Don't assume mmap returns a page-aligned block.

ocaml/runtime/backtrace_nat.c

@@ -141,7 +141,7 @@ void caml_stash_backtrace_wrapper(value exn, char* rsp, char* trapsp) {
    * exception from a segfault... */
   struct stack_info *block = Caml_state->current_stack;
   int page_size = getpagesize();
-  char* protected_low = (char *) block + page_size;
+  char* protected_low = Protected_stack_page(block, page_size);
   char* protected_high = protected_low + page_size;
   if ((rsp >= protected_low) && (rsp < protected_high)) {
     return;

ocaml/runtime/caml/fiber.h

@@ -292,6 +292,15 @@ CAMLnoreturn_end;
 
 value caml_make_unhandled_effect_exn (value effect);
 
+#if defined(NATIVE_CODE) && !defined(STACK_CHECKS_ENABLED)
+// mmap does not seem to guarantee it will always return a page-aligned
+// address as per some man pages from a few years ago (more recent man
+// pages seem to indicate the alignment is guaranteed), hence the last
+// part of the expression below to add an offset guaranteeing alignment
+#define Protected_stack_page(block, page_size) \
+  (((char*) (block)) + (page_size) + (page_size) - ((uintnat) ((char*) (block)) % (page_size)))
+#endif
+
 #endif /* CAML_INTERNALS */
 
 #endif /* CAML_FIBER_H */

ocaml/runtime/fiber.c

@@ -166,7 +166,7 @@ Caml_inline struct stack_info* alloc_for_stack (mlsize_t wosize)
     munmap(block, len);
     return NULL;
   }
-  if (mprotect((char *) block + page_size, page_size, PROT_NONE)) {
+  if (mprotect(Protected_stack_page(block, page_size), page_size, PROT_NONE)) {
     munmap(block, len);
     return NULL;
   }
@@ -884,7 +884,7 @@ void caml_free_stack (struct stack_info* stack)
 #ifndef USE_MMAP_MAP_STACK
 #if defined(NATIVE_CODE) && !defined(STACK_CHECKS_ENABLED)
   int page_size = getpagesize();
-  mprotect((void *) ((char *) stack + page_size),
+  mprotect((void *) Protected_stack_page(stack, page_size),
            page_size,
            PROT_READ | PROT_WRITE);
 #endif

ocaml/runtime/signals_nat.c

@@ -111,7 +111,7 @@ DECLARE_SIGNAL_HANDLER(segv_handler)
   struct stack_info *block = Caml_state->current_stack;
   char* fault_addr = info->si_addr;
   int page_size = getpagesize();
-  char* protected_low = (char *) block + page_size;
+  char* protected_low = Protected_stack_page(block, page_size);
   char* protected_high = protected_low + page_size;
   if ((fault_addr >= protected_low) && (fault_addr < protected_high)) {
     context->uc_mcontext.gregs[REG_RIP]= (greg_t) &caml_raise_stack_overflow_nat;