linux-kernel - Re: [PATCH] fuse: fix possible deadlock if rings are never initialized

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Message-ID: <e3ccbd42-4178-4bc4-ba13-477d1f121868@bsbernd.com>
Date: Thu, 6 Mar 2025 14:48:47 +0100
From: Bernd Schubert <bernd@...ernd.com>
To: Luis Henriques <luis@...lia.com>
Cc: Miklos Szeredi <miklos@...redi.hu>, linux-fsdevel@...r.kernel.org,
 linux-kernel@...r.kernel.org, kernel-dev@...lia.com
Subject: Re: [PATCH] fuse: fix possible deadlock if rings are never
 initialized



On 3/6/25 14:16, Luis Henriques wrote:
> On Thu, Mar 06 2025, Bernd Schubert wrote:
> 
>> On 3/6/25 12:12, Luis Henriques wrote:
>>> When mounting a user-space filesystem using io_uring, the initialization
>>> of the rings is done separately in the server side.  If for some reason
>>> (e.g. a server bug) this step is not performed it will be impossible to
>>> unmount the filesystem if there are already requests waiting.
>>>
>>> This issue is easily reproduced with the libfuse passthrough_ll example,
>>> if the queue depth is set to '0' and a request is queued before trying to
>>> unmount the filesystem.  When trying to force the unmount, fuse_abort_conn()
>>> will try to wake up all tasks waiting in fc->blocked_waitq, but because the
>>> rings were never initialized, fuse_uring_ready() will never return 'true'.
>>>
>>> Fixes: 3393ff964e0f ("fuse: block request allocation until io-uring init is complete")
>>> Signed-off-by: Luis Henriques <luis@...lia.com>
>>> ---
>>>   fs/fuse/dev.c | 2 +-
>>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>>
>>> diff --git a/fs/fuse/dev.c b/fs/fuse/dev.c
>>> index 7edceecedfa5..2fe565e9b403 100644
>>> --- a/fs/fuse/dev.c
>>> +++ b/fs/fuse/dev.c
>>> @@ -77,7 +77,7 @@ void fuse_set_initialized(struct fuse_conn *fc)
>>>   static bool fuse_block_alloc(struct fuse_conn *fc, bool for_background)
>>>   {
>>>   	return !fc->initialized || (for_background && fc->blocked) ||
>>> -	       (fc->io_uring && !fuse_uring_ready(fc));
>>> +	       (fc->io_uring && fc->connected && !fuse_uring_ready(fc));
>>>   }
>>>   
>>>   static void fuse_drop_waiting(struct fuse_conn *fc)
>>>
>>
>> Oh yes, I had missed that.
>>
>> Reviewed-by: Bernd Schubert <bschubert@....com>
> 
> Thanks!  And... by the way, Bernd:
> 
> I know io_uring support in libfuse isn't ready yet, but I think there's
> some error handling missing in your uring branch.  In particular, the
> return of fuse_uring_start() is never checked, and thus if the rings
> initialization fails, the server will not get any error.
> 
> I found that out because I blindly tried the patch below, and I was
> surprised that the server was started just fine.

Thank you! I will work a bit on splitting the uring branch into 
merge-able patches later today, but probably won't finish today (too 
many other things to do).


Thanks,
Bernd