staging: comedi: support vm_access_process for mmap'd buffer

If a process that has mmap'd a COMEDI buffer is being run under a
debugger such as GDB, the buffer contents are inaccessible from the
debugger.  Support the `access()` VM operation to allow the buffer
contents to be accessed by another process.
Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/comedi/comedi_buf.c

@@ -161,6 +161,30 @@ int comedi_buf_map_put(struct comedi_buf_map *bm)
 	return 1;
 }
 
+/* helper for "access" vm operation */
+int comedi_buf_map_access(struct comedi_buf_map *bm, unsigned long offset,
+			  void *buf, int len, int write)
+{
+	unsigned int pgoff = offset & ~PAGE_MASK;
+	unsigned long pg = offset >> PAGE_SHIFT;
+	int done = 0;
+
+	while (done < len && pg < bm->n_pages) {
+		int l = min_t(int, len - done, PAGE_SIZE - pgoff);
+		void *b = bm->page_list[pg].virt_addr + pgoff;
+
+		if (write)
+			memcpy(b, buf, l);
+		else
+			memcpy(buf, b, l);
+		buf += l;
+		done += l;
+		pg++;
+		pgoff = 0;
+	}
+	return done;
+}
+
 /* returns s->async->buf_map and increments its kref refcount */
 struct comedi_buf_map *
 comedi_buf_map_from_subdev_get(struct comedi_subdevice *s)

drivers/staging/comedi/comedi_fops.c

@@ -2165,9 +2165,24 @@ static void comedi_vm_close(struct vm_area_struct *area)
 	comedi_buf_map_put(bm);
 }
 
+static int comedi_vm_access(struct vm_area_struct *vma, unsigned long addr,
+			    void *buf, int len, int write)
+{
+	struct comedi_buf_map *bm = vma->vm_private_data;
+	unsigned long offset =
+	    addr - vma->vm_start + (vma->vm_pgoff << PAGE_SHIFT);
+
+	if (len < 0)
+		return -EINVAL;
+	if (len > vma->vm_end - addr)
+		len = vma->vm_end - addr;
+	return comedi_buf_map_access(bm, offset, buf, len, write);
+}
+
 static const struct vm_operations_struct comedi_vm_ops = {
 	.open = comedi_vm_open,
 	.close = comedi_vm_close,
+	.access = comedi_vm_access,
 };
 
 static int comedi_mmap(struct file *file, struct vm_area_struct *vma)

drivers/staging/comedi/comedi_internal.h

@@ -29,6 +29,8 @@ void comedi_buf_reset(struct comedi_subdevice *s);
 bool comedi_buf_is_mmapped(struct comedi_subdevice *s);
 void comedi_buf_map_get(struct comedi_buf_map *bm);
 int comedi_buf_map_put(struct comedi_buf_map *bm);
+int comedi_buf_map_access(struct comedi_buf_map *bm, unsigned long offset,
+			  void *buf, int len, int write);
 struct comedi_buf_map *comedi_buf_map_from_subdev_get(
 		struct comedi_subdevice *s);
 unsigned int comedi_buf_write_n_available(struct comedi_subdevice *s);