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

[Luis Henriques <luis@igalia.com>]

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@igalia.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@ddn.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.

Cheers,
-- 
Luís

diff --git a/lib/fuse_uring.c b/lib/fuse_uring.c
index 312aa5dbc735..2258cf0d4259 100644
--- a/lib/fuse_uring.c
+++ b/lib/fuse_uring.c
@@ -498,6 +498,11 @@ static struct fuse_ring_pool *fuse_create_ring(struct fuse_session *se)
 		fuse_log(FUSE_LOG_DEBUG, "starting io-uring q-depth=%d\n",
 			 se->uring.q_depth);
 
+	if (!se->uring.q_depth) {
+		fuse_log(FUSE_LOG_ERR, "Invalid ring queue depth value\n");
+		goto err;
+	}
+
 	fuse_ring = calloc(1, sizeof(*fuse_ring));
 	if (fuse_ring == NULL) {
 		fuse_log(FUSE_LOG_ERR, "Allocating the ring failed\n");