unix: Set a guard page at the end of signal stacks

This mitigates possible issues when signal stacks overflow, which could
manifest as segfaults or in unlucky circumstances possible clobbering of
other memory values as stack overflows tend to enable.

src/libstd/sys/unix/stack_overflow.rs

@@ -45,7 +45,7 @@ mod imp {
     use libc::{mmap, munmap};
     use libc::{sigaction, sighandler_t, SA_ONSTACK, SA_SIGINFO, SIGBUS, SIG_DFL};
     use libc::{sigaltstack, SIGSTKSZ, SS_DISABLE};
-    use libc::{MAP_ANON, MAP_PRIVATE, PROT_READ, PROT_WRITE, SIGSEGV};
+    use libc::{MAP_ANON, MAP_PRIVATE, PROT_NONE, PROT_READ, PROT_WRITE, SIGSEGV};
 
     use crate::sys_common::thread_info;
 
@@ -137,11 +137,16 @@ mod imp {
     }
 
     unsafe fn get_stackp() -> *mut libc::c_void {
+        let page_size = crate::sys::unix::os::page_size();
         let stackp =
-            mmap(ptr::null_mut(), SIGSTKSZ, PROT_READ | PROT_WRITE, MAP_PRIVATE | MAP_ANON, -1, 0);
+            mmap(ptr::null_mut(), SIGSTKSZ + page_size, PROT_READ | PROT_WRITE, MAP_PRIVATE | MAP_ANON, -1, 0);
         if stackp == MAP_FAILED {
             panic!("failed to allocate an alternative stack");
         }
+        let guard_result = libc::mprotect(stackp, page_size, PROT_NONE);
+        if guard_result != 0 {
+            panic!("failed to set up alternative stack guard page");
+        }
         stackp
     }