io_uring: prepare fixed rw for dynanic buffers

With dynamic buffer updates, registered buffers in the table may change
at any moment. First of all we want to prevent future races between
updating and importing (i.e. io_import_fixed()), where the latter one
may happen without uring_lock held, e.g. from io-wq.

Save the first loaded io_mapped_ubuf buffer and reuse.
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/21a2302d07766ae956640b6f753292c45200fe8f.1619356238.git.asml.silence@gmail.comSigned-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -839,6 +839,8 @@ struct io_kiocb {
 	struct hlist_node		hash_node;
 	struct async_poll		*apoll;
 	struct io_wq_work		work;
+	/* store used ubuf, so we can prevent reloading */
+	struct io_mapped_ubuf		*imu;
 };
 
 struct io_tctx_node {
@@ -2683,6 +2685,12 @@ static int io_prep_rw(struct io_kiocb *req, const struct io_uring_sqe *sqe)
 		kiocb->ki_complete = io_complete_rw;
 	}
 
+	if (req->opcode == IORING_OP_READ_FIXED ||
+	    req->opcode == IORING_OP_WRITE_FIXED) {
+		req->imu = NULL;
+		io_req_set_rsrc_node(req);
+	}
+
 	req->rw.addr = READ_ONCE(sqe->addr);
 	req->rw.len = READ_ONCE(sqe->len);
 	req->buf_index = READ_ONCE(sqe->buf_index);
@@ -2748,21 +2756,13 @@ static void kiocb_done(struct kiocb *kiocb, ssize_t ret,
 	}
 }
 
-static int io_import_fixed(struct io_kiocb *req, int rw, struct iov_iter *iter)
+static int __io_import_fixed(struct io_kiocb *req, int rw, struct iov_iter *iter,
+			     struct io_mapped_ubuf *imu)
 {
-	struct io_ring_ctx *ctx = req->ctx;
 	size_t len = req->rw.len;
-	struct io_mapped_ubuf *imu;
-	u16 index, buf_index = req->buf_index;
 	u64 buf_end, buf_addr = req->rw.addr;
 	size_t offset;
 
-	if (unlikely(buf_index >= ctx->nr_user_bufs))
-		return -EFAULT;
-	index = array_index_nospec(buf_index, ctx->nr_user_bufs);
-	imu = ctx->user_bufs[index];
-	buf_addr = req->rw.addr;
-
 	if (unlikely(check_add_overflow(buf_addr, (u64)len, &buf_end)))
 		return -EFAULT;
 	/* not inside the mapped region */
@@ -2814,6 +2814,22 @@ static int io_import_fixed(struct io_kiocb *req, int rw, struct iov_iter *iter)
 	return 0;
 }
 
+static int io_import_fixed(struct io_kiocb *req, int rw, struct iov_iter *iter)
+{
+	struct io_ring_ctx *ctx = req->ctx;
+	struct io_mapped_ubuf *imu = req->imu;
+	u16 index, buf_index = req->buf_index;
+
+	if (likely(!imu)) {
+		if (unlikely(buf_index >= ctx->nr_user_bufs))
+			return -EFAULT;
+		index = array_index_nospec(buf_index, ctx->nr_user_bufs);
+		imu = READ_ONCE(ctx->user_bufs[index]);
+		req->imu = imu;
+	}
+	return __io_import_fixed(req, rw, iter, imu);
+}
+
 static void io_ring_submit_unlock(struct io_ring_ctx *ctx, bool needs_lock)
 {
 	if (needs_lock)
@@ -9506,6 +9522,9 @@ static int io_uring_create(unsigned entries, struct io_uring_params *p,
 	ret = io_sq_offload_create(ctx, p);
 	if (ret)
 		goto err;
+	/* always set a rsrc node */
+	io_rsrc_node_switch_start(ctx);
+	io_rsrc_node_switch(ctx, NULL);
 
 	memset(&p->sq_off, 0, sizeof(p->sq_off));
 	p->sq_off.head = offsetof(struct io_rings, sq.head);