dma-buf: add ref counting for module as exporter

Add reference counting on a kernel module that exports dma-buf and
implements its operations. This prevents the module from being unloaded
while DMABUF file is in use.

The original patch [1] was submitted by Tomasz Stanislawski, but this
is a simpler way to do it.

v3: call module_put() as late as possible, per gregkh's comment.
v2: move owner to struct dma_buf, and use DEFINE_DMA_BUF_EXPORT_INFO
macro to simplify the change.

Acked-by: Greg Kroah-Hartman <[e-mail]>
Signed-off-by: Sumit Semwal <[e-mail]>

[1]: https://lkml.org/lkml/2012/8/8/163

drivers/dma-buf/dma-buf.c

···
 #include <linux/anon_inodes.h>
 #include <linux/export.h>
 #include <linux/debugfs.h>
+#include <linux/module.h>
 #include <linux/seq_file.h>
 #include <linux/poll.h>
 #include <linux/reservation.h>
···
 	if (dmabuf->resv == (struct reservation_object *)&dmabuf[1])
 		reservation_object_fini(dmabuf->resv);
 
+	module_put(dmabuf->owner);
 	kfree(dmabuf);
 	return 0;
 }
···
 		return ERR_PTR(-EINVAL);
 	}
 
+	if (!try_module_get(exp_info->owner))
+		return ERR_PTR(-ENOENT);
+
 	dmabuf = kzalloc(alloc_size, GFP_KERNEL);
-	if (dmabuf == NULL)
+	if (!dmabuf) {
+		module_put(exp_info->owner);
 		return ERR_PTR(-ENOMEM);
+	}
 
 	dmabuf->priv = exp_info->priv;
 	dmabuf->ops = exp_info->ops;
 	dmabuf->size = exp_info->size;
 	dmabuf->exp_name = exp_info->exp_name;
+	dmabuf->owner = exp_info->owner;
 	init_waitqueue_head(&dmabuf->poll);
 	dmabuf->cb_excl.poll = dmabuf->cb_shared.poll = &dmabuf->poll;
 	dmabuf->cb_excl.active = dmabuf->cb_shared.active = 0;

include/linux/dma-buf.h

···
  * @attachments: list of dma_buf_attachment that denotes all devices attached.
  * @ops: dma_buf_ops associated with this buffer object.
  * @exp_name: name of the exporter; useful for debugging.
+ * @owner: pointer to exporter module; used for refcounting when exporter is a
+ *         kernel module.
  * @list_node: node for dma_buf accounting and debugging.
  * @priv: exporter specific private data for this buffer object.
  * @resv: reservation object linked to this dma-buf
···
 	unsigned vmapping_counter;
 	void *vmap_ptr;
 	const char *exp_name;
+	struct module *owner;
 	struct list_head list_node;
 	void *priv;
 	struct reservation_object *resv;
···
 
 /**
  * struct dma_buf_export_info - holds information needed to export a dma_buf
- * @exp_name:	name of the exporting module - useful for debugging.
+ * @exp_name:	name of the exporter - useful for debugging.
+ * @owner:	pointer to exporter module - used for refcounting kernel module
  * @ops:	Attach allocator-defined dma buf ops to the new buffer
  * @size:	Size of the buffer
  * @flags:	mode flags for the file
···
  */
 struct dma_buf_export_info {
 	const char *exp_name;
+	struct module *owner;
 	const struct dma_buf_ops *ops;
 	size_t size;
 	int flags;
···
  * helper macro for exporters; zeros and fills in most common values
  */
 #define DEFINE_DMA_BUF_EXPORT_INFO(a)	\
-	struct dma_buf_export_info a = { .exp_name = KBUILD_MODNAME }
+	struct dma_buf_export_info a = { .exp_name = KBUILD_MODNAME, \
+					 .owner = THIS_MODULE }
 
 /**
  * get_dma_buf - convenience wrapper for get_file.