From 2a650deecaa31ee0ae2ba7af9175d975a8b8289b Mon Sep 17 00:00:00 2001
From: Guillaume Bury <guillaume.bury@gmail.com>
Date: Tue, 22 Mar 2022 12:03:36 +0100
Subject: [PATCH] flambda-backend: Backport commit
 fc9534746bf5d08a4c109f22e344cf49d5d46d54 from trunk (#584)

---
 runtime/caml/signals.h |  2 +-
 runtime/signals_byt.c  |  2 +-
 runtime/signals_nat.c  | 25 ++++++++++++++-----------
 3 files changed, 16 insertions(+), 13 deletions(-)

diff --git a/runtime/caml/signals.h b/runtime/caml/signals.h
index 3ff152c26933..285dbd7febdd 100644
--- a/runtime/caml/signals.h
+++ b/runtime/caml/signals.h
@@ -87,7 +87,7 @@ value caml_do_pending_actions_exn (void);
 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 void caml_setup_stack_overflow_detection(void);
+CAMLextern int caml_setup_stack_overflow_detection(void);
 
 CAMLextern void (*caml_enter_blocking_section_hook)(void);
 CAMLextern void (*caml_leave_blocking_section_hook)(void);
diff --git a/runtime/signals_byt.c b/runtime/signals_byt.c
index 2183142da180..38eb5e3a47ac 100644
--- a/runtime/signals_byt.c
+++ b/runtime/signals_byt.c
@@ -81,4 +81,4 @@ int caml_set_signal_action(int signo, int action)
     return 0;
 }
 
-CAMLexport void caml_setup_stack_overflow_detection(void) {}
+CAMLexport int caml_setup_stack_overflow_detection(void) { return 0; }
diff --git a/runtime/signals_nat.c b/runtime/signals_nat.c
index 1be1b45d4209..ca86956783ca 100644
--- a/runtime/signals_nat.c
+++ b/runtime/signals_nat.c
@@ -174,8 +174,6 @@ 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
@@ -269,28 +267,33 @@ 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
 }
 
-CAMLexport 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). */
+
+CAMLexport 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
 }