On Fri, Mar 15, 2024 at 06:49:39PM +0100, Andreas Hindborg wrote: > Ming Lei <ming.lei@xxxxxxxxxx> writes: > > > On Fri, Mar 15, 2024 at 01:46:30PM +0100, Andreas Hindborg wrote: > >> Ming Lei <ming.lei@xxxxxxxxxx> writes: > >> > On Fri, Mar 15, 2024 at 08:52:46AM +0100, Andreas Hindborg wrote: > >> >> Miguel Ojeda <miguel.ojeda.sandonis@xxxxxxxxx> writes: > >> >> > >> >> > On Thu, Mar 14, 2024 at 8:23 PM Andreas Hindborg <nmi@xxxxxxxxxxxx> wrote: > >> >> >> > >> >> >> The way the current code compiles, <kernel::block::mq::Request as > >> >> >> kernel::types::AlwaysRefCounted>::dec_ref` is inlined into the `rnull` > >> >> >> module. A relocation for `rust_helper_blk_mq_free_request_internal` > >> >> >> appears in `rnull_mod.ko`. I didn't test it yet, but if > >> >> >> `__blk_mq_free_request` (or the helper) is not exported, I don't think > >> >> >> this would be possible? > >> >> > > >> >> > Yeah, something needs to be exported since there is a generic > >> >> > involved, but even if you want to go the route of exporting only a > >> >> > different symbol, you would still want to put it in the C header so > >> >> > that you don't get the C missing declaration warning and so that we > >> >> > don't have to write the declaration manually in the helper. > >> >> > >> >> That is what I did: > >> >> > >> >> @@ -703,6 +703,7 @@ int blk_mq_alloc_sq_tag_set(struct blk_mq_tag_set *set, > >> >> unsigned int set_flags); > >> >> void blk_mq_free_tag_set(struct blk_mq_tag_set *set); > >> >> > >> >> +void __blk_mq_free_request(struct request *rq); > >> >> void blk_mq_free_request(struct request *rq); > >> >> int blk_rq_poll(struct request *rq, struct io_comp_batch *iob, > >> >> unsigned int poll_flags); > >> > > >> > Can you explain in detail why one block layer internal helper is > >> > called into rnull driver directly? It never happens in C driver code. > >> > >> It is not the rust null block driver that calls this symbol directly. It > >> is called by the Rust block device driver API. But because of inlining, > >> the symbol is referenced from the loadable object. > > > > What is the exact Rust block device driver API? The key point is that how > > the body of one exported kernel C API(EXPORT_SYMBOL) becomes inlined > > with Rust driver. > > This happens when `ARef<Request<_>>` is dropped. The drop method > (destructor) of this smart pointer decrements the refcount and > potentially calls `__blk_mq_free_request`. > > >> > >> The reason we have to call this symbol directly is to ensure proper > >> lifetime of the `struct request`. For example in C, when a driver > > > > Sounds Rust API still calls into __blk_mq_free_request() directly, right? > > Yes, the Rust block device driver API will call this request if an > `ARef<Request<_>>` is dropped and the refcount goes to 0. > > > If that is the case, the usecase need to be justified, and you need > > to write one standalone patch with the exact story for exporting > > __blk_mq_free_request(). > > Ok, I can do that. > > > > >> converts a tag to a request, the developer makes sure to only ask for > >> requests which are outstanding in the driver. In Rust, for the API to be > >> sound, we must ensure that the developer cannot write safe code that > >> obtains a reference to a request that is not owned by the driver. > >> > >> A similar issue exists in the null block driver when timer completions > >> are enabled. If the request is cancelled and the timer fires after the > >> request has been recycled, there is a problem because the timer holds a > >> reference to the request private data area. > >> > >> To that end, I use the `atomic_t ref` field of the C `struct request` > >> and implement the `AlwaysRefCounted` Rust trait for the request type. > >> This is a smart pointer that owns a reference to the pointee. In this > >> way, the request is not freed and recycled until the smart pointer is > >> dropped. But if the smart pointer holds the last reference when it is > >> dropped, it must be able to free the request, and hence it has to call > >> `__blk_mq_free_request`. > > > > For callbacks(queue_rq, timeout, complete) implemented by driver, block > > layer core guaranteed that the passed request reference is live. > > > > So driver needn't to worry about request lifetime, same with Rust > > driver, I think smart pointer isn't necessary for using request in > > Rust driver. > > Using the C API, there is nothing preventing a driver from using the > request after the lifetime ends. Yes, it is true for C, so will Rust-for-linux need to add refcount for most exported kernel C structure? such as by implementing AlwaysRefCounted traits? > With Rust, we have to make it > impossible.Without the refcount and associated call to > `__blk_mq_free_request`, it would be possible to write Rust code that > access the request after the lifetime ends. This is not sound, and it is > something we need to avoid in the Rust abstractions. > > One concrete way to do write unsound code with a Rust API where lifetime > is not tracked with refcount, is if the null block timer completion > callback fires after the request is completed. Perhaps the driver > cancels the request but forgets to cancel the timer. When the timer > fires, it will access the request via the context pointer, but the > request will be invalid. The issue is less serious for blk-mq request, which is pre-allocated, and one freed request just means it can be re-allocated for other IO in same queue, and the pointed memory won't be really freed. Also as I mentioned, inside driver's ->timeout(), the request is guaranteed to be live by block layer core(won't be re-allocated to other IO), the passed-in request is referenced already, please see bt_iter() which is called from blk_mq_timeout_work(). Here, block layer core just borrows request, then passed the reference to ->timeout(), when request is owned by driver actually. I understand Rust block driver still need to implement ->queue_rq(), ->timeout(), ..., just like C driver, but maybe I am wrong? Or Rust block driver will re-implement part of block layer core code? such as, get one extra reference of request no matter block core has done that. > In C we have to write the driver code so this > cannot happen. In Rust, the API must prevent this from happening. So any > driver written in the safe subset of Rust using this API can never > trigger this behavior. > > By using the refcount, we ensure that the request is alive until all > users who hold a reference to it are dropped. block layer has provided such guarantee if Rust driver follows current block driver model. > > Another concrete example is when a driver calls `blk_mq_tag_to_rq` with > an invalid tag. This can return a reference to an invalid tag, if the > driver is not implemented correctly. By using `req_ref_inc_not_zero` we > can assert that the request is live before we create a Rust reference to > it, and even if the driver code has bugs, it can never access an invalid > request, and thus it can be memory safe. > > We move the responsibility of correctness, in relation to memory safety, > from the driver implementation to the API implementation. After queue_rq(req) is called, request ownership is actually transferred to driver like Rust's move, then driver is free to call blk_mq_tag_to_rq(), and finally return request to block core after the request is completed by driver. The biggest question should be how Rust block driver will be designed & implemented? Will rust block driver follow current C driver's model, such as implementing ->queue_rq(), ->timeout(), ->complete()...? thanks, Ming
