From: Jens Axboe <axboe@kernel.dk>
Date: Wed, 13 Nov 2019 16:12:46 -0700
Subject: io_uring: ensure registered buffer import returns the IO length
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Git-commit: 5e559561a8d7e6d4adfce6aa8fbf3daa3dec1577
Patch-mainline: 5.4-rc8
References: bnc#1151927 5.3.12

A test case was reported where two linked reads with registered buffers
failed the second link always. This is because we set the expected value
of a request in req->result, and if we don't get this result, then we
fail the dependent links. For some reason the registered buffer import
returned -ERROR/0, while the normal import returns -ERROR/length. This
broke linked commands with registered buffers.

Fix this by making io_import_fixed() correctly return the mapped length.

Cc: stable@vger.kernel.org # v5.3
Reported-by: 李通洲 <carter.li@eoitek.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Jiri Slaby <jslaby@suse.cz>
---
 fs/io_uring.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -1179,7 +1179,7 @@ static int io_import_fixed(struct io_rin
 		}
 	}
 
-	return 0;
+	return len;
 }
 
 static ssize_t io_import_iovec(struct io_ring_ctx *ctx, int rw,