Segfault-based memory bounds-checking. (#29)

* Mostly working

* Improve CircleCI releases

* Recover safely from WebAssembly-generated traps

* Add notes about async signal safety

* Improved trap catching with call_protected macro

* Improved test assert_trap code using call_protected macro

* Mostly working

src/apis/emscripten/io.rs

@@ -8,9 +8,8 @@ pub use libc::putchar;
 /// printf
 pub extern "C" fn printf(memory_offset: i32, extra: i32, instance: &Instance) -> i32 {
     debug!("emscripten::printf");
-    let mem = &instance.memories[0];
-    return unsafe {
-        let base_memory_offset = mem.mmap.as_ptr().offset(memory_offset as isize) as *const i8;
-        _printf(base_memory_offset, extra)
-    };
+    unsafe {
+        let addr = instance.memory_offset_addr(0, memory_offset as _) as _;
+        _printf(addr, extra)
+    }
 }

src/apis/emscripten/memory.rs

@@ -21,7 +21,7 @@ pub extern "C" fn _emscripten_memcpy_big(
 /// emscripten: getTotalMemory
 pub extern "C" fn get_total_memory(instance: &mut Instance) -> u32 {
     debug!("emscripten::get_total_memory");
-    instance.memories[0].current_size()
+    instance.memories[0].current_pages()
 }
 
 /// emscripten: enlargeMemory

src/apis/emscripten/varargs.rs

@@ -3,7 +3,7 @@ use std::mem;
 
 #[repr(transparent)]
 pub struct VarArgs {
-    pointer: u32, // assuming 32bit wasm
+    pub pointer: u32, // assuming 32bit wasm
 }
 
 impl VarArgs {

src/webassembly/instance.rs

@@ -31,6 +31,7 @@ use super::module::{Export, ImportableExportable, Module};
 use super::relocation::{Reloc, RelocSink, RelocationType};
 
 type TablesSlice = UncheckedSlice<BoundedSlice<usize>>;
+// TODO: this should be `type MemoriesSlice = UncheckedSlice<UncheckedSlice<u8>>;`, but that crashes for some reason.
 type MemoriesSlice = UncheckedSlice<BoundedSlice<u8>>;
 type GlobalsSlice = UncheckedSlice<u8>;
 
@@ -454,8 +455,14 @@ impl Instance {
             for init in &module.info.data_initializers {
                 debug_assert!(init.base.is_none(), "globalvar base not supported yet");
                 let offset = init.offset;
-                let mem_mut = memories[init.memory_index.index()].as_mut();
-                let to_init = &mut mem_mut[offset..offset + init.data.len()];
+                let mem = &mut memories[init.memory_index.index()];
+                let end_of_init = offset + init.data.len();
+                if end_of_init > mem.current_size() {
+                    let grow_pages = (end_of_init / LinearMemory::WASM_PAGE_SIZE) + 1;
+                    mem.grow(grow_pages as u32)
+                        .expect("failed to grow memory for data initializers");
+                }
+                let to_init = &mut mem[offset..offset + init.data.len()];
                 to_init.copy_from_slice(&init.data);
             }
         }
@@ -477,7 +484,7 @@ impl Instance {
             .map(|mem| {
                 BoundedSlice::new(
                     &mem[..],
-                    mem.current as usize * LinearMemory::WASM_PAGE_SIZE,
+                    mem.current_size(),
                 )
             })
             .collect();
@@ -539,8 +546,9 @@ impl Instance {
     }
 
     pub fn memory_offset_addr(&self, index: usize, offset: usize) -> *const usize {
-        let mem = &self.memories[index];
-        unsafe { mem.mmap.as_ptr().offset(offset as isize) as *const usize }
+        let memories: &[LinearMemory] = &self.memories[..];
+        let mem = &memories[index];
+        unsafe { mem[..].as_ptr().add(offset) as *const usize }
     }
 
     // Shows the value of a global variable.
@@ -568,21 +576,10 @@ extern "C" fn grow_memory(size: u32, memory_index: u32, instance: &mut Instance)
         .grow(size)
         .unwrap_or(-1);
 
-    if old_mem_size != -1 {
-        // Get new memory bytes
-        let new_mem_bytes = (old_mem_size as usize + size as usize) * LinearMemory::WASM_PAGE_SIZE;
-        // Update data_pointer
-        instance
-            .data_pointers
-            .memories
-            .get_unchecked_mut(memory_index as usize)
-            .len = new_mem_bytes;
-    }
-
     old_mem_size
 }
 
 extern "C" fn current_memory(memory_index: u32, instance: &mut Instance) -> u32 {
     let memory = &instance.memories[memory_index as usize];
-    memory.current_size() as u32
+    memory.current_pages() as u32
 }

src/webassembly/memory.rs

@@ -3,27 +3,27 @@
 //! webassembly::Instance.
 //! A memory created by Rust or in WebAssembly code will be accessible and
 //! mutable from both Rust and WebAssembly.
-use memmap::MmapMut;
-use std::fmt;
+use nix::sys::mman::{mmap, MapFlags, ProtFlags};
+use nix::libc::{c_void, mprotect, PROT_READ, PROT_WRITE};
+use std::slice;
 use std::ops::{Deref, DerefMut};
 
 const PAGE_SIZE: u32 = 65536;
 const MAX_PAGES: u32 = 65536;
 
 /// A linear memory instance.
-///
+//
+#[derive(Debug)]
 pub struct LinearMemory {
-    pub mmap: MmapMut,
-    // The initial size of the WebAssembly Memory, in units of
-    // WebAssembly pages.
-    pub current: u32,
+    base: *mut c_void, // The size will always be `LinearMemory::DEFAULT_SIZE`
+    current: u32, // current number of wasm pages
     // The maximum size the WebAssembly Memory is allowed to grow
     // to, in units of WebAssembly pages.  When present, the maximum
     // parameter acts as a hint to the engine to reserve memory up
     // front.  However, the engine may ignore or clamp this reservation
     // request.  In general, most WebAssembly modules shouldn't need
     // to set a maximum.
-    pub maximum: Option<u32>,
+    maximum: Option<u32>,
 }
 
 /// It holds the raw bytes of memory accessed by a WebAssembly Instance
@@ -45,11 +45,30 @@ impl LinearMemory {
         );
 
         // TODO: Investigate if memory is zeroed out
-        let mmap = MmapMut::map_anon(LinearMemory::DEFAULT_HEAP_SIZE).unwrap();
+        let base = unsafe {
+            mmap(
+                0 as _,
+                LinearMemory::DEFAULT_SIZE,
+                ProtFlags::PROT_NONE,
+                MapFlags::MAP_ANON | MapFlags::MAP_SHARED,
+                -1,
+                0,
+            ).unwrap()
+        };
+
+        if initial > 0 {
+            assert_eq!(unsafe {
+                mprotect(
+                    base,
+                    (initial * PAGE_SIZE) as _,
+                    PROT_READ | PROT_WRITE,
+                )
+            }, 0);
+        }
 
         debug!("LinearMemory instantiated");
         Self {
-            mmap,
+            base,
             current: initial,
             maximum,
         }
@@ -57,11 +76,15 @@ impl LinearMemory {
 
     /// Returns an base address of this linear memory.
     pub fn base_addr(&mut self) -> *mut u8 {
-        self.mmap.as_mut_ptr()
+        self.base as _
     }
 
     /// Returns a number of allocated wasm pages.
-    pub fn current_size(&self) -> u32 {
+    pub fn current_size(&self) -> usize {
+        (self.current * PAGE_SIZE) as _
+    }
+
+    pub fn current_pages(&self) -> u32 {
         self.current
     }
 
@@ -76,7 +99,10 @@ impl LinearMemory {
     /// of pages.
     pub fn grow(&mut self, add_pages: u32) -> Option<i32> {
         debug!("grow_memory called!");
-        debug!("old memory = {:?}", self.mmap);
+        if add_pages == 0 {
+            return Some(self.current as _);
+        }
+
         let prev_pages = self.current;
 
         let new_pages = match self.current.checked_add(add_pages) {
@@ -91,18 +117,23 @@ impl LinearMemory {
         // Wasm linear memories are never allowed to grow beyond what is
         // indexable. If the memory has no maximum, enforce the greatest
         // limit here.
-        } else if new_pages >= 65536 {
+        } else if new_pages >= MAX_PAGES {
             return None;
         }
 
         let prev_bytes = (prev_pages * PAGE_SIZE) as usize;
         let new_bytes = (new_pages * PAGE_SIZE) as usize;
 
-        // Updating self.current if new_bytes > prev_bytes
-        if new_bytes > prev_bytes {
-            self.current = new_pages;
+        unsafe {
+            assert_eq!(mprotect(
+                self.base.add(prev_bytes),
+                new_bytes - prev_bytes,
+                PROT_READ | PROT_WRITE,
+            ), 0);
         }
 
+        self.current = new_pages;
+
         Some(prev_pages as i32)
     }
 
@@ -119,16 +150,6 @@ impl LinearMemory {
     }
 }
 
-impl fmt::Debug for LinearMemory {
-    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
-        f.debug_struct("LinearMemory")
-            .field("mmap", &self.mmap)
-            .field("current", &self.current)
-            .field("maximum", &self.maximum)
-            .finish()
-    }
-}
-
 // Not comparing based on memory content. That would be inefficient.
 impl PartialEq for LinearMemory {
     fn eq(&self, other: &LinearMemory) -> bool {
@@ -136,42 +157,20 @@ impl PartialEq for LinearMemory {
     }
 }
 
-impl AsRef<[u8]> for LinearMemory {
-    fn as_ref(&self) -> &[u8] {
-        &self.mmap
-    }
-}
-
-impl AsMut<[u8]> for LinearMemory {
-    fn as_mut(&mut self) -> &mut [u8] {
-        &mut self.mmap
-    }
-}
 
 impl Deref for LinearMemory {
     type Target = [u8];
     fn deref(&self) -> &[u8] {
-        &*self.mmap
+        unsafe {
+            slice::from_raw_parts(self.base as _, (self.current * PAGE_SIZE) as _)
+        }
     }
 }
 
 impl DerefMut for LinearMemory {
     fn deref_mut(&mut self) -> &mut [u8] {
-        &mut *self.mmap
+        unsafe {
+            slice::from_raw_parts_mut(self.base as _, (self.current * PAGE_SIZE) as _)
+        }
     }
 }
-
-// impl Clone for LinearMemory {
-//     fn clone(&self) -> LinearMemory {
-//         let mut mmap = MmapMut::map_anon(self.maximum.unwrap_or(self.current) as usize).unwrap();
-//         let mut base_mmap = &self.mmap;
-//         let to_init = &mut mmap[0..self.current as usize];
-//         to_init.copy_from_slice(&self.mmap);
-
-//         return LinearMemory {
-//             mmap: mmap,
-//             current: self.current,
-//             maximum: self.maximum,
-//         };
-//     }
-// }

src/webassembly/mod.rs

@@ -7,7 +7,7 @@ pub mod module;
 pub mod relocation;
 pub mod utils;
 
-use cranelift_codegen::{isa, settings};
+use cranelift_codegen::{isa, settings::{self, Configurable}};
 use std::panic;
 use std::str::FromStr;
 use target_lexicon;
@@ -47,7 +47,16 @@ pub fn instantiate(
     buffer_source: Vec<u8>,
     import_object: ImportObject<&str, &str>,
 ) -> Result<ResultObject, ErrorKind> {
-    let flags = settings::Flags::new(settings::builder());
+
+    let flags = {
+        let mut builder = settings::builder();
+        builder.set("opt_level", "best")
+            .unwrap();
+
+        let flags = settings::Flags::new(builder);
+        debug_assert_eq!(flags.opt_level(), settings::OptLevel::Best);
+        flags
+    };
     let isa = isa::lookup(triple!("x86_64")).unwrap().finish(flags);
 
     let module = compile(buffer_source)?;

src/webassembly/module.rs

@@ -401,9 +401,6 @@ impl<'environment> FuncEnvironmentTrait for FuncEnvironment<'environment> {
     }
 
     // TODO: offsets should be based on the architecture the wasmer was compiled for.
-    //      e.g., BoundedSlice.len will be 32-bit (4 bytes) when wasmer is compiled for a 32-bit arch,
-    //      however the 32-bit wasmer may be running on 64-bit arch, which means ptr_size here will
-    //      be 8 bytes. That will definitely gove the wrong offset values
     fn make_heap(&mut self, func: &mut ir::Function, memory_index: MemoryIndex) -> ir::Heap {
         debug_assert_eq!(
             memory_index.index(),
@@ -423,8 +420,7 @@ impl<'environment> FuncEnvironmentTrait for FuncEnvironment<'environment> {
         });
 
         // Based on the index provided, we need to know the offset into memories array
-        // Each element in the memories array has a size of (ptr_size * 2)
-        let memory_data_offset = 0; // (memory_index as usize * ptr_size * 2) as i32;
+        let memory_data_offset = (memory_index.index() * ptr_size as usize) as i32;
 
         // Load value at the (base + memory_data_offset)
         // which is the address of data_pointer.memories[index].data
@@ -435,21 +431,14 @@ impl<'environment> FuncEnvironmentTrait for FuncEnvironment<'environment> {
             readonly: true,
         });
 
-        // Load value at the (base + memory_data_offset)
-        // which is the value of data_pointer.memories[index].len
-        let bound_gv = func.create_global_value(ir::GlobalValueData::Load {
-            base,
-            offset: Offset32::new(memory_data_offset + ptr_size as i32),
-            global_type: I32,
-            readonly: false,
-        });
-
         // Create table based on the data above
         let heap = func.create_heap(ir::HeapData {
             base: heap_base,
             min_size: 0.into(),
             guard_size: (LinearMemory::DEFAULT_GUARD_SIZE as i64).into(),
-            style: ir::HeapStyle::Dynamic { bound_gv },
+            style: ir::HeapStyle::Static {
+                bound: (LinearMemory::DEFAULT_HEAP_SIZE as i64).into(),
+            },
             index_type: I32,
         });
 

src/webassembly/utils.rs

@@ -22,13 +22,9 @@ pub fn print_instance_offsets(instance: &Instance) {
     let memories_pointer_address_0 = memories_pointer_address_ptr_0 as usize;
 
     let memories_pointer_address_ptr_0_data: *const usize =
-        unsafe { transmute(&data_ptr.memories.get_unchecked(0).data) };
+        unsafe { transmute(&data_ptr.memories.get_unchecked(0)) };
     let memories_pointer_address_0_data = memories_pointer_address_ptr_0_data as usize;
 
-    let memories_pointer_address_ptr_0_len: *const usize =
-        unsafe { transmute(&data_ptr.memories.get_unchecked(0).len) };
-    let memories_pointer_address_0_len = memories_pointer_address_ptr_0_len as usize;
-
     let globals_pointer_address_ptr: *const usize = unsafe { transmute(&data_ptr.globals) };
     let globals_pointer_address = globals_pointer_address_ptr as usize;
 
@@ -40,7 +36,6 @@ instance.data_pointers.tables \t- {:X} | offset - {:?}
 instance.data_pointers.memories\t- {:X} | offset - {:?}
     .memories[0] \t\t- {:X} | offset - {:?}
     .memories[0].data\t\t- {:X} | offset - {:?}
-    .memories[0].len({:?})\t- {:X} | offset - {:?}
 instance.data_pointers.globals \t- {:X} | offset - {:?}
 ====== INSTANCE OFFSET TABLE ======
         ",
@@ -54,9 +49,6 @@ instance.data_pointers.globals \t- {:X} | offset - {:?}
         0,
         memories_pointer_address_0_data,
         memories_pointer_address_0_data - memories_pointer_address_0_data,
-        data_ptr.memories.get_unchecked(0).len,
-        memories_pointer_address_0_len,
-        memories_pointer_address_0_len - memories_pointer_address_0_data,
         globals_pointer_address,
         globals_pointer_address - instance_address,
     );