auto merge of #15588 : alexcrichton/rust/issue-15478, r=cmr

If modified, you can safely unmap arbitrary memory. These fields are not
intended to be modified, so read-only accessors are the only ones that are
provided.

Closes #15478

src/libgreen/stack.rs

@@ -8,14 +8,15 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
+use std::ptr;
 use std::sync::atomics;
 use std::os::{errno, page_size, MemoryMap, MapReadable, MapWritable,
-              MapNonStandardFlags, MapVirtual, getenv};
+              MapNonStandardFlags, getenv};
 use libc;
 
 /// A task's stack. The name "Stack" is a vestige of segmented stacks.
 pub struct Stack {
-    buf: MemoryMap,
+    buf: Option<MemoryMap>,
     min_size: uint,
     valgrind_id: libc::c_uint,
 }
@@ -52,11 +53,11 @@ impl Stack {
         // guaranteed to be aligned properly.
         if !protect_last_page(&stack) {
             fail!("Could not memory-protect guard page. stack={}, errno={}",
-                  stack.data, errno());
+                  stack.data(), errno());
         }
 
         let mut stk = Stack {
-            buf: stack,
+            buf: Some(stack),
             min_size: size,
             valgrind_id: 0
         };
@@ -71,7 +72,7 @@ impl Stack {
     /// Create a 0-length stack which starts (and ends) at 0.
     pub unsafe fn dummy_stack() -> Stack {
         Stack {
-            buf: MemoryMap { data: 0 as *mut u8, len: 0, kind: MapVirtual },
+            buf: None,
             min_size: 0,
             valgrind_id: 0
         }
@@ -79,14 +80,15 @@ impl Stack {
 
     /// Point to the low end of the allocated stack
     pub fn start(&self) -> *const uint {
-        self.buf.data as *const uint
+        self.buf.as_ref().map(|m| m.data() as *const uint)
+            .unwrap_or(ptr::null())
     }
 
     /// Point one uint beyond the high end of the allocated stack
     pub fn end(&self) -> *const uint {
-        unsafe {
-            self.buf.data.offset(self.buf.len as int) as *const uint
-        }
+        self.buf.as_ref().map(|buf| unsafe {
+            buf.data().offset(buf.len() as int) as *const uint
+        }).unwrap_or(ptr::null())
     }
 }
 
@@ -96,7 +98,7 @@ fn protect_last_page(stack: &MemoryMap) -> bool {
         // This may seem backwards: the start of the segment is the last page?
         // Yes! The stack grows from higher addresses (the end of the allocated
         // block) to lower addresses (the start of the allocated block).
-        let last_page = stack.data as *mut libc::c_void;
+        let last_page = stack.data() as *mut libc::c_void;
         libc::mprotect(last_page, page_size() as libc::size_t,
                        libc::PROT_NONE) != -1
     }
@@ -106,7 +108,7 @@ fn protect_last_page(stack: &MemoryMap) -> bool {
 fn protect_last_page(stack: &MemoryMap) -> bool {
     unsafe {
         // see above
-        let last_page = stack.data as *mut libc::c_void;
+        let last_page = stack.data() as *mut libc::c_void;
         let mut old_prot: libc::DWORD = 0;
         libc::VirtualProtect(last_page, page_size() as libc::SIZE_T,
                              libc::PAGE_NOACCESS,

src/libstd/os.rs

@@ -1277,12 +1277,9 @@ pub fn page_size() -> uint {
 /// The memory map is released (unmapped) when the destructor is run, so don't
 /// let it leave scope by accident if you want it to stick around.
 pub struct MemoryMap {
-    /// Pointer to the memory created or modified by this map.
-    pub data: *mut u8,
-    /// Number of bytes this map applies to
-    pub len: uint,
-    /// Type of mapping
-    pub kind: MemoryMapKind,
+    data: *mut u8,
+    len: uint,
+    kind: MemoryMapKind,
 }
 
 /// Type of memory map
@@ -1617,6 +1614,15 @@ impl Drop for MemoryMap {
     }
 }
 
+impl MemoryMap {
+    /// Returns the pointer to the memory created or modified by this map.
+    pub fn data(&self) -> *mut u8 { self.data }
+    /// Returns the number of bytes this map applies to.
+    pub fn len(&self) -> uint { self.len }
+    /// Returns the type of mapping this represents.
+    pub fn kind(&self) -> MemoryMapKind { self.kind }
+}
+
 #[cfg(target_os = "linux")]
 pub mod consts {
     pub use os::arch_consts::ARCH;