std::thread update freebsd stack guard handling.

up to now, it had been assumed the stack guard setting default is not
touched in the field but some user might just want to disable it or
increase it. checking it once at runtime should be enough.

library/std/src/sys/pal/unix/thread.rs

@@ -847,11 +847,31 @@ pub mod guard {
             let stackptr = get_stack_start_aligned()?;
             let guardaddr = stackptr.addr();
             // Technically the number of guard pages is tunable and controlled
-            // by the security.bsd.stack_guard_page sysctl, but there are
-            // few reasons to change it from the default. The default value has
-            // been 1 ever since FreeBSD 11.1 and 10.4.
-            const GUARD_PAGES: usize = 1;
-            let guard = guardaddr..guardaddr + GUARD_PAGES * page_size;
+            // by the security.bsd.stack_guard_page sysctl.
+            // By default it is 1, checking once is enough since it is
+            // a boot time config value.
+            static LOCK: crate::sync::OnceLock<usize> = crate::sync::OnceLock::new();
+            let guard = guardaddr
+                ..guardaddr
+                    + *LOCK.get_or_init(|| {
+                        use crate::sys::weak::dlsym;
+                        dlsym!(fn sysctlbyname(*const libc::c_char, *mut libc::c_void, *mut libc::size_t, *const libc::c_void, libc::size_t) -> libc::c_int);
+                        let mut guard: usize = 0;
+                        let mut size = crate::mem::size_of_val(&guard);
+                        let oid = crate::ffi::CStr::from_bytes_with_nul(
+                            b"security.bsd.stack_guard_page\0",
+                        )
+                        .unwrap();
+                        match sysctlbyname.get() {
+                            Some(fcn) => {
+                                if fcn(oid.as_ptr(), &mut guard as *mut _ as *mut _, &mut size as *mut _ as *mut _, crate::ptr::null_mut(), 0) == 0 {
+                                    return guard;
+                                }
+                                return 1;
+                            },
+                            _ => { return 1; }
+                        }
+                    }) * page_size;
             Some(guard)
         } else if cfg!(target_os = "openbsd") {
             // OpenBSD stack already includes a guard page, and stack is