NOMMU: implement access_remote_vm

Recent vm changes brought in a new function which the core procfs code
utilizes. So implement it for nommu systems too to avoid link failures.

Signed-off-by: Mike Frysinger <[e-mail]>
Signed-off-by: David Howells <[e-mail]>
Tested-by: Simon Horman <[e-mail]>
Tested-by: Ithamar Adema <[e-mail]>
Acked-by: Greg Ungerer <[e-mail]>

mm/nommu.c

···
 }
 EXPORT_SYMBOL(filemap_fault);
 
-/*
- * Access another process' address space.
- * - source/target buffer must be kernel space
- */
-int access_process_vm(struct task_struct *tsk, unsigned long addr, void *buf, int len, int write)
+static int __access_remote_vm(struct task_struct *tsk, struct mm_struct *mm,
+		unsigned long addr, void *buf, int len, int write)
 {
 	struct vm_area_struct *vma;
-	struct mm_struct *mm;
-
-	if (addr + len < addr)
-		return 0;
-
-	mm = get_task_mm(tsk);
-	if (!mm)
-		return 0;
 
 	down_read(&mm->mmap_sem);
 
···
 	}
 
 	up_read(&mm->mmap_sem);
+
+	return len;
+}
+
+/**
+ * @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
+ */
+int access_process_vm(struct task_struct *tsk, unsigned long addr, void *buf, int len, int write)
+{
+	struct mm_struct *mm;
+
+	if (addr + len < addr)
+		return 0;
+
+	mm = get_task_mm(tsk);
+	if (!mm)
+		return 0;
+
+	len = __access_remote_vm(tsk, mm, addr, buf, len, write);
+
 	mmput(mm);
 	return len;
 }