Add comments and unify guard page setup.

While currently only NetBSD seems to be affected, all systems
implementing PAX MPROTECT in strict mode need this treatment,
and it does not hurt others.

Author: MartinHusemann

src/libstd/sys/unix/thread.rs

@@ -326,23 +326,20 @@ pub mod guard {
             // Reallocate the last page of the stack.
             // This ensures SIGBUS will be raised on
             // stack overflow.
-            if cfg!(target_os = "netbsd") {
-                let result = mmap(stackaddr, PAGE_SIZE, PROT_READ | PROT_WRITE,
-                                   MAP_PRIVATE | MAP_ANON | MAP_FIXED, -1, 0);
-                if result != stackaddr || result == MAP_FAILED {
-                    panic!("failed to allocate a guard page");
-                }
-                let result = mprotect(stackaddr, PAGE_SIZE, 0);
+            // Systems which enforce strict PAX MPROTECT do not allow
+            // to mprotect() a mapping with less restrictive permissions
+            // than the initial mmap() used, so we mmap() here with
+            // read/write permissions and only then mprotect() it to
+            // no permissions at all. See issue #50313.
+            let result = mmap(stackaddr, PAGE_SIZE, PROT_READ | PROT_WRITE,
+                              MAP_PRIVATE | MAP_ANON | MAP_FIXED, -1, 0);
+            if result != stackaddr || result == MAP_FAILED {
+                panic!("failed to allocate a guard page");
+            }
 
-                if result != 0 {
-                    panic!("unable to protect the guard page");
-                }
-            } else {
-                let result = mmap(stackaddr, PAGE_SIZE, PROT_NONE,
-                                  MAP_PRIVATE | MAP_ANON | MAP_FIXED, -1, 0);
-                if result != stackaddr || result == MAP_FAILED {
-                    panic!("failed to allocate a guard page");
-                }
+            let result = mprotect(stackaddr, PAGE_SIZE, PROT_NONE);
+            if result != 0 {
+                panic!("failed to protect the guard page");
             }
 
             let guardaddr = stackaddr as usize;