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JS: Recognise that a less-than test is as good as a non-equal test for mitigating off-by-one array access #20423

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Sep 15, 2025
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JS: Recognise that a less-than test is as good as a non-equal test for mitigating off-by-one array access #20423

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4fb133a
Recognise that a less-than test is as good as a non-equal test for mi…
smowton Sep 12, 2025
f5780ae
Amend docstring
smowton Sep 12, 2025
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Change note
smowton Sep 12, 2025
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17 changes: 15 additions & 2 deletions javascript/ql/src/LanguageFeatures/LengthComparisonOffByOne.ql
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Original file line number Diff line number Diff line change
Expand Up @@ -33,6 +33,18 @@ ConditionGuardNode getLengthLEGuard(Variable index, Variable array) {
)
}

/**
* Gets a condition that checks that `index` is less than `array.length`.
*/
ConditionGuardNode getLengthLTGuard(Variable index, Variable array) {
exists(RelationalComparison cmp | cmp instanceof GTExpr or cmp instanceof LTExpr |
cmp = result.getTest() and
result.getOutcome() = true and
cmp.getGreaterOperand() = arrayLen(array) and
cmp.getLesserOperand() = index.getAnAccess()
)
}

/**
* Gets a condition that checks that `index` is not equal to `array.length`.
*/
Expand Down Expand Up @@ -62,7 +74,8 @@ where
elementRead(ea, array, index, bb) and
// and the read is guarded by the comparison
cond.dominates(bb) and
// but the read is not guarded by another check that `index != array.length`
not getLengthNEGuard(index, array).dominates(bb)
// but the read is not guarded by another check that `index != array.length` or `index < array.length`
not getLengthNEGuard(index, array).dominates(bb) and
not getLengthLTGuard(index, array).dominates(bb)
select cond.getTest(), "Off-by-one index comparison against length may lead to out-of-bounds $@.",
ea, "read"
4 changes: 4 additions & 0 deletions javascript/ql/src/change-notes/2025-09-12-off-by-one.md
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@@ -0,0 +1,4 @@
---
category: minorAnalysis
---
* Query `js/index-out-of-bounds` no longer produces a false-positive when a strictly-less-than check overrides a previous less-than-or-equal test.
8 changes: 8 additions & 0 deletions javascript/ql/test/query-tests/LanguageFeatures/LengthComparisonOffByOne/tst.js
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Expand Up @@ -55,3 +55,11 @@ function badContains(a, elt) {
return true;
return false;
}

// OK - incorrect upper bound, but extra check
function badContains2(a, elt) {
for (let i = 0; i <= a.length; ++i)
if (i < a.length && a[i] === elt)
return true;
return false;
}