make a more stable exit strategy for fuzzing with loops

boa_engine/src/error.rs

@@ -503,6 +503,17 @@ impl JsNativeError {
         Self::new(JsNativeErrorKind::Uri, Box::default(), None)
     }
 
+    /// Creates a new `JsNativeError` that indicates that the context hit its execution limit. This
+    /// is only used in a fuzzing context.
+    #[cfg(feature = "fuzz")]
+    pub fn no_instructions_remain() -> Self {
+        Self::new(
+            JsNativeErrorKind::NoInstructionsRemain,
+            Box::default(),
+            None,
+        )
+    }
+
     /// Sets the message of this error.
     ///
     /// # Examples
@@ -619,6 +630,12 @@ impl JsNativeError {
             }
             JsNativeErrorKind::Type => (constructors.type_error().prototype(), ErrorKind::Type),
             JsNativeErrorKind::Uri => (constructors.uri_error().prototype(), ErrorKind::Uri),
+            #[cfg(feature = "fuzz")]
+            JsNativeErrorKind::NoInstructionsRemain => {
+                // we can propagate out from try/catch since the catch block will also perform some
+                // operation
+                (constructors.error().prototype(), ErrorKind::Error)
+            }
         };
 
         let o = JsObject::from_proto_and_data(prototype, ObjectData::error(tag));
@@ -747,6 +764,10 @@ pub enum JsNativeErrorKind {
     /// [e_uri]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/encodeURI
     /// [d_uri]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/decodeURI
     Uri,
+    /// Error thrown when no instructions remain. Only used in a fuzzing context; not a valid JS
+    /// error variant.
+    #[cfg(feature = "fuzz")]
+    NoInstructionsRemain,
 }
 
 impl std::fmt::Display for JsNativeErrorKind {
@@ -760,6 +781,8 @@ impl std::fmt::Display for JsNativeErrorKind {
             JsNativeErrorKind::Syntax => "SyntaxError",
             JsNativeErrorKind::Type => "TypeError",
             JsNativeErrorKind::Uri => "UriError",
+            #[cfg(feature = "fuzz")]
+            JsNativeErrorKind::NoInstructionsRemain => "NoInstructionsRemain",
         }
         .fmt(f)
     }

boa_engine/src/vm/mod.rs

@@ -5,7 +5,7 @@
 use crate::{
     builtins::async_generator::{AsyncGenerator, AsyncGeneratorState},
     vm::{call_frame::CatchAddresses, code_block::Readable},
-    Context, JsResult, JsValue,
+    Context, JsError, JsNativeError, JsResult, JsValue,
 };
 use boa_interner::ToInternedString;
 use boa_profiler::Profiler;
@@ -114,7 +114,7 @@ impl Context {
     fn execute_instruction(&mut self) -> JsResult<ShouldExit> {
         #[cfg(feature = "fuzz")]
         if self.insns_remaining == 0 {
-            return Ok(ShouldExit::True);
+            return Err(JsError::from_native(JsNativeError::no_instructions_remain()));
         } else {
             self.insns_remaining -= 1;
         }

fuzz/fuzz_targets/vm-implied.rs

@@ -3,22 +3,30 @@
 mod common;
 
 use crate::common::FuzzSource;
-use boa_engine::{Context, JsResult, JsValue};
+use boa_engine::{Context, JsNativeErrorKind};
 use libfuzzer_sys::fuzz_target;
 use libfuzzer_sys::Corpus;
 
-fn do_fuzz(original: FuzzSource) -> JsResult<JsValue> {
+fn do_fuzz(original: FuzzSource) -> Corpus {
     let mut ctx = Context::builder()
         .interner(original.interner)
         .insns_remaining(1 << 16)
         .build();
-    ctx.eval(&original.source)
+    match ctx.eval(&original.source) {
+        Ok(_) => Corpus::Keep,
+        Err(e) => match e.try_native(&mut ctx) {
+            Ok(e) => {
+                if matches!(e.kind, JsNativeErrorKind::NoInstructionsRemain) {
+                    // most interesting loops will initially cause an infinite loop
+                    // this is okay as long as it is explicitly handled
+                    Corpus::Keep
+                } else {
+                    Corpus::Reject
+                }
+            }
+            Err(_) => Corpus::Reject,
+        },
+    }
 }
 
-fuzz_target!(|original: FuzzSource| -> Corpus {
-    if do_fuzz(original).is_ok() {
-        Corpus::Keep
-    } else {
-        Corpus::Reject
-    }
-});
+fuzz_target!(|original: FuzzSource| -> Corpus { do_fuzz(original) });