From 8de23c018fe6ec5b1f5f492a2532051bc15fce49 Mon Sep 17 00:00:00 2001
From: Mark Shannon <mark@hotpy.org>
Date: Wed, 24 Sep 2025 12:35:54 +0100
Subject: [PATCH] Fix C stack limits by factoring out finding hardware stack
 limits

---
 Python/ceval.c | 60 +++++++++++++++++++++++++++-----------------------
 1 file changed, 32 insertions(+), 28 deletions(-)

diff --git a/Python/ceval.c b/Python/ceval.c
index 0ccaacaf3ed5b1..1b52128c858ecb 100644
--- a/Python/ceval.c
+++ b/Python/ceval.c
@@ -438,31 +438,26 @@ int pthread_attr_destroy(pthread_attr_t *a)
 
 #endif
 
-
-void
-_Py_InitializeRecursionLimits(PyThreadState *tstate)
+static void
+hardware_stack_limits(uintptr_t *top, uintptr_t *base)
 {
-    _PyThreadStateImpl *_tstate = (_PyThreadStateImpl *)tstate;
 #ifdef WIN32
     ULONG_PTR low, high;
     GetCurrentThreadStackLimits(&low, &high);
-    _tstate->c_stack_top = (uintptr_t)high;
+    *top = (uintptr_t)high;
     ULONG guarantee = 0;
     SetThreadStackGuarantee(&guarantee);
-    _tstate->c_stack_hard_limit = ((uintptr_t)low) + guarantee + _PyOS_STACK_MARGIN_BYTES;
-    _tstate->c_stack_soft_limit = _tstate->c_stack_hard_limit + _PyOS_STACK_MARGIN_BYTES;
+    *base = (uintptr_t)low + guarantee;
 #elif defined(__APPLE__)
     pthread_t this_thread = pthread_self();
     void *stack_addr = pthread_get_stackaddr_np(this_thread); // top of the stack
     size_t stack_size = pthread_get_stacksize_np(this_thread);
-    _tstate->c_stack_top = (uintptr_t)stack_addr;
-    _tstate->c_stack_hard_limit = _tstate->c_stack_top - stack_size;
-    _tstate->c_stack_soft_limit = _tstate->c_stack_hard_limit + _PyOS_STACK_MARGIN_BYTES;
+    *top = (uintptr_t)stack_addr;
+    *base = ((uintptr_t)stack_addr) - stack_size;
 #else
-    uintptr_t here_addr = _Py_get_machine_stack_pointer();
-/// XXX musl supports HAVE_PTHRED_GETATTR_NP, but the resulting stack size
-/// (on alpine at least) is much smaller than expected and imposes undue limits
-/// compared to the old stack size estimation.  (We assume musl is not glibc.)
+    /// XXX musl supports HAVE_PTHRED_GETATTR_NP, but the resulting stack size
+    /// (on alpine at least) is much smaller than expected and imposes undue limits
+    /// compared to the old stack size estimation.  (We assume musl is not glibc.)
 #  if defined(HAVE_PTHREAD_GETATTR_NP) && !defined(_AIX) && \
         !defined(__NetBSD__) && (defined(__GLIBC__) || !defined(__linux__))
     size_t stack_size, guard_size;
@@ -475,26 +470,35 @@ _Py_InitializeRecursionLimits(PyThreadState *tstate)
         err |= pthread_attr_destroy(&attr);
     }
     if (err == 0) {
-        uintptr_t base = ((uintptr_t)stack_addr) + guard_size;
-        _tstate->c_stack_top = base + stack_size;
-#ifdef _Py_THREAD_SANITIZER
-        // Thread sanitizer crashes if we use a bit more than half the stack.
-        _tstate->c_stack_soft_limit = base + (stack_size / 2);
-#else
-        _tstate->c_stack_soft_limit = base + _PyOS_STACK_MARGIN_BYTES * 2;
-#endif
-        _tstate->c_stack_hard_limit = base + _PyOS_STACK_MARGIN_BYTES;
-        assert(_tstate->c_stack_soft_limit < here_addr);
-        assert(here_addr < _tstate->c_stack_top);
+        *base = ((uintptr_t)stack_addr) + guard_size;
+        *top = (uintptr_t)stack_addr + stack_size;
         return;
     }
 #  endif
-    _tstate->c_stack_top = _Py_SIZE_ROUND_UP(here_addr, 4096);
-    _tstate->c_stack_soft_limit = _tstate->c_stack_top - Py_C_STACK_SIZE;
-    _tstate->c_stack_hard_limit = _tstate->c_stack_top - (Py_C_STACK_SIZE + _PyOS_STACK_MARGIN_BYTES);
+    uintptr_t here_addr = _Py_get_machine_stack_pointer();
+    uintptr_t top_addr = _Py_SIZE_ROUND_UP(here_addr, 4096);
+    *top = top_addr;
+    *base = top_addr - Py_C_STACK_SIZE;
 #endif
 }
 
+void
+_Py_InitializeRecursionLimits(PyThreadState *tstate)
+{
+    uintptr_t top;
+    uintptr_t base;
+    hardware_stack_limits(&top, &base);
+#ifdef _Py_THREAD_SANITIZER
+    // Thread sanitizer crashes if we use more than half the stack.
+    uintptr_t stacksize = top - base;
+    base += stacksize/2;
+#endif
+    _PyThreadStateImpl *_tstate = (_PyThreadStateImpl *)tstate;
+    _tstate->c_stack_top = top;
+    _tstate->c_stack_hard_limit = base + _PyOS_STACK_MARGIN_BYTES;
+    _tstate->c_stack_soft_limit = base + _PyOS_STACK_MARGIN_BYTES * 2;
+}
+
 /* The function _Py_EnterRecursiveCallTstate() only calls _Py_CheckRecursiveCall()
    if the recursion_depth reaches recursion_limit. */
 int