Pass down "stack start" variables from closer to the top of the stack

The implementation of `native_thread_init_stack` for the various
threading models can use the address of a local variable as part of the
calculation of the machine stack extents:

* pthreads uses it as a lower-bound on the start of the stack, because
  glibc (and maybe other libcs) can store its own data on the stack
  before calling into user code on thread creation.
* win32 uses it as an argument to VirtualQuery, which gets the extent of
  the memory mapping which contains the variable

However, the local being used for this is actually allocated _inside_
the `native_thread_init_stack` frame; that means the caller might
allocate a VALUE on the stack that actually lies outside the bounds
stored in machine.stack_{start,end}.

A local variable from one level above the topmost frame that stores
VALUEs on the stack must be drilled down into the call to
`native_thread_init_stack` to be used in the calculation. This probably
doesn't _really_ matter for the win32 case (they'll be in the same
memory mapping so VirtualQuery should return the same thing), but
definitely could matter for the pthreads case.

[Bug #20001]

eval.c

@@ -70,7 +70,7 @@ ruby_setup(void)
     if (GET_VM())
         return 0;
 
-    ruby_init_stack((void *)&state);
+    ruby_init_stack(&state);
 
     /*
      * Disable THP early before mallocs happen because we want this to
@@ -79,7 +79,7 @@ ruby_setup(void)
 #if defined(__linux__) && defined(PR_SET_THP_DISABLE)
     prctl(PR_SET_THP_DISABLE, 1, 0, 0, 0);
 #endif
-    Init_BareVM();
+    Init_BareVM(&state);
     Init_heap();
     rb_vm_encoded_insn_data_table_init();
     Init_vm_objects();

include/ruby/internal/interpreter.h

@@ -141,7 +141,7 @@ void ruby_show_copyright(void);
  *
  * @param[in]  addr  A pointer somewhere on the stack, near its bottom.
  */
-void ruby_init_stack(volatile VALUE *addr);
+void ruby_init_stack(volatile void *addr);
 
 /**
  * Initializes the VM and builtin libraries.

internal/inits.h

@@ -29,7 +29,7 @@ int Init_enc_set_filesystem_encoding(void);
 void Init_newline(void);
 
 /* vm.c */
-void Init_BareVM(void);
+void Init_BareVM(void *local_in_parent_frame);
 void Init_vm_objects(void);
 
 /* vm_backtrace.c */

thread.c

@@ -522,9 +522,9 @@ static VALUE rb_threadptr_raise(rb_thread_t *, int, VALUE *);
 static VALUE rb_thread_to_s(VALUE thread);
 
 void
-ruby_thread_init_stack(rb_thread_t *th)
+ruby_thread_init_stack(rb_thread_t *th, void *local_in_parent_frame)
 {
-    native_thread_init_stack(th);
+    native_thread_init_stack(th, local_in_parent_frame);
 }
 
 const VALUE *

thread_none.c

@@ -140,12 +140,12 @@ ruby_mn_threads_params(void)
 }
 
 void
-ruby_init_stack(volatile VALUE *addr)
+ruby_init_stack(volatile void *addr)
 {
 }
 
 static int
-native_thread_init_stack(rb_thread_t *th)
+native_thread_init_stack(rb_thread_t *th, void *local_in_parent_frame)
 {
 #if defined(__wasm__) && !defined(__EMSCRIPTEN__)
     th->ec->machine.stack_start = (VALUE *)rb_wasm_stack_get_base();

thread_pthread.c

@@ -1964,7 +1964,7 @@ reserve_stack(volatile char *limit, size_t size)
 
 #undef ruby_init_stack
 void
-ruby_init_stack(volatile VALUE *addr)
+ruby_init_stack(volatile void *addr)
 {
     native_main_thread.id = pthread_self();
 
@@ -2049,7 +2049,7 @@ ruby_init_stack(volatile VALUE *addr)
     {int err = (expr); if (err) {rb_bug_errno(#expr, err);}}
 
 static int
-native_thread_init_stack(rb_thread_t *th)
+native_thread_init_stack(rb_thread_t *th, void *local_in_parent_frame)
 {
     rb_nativethread_id_t curr = pthread_self();
 
@@ -2064,8 +2064,8 @@ native_thread_init_stack(rb_thread_t *th)
             size_t size;
 
             if (get_stack(&start, &size) == 0) {
-                uintptr_t diff = (uintptr_t)start - (uintptr_t)&curr;
-                th->ec->machine.stack_start = (VALUE *)&curr;
+                uintptr_t diff = (uintptr_t)start - (uintptr_t)local_in_parent_frame;
+                th->ec->machine.stack_start = (uintptr_t)local_in_parent_frame;
                 th->ec->machine.stack_maxsize = size - diff;
             }
         }
@@ -2185,8 +2185,19 @@ native_thread_create_dedicated(rb_thread_t *th)
 static void
 call_thread_start_func_2(rb_thread_t *th)
 {
-    native_thread_init_stack(th);
+    /* Capture the address of a local in this stack frame to mark the beginning of the
+       machine stack for this thread. This is required even if we can tell the real
+       stack beginning from the pthread API in native_thread_init_stack, because
+       glibc stores some of its own data on the stack before calling into user code
+       on a new thread, and replacing that data on fiber-switch would break it (see
+       bug #13887) */
+    VALUE stack_start = 0;
+    VALUE *stack_start_addr = &stack_start;
+    native_thread_init_stack(th, stack_start_addr);
     thread_start_func_2(th, th->ec->machine.stack_start);
+
+    /* Ensure that stack_start really was spilled to the stack */
+    RB_GC_GUARD(stack_start)
 }
 
 static void *

thread_win32.c

@@ -582,7 +582,7 @@ rb_native_cond_destroy(rb_nativethread_cond_t *cond)
 }
 
 void
-ruby_init_stack(volatile VALUE *addr)
+ruby_init_stack(volatile void *addr)
 {
 }
 
@@ -594,20 +594,20 @@ COMPILER_WARNING_PUSH
 COMPILER_WARNING_IGNORED(-Wmaybe-uninitialized)
 #endif
 static inline SIZE_T
-query_memory_basic_info(PMEMORY_BASIC_INFORMATION mi)
+query_memory_basic_info(PMEMORY_BASIC_INFORMATION mi, void *local_in_parent_frame)
 {
-    return VirtualQuery(mi, mi, sizeof(*mi));
+    return VirtualQuery(local_in_parent_frame, mi, sizeof(*mi));
 }
 COMPILER_WARNING_POP
 
 static void
-native_thread_init_stack(rb_thread_t *th)
+native_thread_init_stack(rb_thread_t *th, void *local_in_parent_frame)
 {
     MEMORY_BASIC_INFORMATION mi;
     char *base, *end;
     DWORD size, space;
 
-    CHECK_ERR(query_memory_basic_info(&mi));
+    CHECK_ERR(query_memory_basic_info(&mi, local_in_parent_frame));
     base = mi.AllocationBase;
     end = mi.BaseAddress;
     end += mi.RegionSize;
@@ -638,7 +638,7 @@ thread_start_func_1(void *th_ptr)
     rb_thread_t *th = th_ptr;
     volatile HANDLE thread_id = th->nt->thread_id;
 
-    native_thread_init_stack(th);
+    native_thread_init_stack(th, &th);
     th->nt->interrupt_event = CreateEvent(0, TRUE, FALSE, 0);
 
     /* run */

vm.c

@@ -4174,7 +4174,7 @@ rb_vm_set_progname(VALUE filename)
 extern const struct st_hash_type rb_fstring_hash_type;
 
 void
-Init_BareVM(void)
+Init_BareVM(void *local_in_parent_frame)
 {
     /* VM bootstrap: phase 1 */
     rb_vm_t * vm = ruby_mimmalloc(sizeof(*vm));
@@ -4204,7 +4204,7 @@ Init_BareVM(void)
     th_init(th, 0, vm);
 
     rb_ractor_set_current_ec(th->ractor, th->ec);
-    ruby_thread_init_stack(th);
+    ruby_thread_init_stack(th, local_in_parent_frame);
 
     // setup ractor system
     rb_native_mutex_initialize(&vm->ractor.sync.lock);

vm_core.h

@@ -1842,7 +1842,7 @@ rb_control_frame_t *rb_vm_get_binding_creatable_next_cfp(const rb_execution_cont
 VALUE *rb_vm_svar_lep(const rb_execution_context_t *ec, const rb_control_frame_t *cfp);
 int rb_vm_get_sourceline(const rb_control_frame_t *);
 void rb_vm_stack_to_heap(rb_execution_context_t *ec);
-void ruby_thread_init_stack(rb_thread_t *th);
+void ruby_thread_init_stack(rb_thread_t *th, void *local_in_parent_frame);
 rb_thread_t * ruby_thread_from_native(void);
 int ruby_thread_set_native(rb_thread_t *th);
 int rb_vm_control_frame_id_and_class(const rb_control_frame_t *cfp, ID *idp, ID *called_idp, VALUE *klassp);