Dynamically allocate the alternate signal stack (ocaml#10266)

In Glibc 2.34 and later, SIGSTKSZ may not be a compile-time constant.
It is no longer possible to statically allocate the alternate signal
stack for the main thread, as we've been doing for the last 25 years.

This commit implements dynamic allocation of the alternate signal stack
even for the main thread.  It reuses the code already in place to allocate
the alternate signal stack for other threads.

Fixes: ocaml#10250.

runtime/caml/signals.h

@@ -82,7 +82,7 @@ void caml_set_action_pending (void);
 value caml_do_pending_actions_exn (void);
 value caml_process_pending_actions_with_root (value extra_root); // raises
 int caml_set_signal_action(int signo, int action);
-void caml_setup_stack_overflow_detection(void);
+int caml_setup_stack_overflow_detection(void);
 
 CAMLextern void (*caml_enter_blocking_section_hook)(void);
 CAMLextern void (*caml_leave_blocking_section_hook)(void);

runtime/signals_byt.c

@@ -86,4 +86,4 @@ int caml_set_signal_action(int signo, int action)
     return 0;
 }
 
-void caml_setup_stack_overflow_detection(void) {}
+int caml_setup_stack_overflow_detection(void) {}

runtime/signals_nat.c

@@ -195,8 +195,6 @@ DECLARE_SIGNAL_HANDLER(trap_handler)
 #error "CONTEXT_SP is required if HAS_STACK_OVERFLOW_DETECTION is defined"
 #endif
 
-static char sig_alt_stack[SIGSTKSZ];
-
 /* Code compiled with ocamlopt never accesses more than
    EXTRA_STACK bytes below the stack pointer. */
 #define EXTRA_STACK 256
@@ -282,28 +280,33 @@ void caml_init_signals(void)
 #endif
 
 #ifdef HAS_STACK_OVERFLOW_DETECTION
-  {
-    stack_t stk;
+  if (caml_setup_stack_overflow_detection() != -1) {
     struct sigaction act;
-    stk.ss_sp = sig_alt_stack;
-    stk.ss_size = SIGSTKSZ;
-    stk.ss_flags = 0;
     SET_SIGACT(act, segv_handler);
     act.sa_flags |= SA_ONSTACK | SA_NODEFER;
     sigemptyset(&act.sa_mask);
-    if (sigaltstack(&stk, NULL) == 0) { sigaction(SIGSEGV, &act, NULL); }
+    sigaction(SIGSEGV, &act, NULL);
   }
 #endif
 }
 
-void caml_setup_stack_overflow_detection(void)
+/* Allocate and select an alternate stack for handling signals,
+   especially SIGSEGV signals.
+   Each thread needs its own alternate stack.
+   The alternate stack used to be statically-allocated for the main thread,
+   but this is incompatible with Glibc 2.34 and newer, where SIGSTKSZ
+   may not be a compile-time constant (issue #10250). */
+
+int caml_setup_stack_overflow_detection(void)
 {
 #ifdef HAS_STACK_OVERFLOW_DETECTION
   stack_t stk;
   stk.ss_sp = malloc(SIGSTKSZ);
+  if (stk.ss_sp == NULL) return -1;
   stk.ss_size = SIGSTKSZ;
   stk.ss_flags = 0;
-  if (stk.ss_sp)
-    sigaltstack(&stk, NULL);
+  return sigaltstack(&stk, NULL);
+#else
+  return 0;
 #endif
 }