runtime: use PAGE_NO­ACCESS in sysReseve

[alexbrainman]

Windows version of runtime.sysReseve calls VirtualAlloc with MEM_RESERVE and PAGE_READWRITE, but it should use PAGE_NO­ACCESS when using MEM_RESERVE.

According to https://devblogs.microsoft.com/oldnewthing/20171227-00/?p=97656

Why do you have to pass a valid value even if the system doesn’t use it?

This is an artifact of how the front-end parameter validation is done. The Virtual­Alloc function does parameter validation by checking each parameter individually to confirm that the value is among the valid values.

and in particular

Bonus chatter: You would think that the flProtect would not be used when reserving address space with MEM_RESERVE, but you’d be wrong. If reserving regular address space, then the protection should be PAGE_NO­ACCESS.

Someone would have to try and see what effect this change have on memory used by Go executable before making this change. Maybe by using vmmap or rammap tools.

I am creating this issue before I forget.

/cc @aclements and @zx2c4 because you might be interested.

Alex

[alexbrainman]

I tried changing

diff --git a/src/runtime/mem_windows.go b/src/runtime/mem_windows.go
index 165062e..3ccb690 100644
--- a/src/runtime/mem_windows.go
+++ b/src/runtime/mem_windows.go
@@ -115,13 +115,13 @@ func sysReserve(v unsafe.Pointer, n uintptr) unsafe.Pointer {
 	// v is just a hint.
 	// First try at v.
 	// This will fail if any of [v, v+n) is already reserved.
-	v = unsafe.Pointer(stdcall4(_VirtualAlloc, uintptr(v), n, _MEM_RESERVE, _PAGE_READWRITE))
+	v = unsafe.Pointer(stdcall4(_VirtualAlloc, uintptr(v), n, _MEM_RESERVE, _PAGE_NOACCESS))
 	if v != nil {
 		return v
 	}
 
 	// Next let the kernel choose the address.
-	return unsafe.Pointer(stdcall4(_VirtualAlloc, 0, n, _MEM_RESERVE, _PAGE_READWRITE))
+	return unsafe.Pointer(stdcall4(_VirtualAlloc, 0, n, _MEM_RESERVE, _PAGE_NOACCESS))
 }
 
 func sysMap(v unsafe.Pointer, n uintptr, sysStat *uint64) {

and I run this program

package main

import (
	"time"
)

func main() {
	time.Sleep(time.Hour)
}

and I used vmmap to compare memory map before and after the change. And I don't see any differences.

I use up to date version of Windows 10.

Alex