proc: use mm_access() instead of ptrace_may_access()

mm_access() handles this much better, and avoids some race conditions.
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Cc: Hugh Dickins <hughd@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/proc/base.c

@@ -2022,11 +2022,8 @@ static int map_files_d_revalidate(struct dentry *dentry, struct nameidata *nd)
 	if (!task)
 		goto out_notask;
 
-	if (!ptrace_may_access(task, PTRACE_MODE_READ))
-		goto out;
-
-	mm = get_task_mm(task);
-	if (!mm)
+	mm = mm_access(task, PTRACE_MODE_READ);
+	if (IS_ERR_OR_NULL(mm))
 		goto out;
 
 	if (!dname_to_vma_addr(dentry, &vm_start, &vm_end)) {