diff --git a/compiler-rt/lib/safestack/safestack.cc b/compiler-rt/lib/safestack/safestack.cc
--- a/compiler-rt/lib/safestack/safestack.cc
+++ b/compiler-rt/lib/safestack/safestack.cc
@@ -228,6 +228,8 @@
   size = RoundUpTo(size, kStackAlign);
 
   void *addr = unsafe_stack_alloc(size, guard);
+  // Put tinfo at the end of the buffer. guard may be not page aligned.
+  // If that is so then some bytest after addr can be mprotected.
   struct tinfo *tinfo =
       (struct tinfo *)(((char *)addr) + size - sizeof(struct tinfo));
   tinfo->start_routine = start_routine;