mm: implement access_remote_vm

Provide an alternative to access_process_vm that allows the caller to obtain a
reference to the supplied mm_struct.
Signed-off-by: Stephen Wilson <wilsons@start.ca>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

include/linux/mm.h

@@ -971,6 +971,8 @@ static inline int handle_mm_fault(struct mm_struct *mm,
 
 extern int make_pages_present(unsigned long addr, unsigned long end);
 extern int access_process_vm(struct task_struct *tsk, unsigned long addr, void *buf, int len, int write);
+extern int access_remote_vm(struct mm_struct *mm, unsigned long addr,
+		void *buf, int len, int write);
 
 int __get_user_pages(struct task_struct *tsk, struct mm_struct *mm,
 		     unsigned long start, int len, unsigned int foll_flags,

mm/memory.c

@@ -3711,6 +3711,22 @@ static int __access_remote_vm(struct task_struct *tsk, struct mm_struct *mm,
 	return buf - old_buf;
 }
 
+/**
+ * @access_remote_vm - access another process' address space
+ * @mm:		the mm_struct of the target address space
+ * @addr:	start address to access
+ * @buf:	source or destination buffer
+ * @len:	number of bytes to transfer
+ * @write:	whether the access is a write
+ *
+ * The caller must hold a reference on @mm.
+ */
+int access_remote_vm(struct mm_struct *mm, unsigned long addr,
+		void *buf, int len, int write)
+{
+	return __access_remote_vm(NULL, mm, addr, buf, len, write);
+}
+
 /*
  * Access another process' address space.
  * Source/target buffer must be kernel space,