even between
>> different devices within a single bus_type (e.g. DMA masks).
>>
>> In the case of io-pgtable at least, there's no explicit refcounting
>> since the struct device must be the one representing the physical
>> platform/PCI/etc. device consuming the pagetable, so if that were to
>> disappear from underneath its driver while the pagetable is still in
>> use, things would already have gone very very wrong indeed :)
> 
> There's no terribly good way to encode this relationship in safe Rust as
> far as I know. So although it might be "obvious" (and I think my driver
> can never violate it as it is currently designed), this means the Rust
> abstraction will have to take the device reference if the C side does
> not, because safe rust abstractions have to actually make these bugs
> impossible and nothing stops a Rust driver from, say, stashing an
> io_pgtable reference into a global and letting the device go away.

If someone did that, then simply holding a struct device reference 
wouldn't guarantee much, since it only prevents the pointer itself from 
becoming invalid - it still doesn't say any of the data *in* the 
structure is still valid and "safe" for what a DMA API call might do 
with it.

At the very least you'd probably have to somehow also guarantee that the 
device has a driver bound (which is the closest thing to a general 
indication of valid DMA ops across all architectures) and block it from 
unbinding for the lifetime of the reference, but that would then mean a 
simple driver which expects to tear down its io-pgtable from its .remove 
callback could never be unbound due to the circular dependency :/

Cheers,
Robin.

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