Changeset 203925 in webkit

Timestamp:

Jul 29, 2016 4:42:08 PM (8 years ago)

Author:

commit-queue@webkit.org

Message:

Undefined Behavior in JSValue cast from NaN
​https://bugs.webkit.org/show_bug.cgi?id=160322

Patch by Jonathan Bedard <Jonathan Bedard> on 2016-07-29
Reviewed by Mark Lam.

JSValues can be constructed from doubles, and in some cases, are deliberately constructed with NaN values.

In circumstances where NaN is bound through the default JSValue constructor, however, an undefined conversion
to int32_t occurs. While the subsequent if statement should fail and construct the JSValue through the explicit
double constructor, given that the deliberate use of NaN is fairly common, it seems that the jsNaN() function
should immediately call the explicit double constructor both for efficiency and to prevent inadvertent
suppressing of any other bugs which may be instantiating a JSValue with a NaN double.

• runtime/JSCJSValueInlines.h:

(JSC::jsNaN): Explicit double construction for NaN JSValues to avoid undefined behavior.

Location:

trunk/Source/JavaScriptCore

Files:

• ChangeLog (modified) (1 diff)
• runtime/JSCJSValueInlines.h (modified) (2 diffs)

trunk/Source/JavaScriptCore/ChangeLog

@@ +1 @@
+2016-07-29  Jonathan Bedard  <jbedard@apple.com>
+
+        Undefined Behavior in JSValue cast from NaN
+        https://bugs.webkit.org/show_bug.cgi?id=160322
+
+        Reviewed by Mark Lam.
+
+        JSValues can be constructed from doubles, and in some cases, are deliberately constructed with NaN values.
+
+        In circumstances where NaN is bound through the default JSValue constructor, however, an undefined conversion
+        to int32_t occurs.  While the subsequent if statement should fail and construct the JSValue through the explicit
+        double constructor, given that the deliberate use of NaN is fairly common, it seems that the jsNaN() function
+        should immediately call the explicit double constructor both for efficiency and to prevent inadvertent
+        suppressing of any other bugs which may be instantiating a JSValue with a NaN double.
+
+        * runtime/JSCJSValueInlines.h:
+        (JSC::jsNaN): Explicit double construction for NaN JSValues to avoid undefined behavior.
+
 2016-07-29  Michael Saboff  <msaboff@apple.com>
 

trunk/Source/JavaScriptCore/runtime/JSCJSValueInlines.h

@@ -71 +71 @@
 inline JSValue jsNaN()
 {
-    return JSValue(PNaN);
+    return JSValue(JSValue::EncodeAsDouble, PNaN);
 }
 
@@ -141 +141 @@
 inline JSValue::JSValue(double d)
 {
+    // Note: while this behavior is undefined for NaN and inf, the subsequent statement will catch these cases.
     const int32_t asInt32 = static_cast<int32_t>(d);
     if (asInt32 != d || (!asInt32 && std::signbit(d))) { // true for -0.0