t/io_uring: fix passthrough fixed buffer support

A previous commit changed t/io_uring to register a single region for
all of the registered buffers, and while it updated non-passthrough IO
for that change, the passthrough path still sets a specific buffer
index. This makes passthrough with fixed buffers fail for any buffer
but the first one, as it's asking for a buffer that doesn't exist rather
than index the first one. That causes -EFAULT completions.

Ensure the buf_index is set to 0 for passthrough as well.

Fixes: 21f461f ("t/io_uring: register single buffer for whole IO region")
Signed-off-by: Jens Axboe <[email protected]>

Author: axboe

t/io_uring.c

@@ -628,7 +628,7 @@ static void init_io_pt(struct submitter *s, unsigned index)
 	cmd->data_len = bs;
 	if (fixedbufs) {
 		sqe->uring_cmd_flags = IORING_URING_CMD_FIXED;
-		sqe->buf_index = index;
+		sqe->buf_index = 0;
 	}
 	cmd->nsid = f->nsid;
 	cmd->opcode = 2;