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Constant length comparison

ID: go/constant-length-comparison
Kind: problem
Security severity: 
Severity: warning
Precision: high
Tags:
   - correctness
Query suites:
   - go-security-and-quality.qls

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Indexing operations on arrays, slices, or strings should use an index at most one less than the length. If the operation uses a variable index but checks the length against a constant, this may indicate a logic error which could lead to an out-of-bounds access.

Recommendation

Inspect the code closely to determine whether the length should be compared to the index variable instead. For loops that iterate over every element, using a range loop is better than explicit index manipulation.

Example

The following example shows a method which checks whether slice xs is a prefix of slice ys:

package main

func isPrefixOf(xs, ys []int) bool {
	for i := 0; i < len(xs); i++ {
		if len(ys) == 0 || xs[i] != ys[i] {
			return false
		}
	}
	return true
}

A loop using an index variable i is used to iterate over the elements of xs and compare them to the corresponding elements of ys. However, the check to ensure that i is a valid index into ys is incorrectly specified as len(ys) == 0. Instead, the check should ensure that len(ys) is greater than i:

package main

func isPrefixOfGood(xs, ys []int) bool {
	for i := 0; i < len(xs); i++ {
		if len(ys) <= i || xs[i] != ys[i] {
			return false
		}
	}
	return true
}

References

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