Recover safely from WebAssembly-generated traps

src/main.rs

@@ -28,6 +28,7 @@ use structopt::StructOpt;
 mod macros;
 pub mod apis;
 pub mod common;
+mod recovery;
 pub mod sighandler;
 #[cfg(test)]
 mod spectests;
@@ -95,9 +96,7 @@ fn execute_wasm(wasm_path: PathBuf) -> Result<(), String> {
                     Some(&webassembly::Export::Function(index)) => index,
                     _ => panic!("Main function not found"),
                 });
-        let main: extern "C" fn(&webassembly::Instance) =
-            get_instance_function!(instance, func_index);
-        main(&instance);
+        instance.start_func(func_index).unwrap();
     }
 
     Ok(())

src/recovery.rs

@@ -0,0 +1,41 @@
+//! When a WebAssembly module triggers any traps, we perform recovery here.
+
+use std::cell::UnsafeCell;
+
+extern "C" {
+    fn setjmp(env: *mut ::nix::libc::c_void) -> ::nix::libc::c_int;
+    fn longjmp(env: *mut ::nix::libc::c_void, val: ::nix::libc::c_int) -> !;
+}
+
+const SETJMP_BUFFER_LEN: usize = 27;
+
+thread_local! {
+    static SETJMP_BUFFER: UnsafeCell<[::nix::libc::c_int; SETJMP_BUFFER_LEN]> = UnsafeCell::new([0; SETJMP_BUFFER_LEN]);
+}
+
+/// Calls a function with longjmp receiver installed. The function must be compiled from WebAssembly;
+/// Otherwise, the behavior is undefined.
+pub unsafe fn protected_call<T, R>(f: fn(T) -> R, p: T) -> Result<R, i32> {
+    let jmp_buf = SETJMP_BUFFER.with(|buf| buf.get());
+    let prev_jmp_buf = *jmp_buf;
+
+    let signum = setjmp(jmp_buf as *mut ::nix::libc::c_void);
+    if signum != 0 {
+        *jmp_buf = prev_jmp_buf;
+        Err(signum)
+    } else {
+        let ret = f(p);
+        *jmp_buf = prev_jmp_buf;
+        Ok(ret)
+    }
+}
+
+/// Unwinds to last protected_call.
+pub unsafe fn do_unwind(signum: i32) -> ! {
+    let jmp_buf = SETJMP_BUFFER.with(|buf| buf.get());
+    if *jmp_buf == [0; SETJMP_BUFFER_LEN] {
+        ::std::process::abort();
+    }
+
+    longjmp(jmp_buf as *mut ::nix::libc::c_void, signum)
+}

src/sighandler.rs

@@ -4,12 +4,11 @@
 //!
 //! Please read more about this here: https://github.com/CraneStation/wasmtime/issues/15
 //! This code is inspired by: https://github.com/pepyakin/wasmtime/commit/625a2b6c0815b21996e111da51b9664feb174622
+use super::recovery;
 use nix::sys::signal::{
-    sigaction, SaFlags, SigAction, SigHandler, SigSet, Signal, SIGBUS, SIGFPE, SIGILL, SIGSEGV,
+    sigaction, SaFlags, SigAction, SigHandler, SigSet, SIGBUS, SIGFPE, SIGILL, SIGSEGV,
 };
 
-static mut SETJMP_BUFFER: [::nix::libc::c_int; 27] = [0; 27];
-
 pub unsafe fn install_sighandler() {
     let sa = SigAction::new(
         SigHandler::Handler(signal_trap_handler),
@@ -20,29 +19,10 @@ pub unsafe fn install_sighandler() {
     sigaction(SIGILL, &sa).unwrap();
     sigaction(SIGSEGV, &sa).unwrap();
     sigaction(SIGBUS, &sa).unwrap();
-    let signum = setjmp((&mut SETJMP_BUFFER[..]).as_mut_ptr() as *mut ::nix::libc::c_void);
-    if signum != 0 {
-        let signal = Signal::from_c_int(signum).unwrap();
-        match signal {
-            SIGFPE => panic!("floating-point exception"),
-            SIGILL => panic!("illegal instruction"),
-            SIGSEGV => panic!("segmentation violation"),
-            SIGBUS => panic!("bus error"),
-            _ => panic!("signal error: {:?}", signal),
-        };
-    }
-}
-
-extern "C" {
-    fn setjmp(env: *mut ::nix::libc::c_void) -> ::nix::libc::c_int;
-    fn longjmp(env: *mut ::nix::libc::c_void, val: ::nix::libc::c_int);
 }
 
 extern "C" fn signal_trap_handler(signum: ::nix::libc::c_int) {
     unsafe {
-        longjmp(
-            (&mut SETJMP_BUFFER).as_mut_ptr() as *mut ::nix::libc::c_void,
-            signum,
-        );
+        recovery::do_unwind(signum);
     }
 }

src/webassembly/instance.rs

@@ -22,6 +22,7 @@ use std::slice;
 use std::sync::Arc;
 
 use super::super::common::slice::{BoundedSlice, UncheckedSlice};
+use super::super::recovery;
 use super::errors::ErrorKind;
 use super::import_object::{ImportObject, ImportValue};
 use super::math_intrinsics;
@@ -225,7 +226,8 @@ impl Instance {
                     // let r = *Arc::from_raw(isa_ptr);
                     compile_function(&*options.isa, function_body).unwrap()
                     // unimplemented!()
-                }).collect();
+                })
+                .collect();
 
             for compiled_func in compiled_funcs.into_iter() {
                 let CompiledFunction {
@@ -475,7 +477,8 @@ impl Instance {
                     &mem[..],
                     mem.current as usize * LinearMemory::WASM_PAGE_SIZE,
                 )
-            }).collect();
+            })
+            .collect();
         let globals_pointer: GlobalsSlice = globals[..].into();
 
         let data_pointers = DataPointers {
@@ -513,10 +516,16 @@ impl Instance {
         get_function_addr(&func_index, &self.import_functions, &self.functions)
     }
 
-    pub fn start(&self) {
+    pub fn start_func(&self, func_index: FuncIndex) -> Result<(), i32> {
+        let func: fn(&Instance) = get_instance_function!(&self, func_index);
+        unsafe { recovery::protected_call(func, self) }
+    }
+
+    pub fn start(&self) -> Result<(), i32> {
         if let Some(func_index) = self.start_func {
-            let func: fn(&Instance) = get_instance_function!(&self, func_index);
-            func(self)
+            self.start_func(func_index)
+        } else {
+            panic!("start func not found")
         }
     }