Free the alternate signal stack when thread / process stops

Commit fc95347 introduced dynamic allocation of the alternate stack
used to process signals.

This commit makes sure the dynamically-allocated stack is freed when
the main OCaml code terminates or an OCaml thread terminates.

In passing, we also reset relevant signal handlers to their default behavior
when the main OCaml code terminates.

Fixes: ocaml#10698

otherlibs/systhreads/st_stubs.c

@@ -550,6 +550,7 @@ static ST_THREAD_FUNCTION caml_thread_start(void * arg)
 #ifdef NATIVE_CODE
   }
 #endif
+  caml_stop_stack_overflow_detection();
   /* The thread now stops running */
   return 0;
 }

runtime/caml/signals.h

@@ -88,6 +88,7 @@ value caml_process_pending_actions_with_root (value extra_root); // raises
 value caml_process_pending_actions_with_root_exn (value extra_root);
 int caml_set_signal_action(int signo, int action);
 CAMLextern int caml_setup_stack_overflow_detection(void);
+CAMLextern int caml_stop_stack_overflow_detection(void);
 
 CAMLextern void (*caml_enter_blocking_section_hook)(void);
 CAMLextern void (*caml_leave_blocking_section_hook)(void);

runtime/signals_byt.c

@@ -82,3 +82,4 @@ int caml_set_signal_action(int signo, int action)
 }
 
 CAMLexport int caml_setup_stack_overflow_detection(void) { return 0; }
+CAMLexport int caml_stop_stack_overflow_detection(void) { return 0; }

runtime/signals_nat.c

@@ -288,6 +288,36 @@ void caml_init_signals(void)
 #endif
 }
 
+/* Termination of signal stuff */
+
+#if defined(TARGET_power) || defined(TARGET_s390x) \
+    || defined(HAS_STACK_OVERFLOW_DETECTION)
+static void set_signal_default(int signum)
+{
+  struct sigaction act;
+  sigemptyset(&act.sa_mask);
+  act.sa_handler = SIG_DFL;
+  act.sa_flags = 0;
+  sigaction(signum, &act, NULL);
+}
+#endif
+
+void caml_terminate_signals(void)
+{
+#if defined(TARGET_power)
+  set_signal_default(SIGTRAP);
+#endif
+
+#if defined(TARGET_s390x)
+  set_signal_default(SIGFPE);
+#endif
+
+#ifdef HAS_STACK_OVERFLOW_DETECTION
+  set_signal_default(SIGSEGV);
+  caml_stop_stack_overflow_detection();
+#endif
+}
+
 /* Allocate and select an alternate stack for handling signals,
    especially SIGSEGV signals.
    Each thread needs its own alternate stack.
@@ -308,3 +338,19 @@ CAMLexport int caml_setup_stack_overflow_detection(void)
   return 0;
 #endif
 }
+
+CAMLexport int caml_stop_stack_overflow_detection(void)
+{
+#ifdef HAS_STACK_OVERFLOW_DETECTION
+  stack_t oldstk, stk;
+  stk.ss_flags = SS_DISABLE;
+  if (sigaltstack(&stk, &oldstk) == -1) return -1;
+  /* If caml_setup_stack_overflow_detection failed, we are not using
+     an alternate signal stack.  SS_DISABLE will be set in oldstk,
+     and there is nothing to free in this case. */
+  if (! (oldstk.ss_flags & SS_DISABLE)) free(oldstk.ss_sp);
+  return 0;
+#else
+  return 0;
+#endif
+}

runtime/startup_nat.c

@@ -92,6 +92,7 @@ void (*caml_termination_hook)(void *) = NULL;
 
 extern value caml_start_program (caml_domain_state*);
 extern void caml_init_signals (void);
+extern void caml_terminate_signals (void);
 #ifdef _WIN32
 extern void caml_win32_overflow_detection (void);
 #endif
@@ -106,6 +107,7 @@ extern void caml_install_invalid_parameter_handler();
 value caml_startup_common(char_os **argv, int pooling)
 {
   char_os * exe_name, * proc_self_exe;
+  value res;
   char tos;
 
   /* Initialize the domain */
@@ -155,7 +157,9 @@ value caml_startup_common(char_os **argv, int pooling)
     if (caml_termination_hook != NULL) caml_termination_hook(NULL);
     return Val_unit;
   }
-  return caml_start_program(Caml_state);
+  res = caml_start_program(Caml_state);
+  caml_terminate_signals();
+  return res;
 }
 
 value caml_startup_exn(char_os **argv)

tools/ci/inria/sanitizers/lsan-suppr.txt

@@ -1,4 +1,2 @@
 # ocamlyacc doesn't clean memory on exit
 leak:ocamlyacc
-# Alternate signal stacks are currently never freed (see #10266)
-leak:caml_setup_stack_overflow_detection