Merge pull request #380 from asger-semmle/generalize-useless-conditional

Approved by esben-semmle

Author: semmle-qlci

change-notes/1.19/analysis-javascript.md

@@ -39,6 +39,7 @@
 | User-controlled bypass of security check | Fewer results | This rule no longer flags conditions that guard early returns. The precision of this rule has been revised to "medium". Results are no longer shown on LGTM by default. | 
 | Whitespace contradicts operator precedence | Fewer false-positive results | This rule no longer flags operators with asymmetric whitespace. |
 | Unused import | Fewer false-positive results | This rule no longer flags imports used by the `transform-react-jsx` Babel plugin. |
+| Useless conditional | More true-positive results | This rule now flags useless conditions in more cases. |
 
 ## Changes to QL libraries
 

javascript/ql/src/Statements/UselessConditional.ql

@@ -115,25 +115,21 @@ predicate whitelist(Expr e) {
 }
 
 /**
- * Holds if `e` is part of a conditional node `cond` that evaluates
- * `e` and checks its value for truthiness.
+ * Gets the `&&` expression that defines this guard node, if any.
  */
-predicate isConditional(ASTNode cond, Expr e) {
-  e = cond.(IfStmt).getCondition() or
-  e = cond.(ConditionalExpr).getCondition() or
-  e = cond.(LogAndExpr).getLeftOperand() or
-  e = cond.(LogOrExpr).getLeftOperand()
+LogAndExpr getLogAndExpr(ConditionGuardNode guard) {
+  result.getLeftOperand().stripParens() = guard.getTest()
 }
 
-from ASTNode cond, DataFlow::AnalyzedNode op, boolean cv, ASTNode sel, string msg
-where isConditional(cond, op.asExpr()) and
+from ConditionGuardNode guard, DataFlow::AnalyzedNode op, boolean cv, ASTNode sel, string msg
+where guard.getTest() = op.asExpr() and
       cv = op.getTheBooleanValue()and
       not whitelist(op.asExpr()) and
 
       // if `cond` is of the form `<non-trivial truthy expr> && <something>`,
       // we suggest replacing it with `<non-trivial truthy expr>, <something>`
-      if cond instanceof LogAndExpr and cv = true and not op.asExpr().isPure() then
-        (sel = cond and msg = "This logical 'and' expression can be replaced with a comma expression.")
+      if exists(getLogAndExpr(guard)) and cv = true and not op.asExpr().isPure() then
+        (sel = getLogAndExpr(guard) and msg = "This logical 'and' expression can be replaced with a comma expression.")
 
       // otherwise we just report that `op` always evaluates to `cv`
       else (

javascript/ql/test/query-tests/Statements/UselessConditional/UselessConditional.expected

@@ -1,4 +1,4 @@
-| UselessConditional.js:5:7:5:12 | !lines | This expression always evaluates to false. |
+| UselessConditional.js:5:8:5:12 | lines | Variable 'lines' always evaluates to true here. |
 | UselessConditional.js:12:34:12:79 | (v = ne ... k] = v) | This logical 'and' expression can be replaced with a comma expression. |
 | UselessConditional.js:17:9:17:9 | a | Variable 'a' always evaluates to false here. |
 | UselessConditional.js:18:9:18:9 | b | Variable 'b' always evaluates to false here. |
@@ -15,7 +15,8 @@
 | UselessConditional.js:65:5:65:5 | x | Variable 'x' always evaluates to true here. |
 | UselessConditional.js:76:13:76:13 | x | Variable 'x' always evaluates to true here. |
 | UselessConditional.js:82:13:82:13 | x | Variable 'x' always evaluates to true here. |
-| UselessConditional.js:89:10:89:16 | x, true | This expression always evaluates to true. |
+| UselessConditional.js:94:16:94:16 | x | Variable 'x' always evaluates to false here. |
+| UselessConditional.js:101:18:101:18 | x | Variable 'x' always evaluates to false here. |
 | UselessConditionalGood.js:58:12:58:13 | x2 | Variable 'x2' always evaluates to false here. |
 | UselessConditionalGood.js:69:12:69:13 | xy | Variable 'xy' always evaluates to false here. |
 | UselessConditionalGood.js:85:12:85:13 | xy | Variable 'xy' always evaluates to false here. |

javascript/ql/test/query-tests/Statements/UselessConditional/UselessConditional.js

@@ -89,4 +89,17 @@ async function awaitFlow(){
     if ((x, true));
 });
 
+(function (x, y) {
+    if (!x) {
+        while (x) { // NOT OK
+            f();
+        }
+        while (true) { // OK
+            break;
+        }
+        if (true && true) {} // OK
+        if (y && x) {} // NOT OK
+    }
+});
+
 // semmle-extractor-options: --experimental