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Constant propagation

This analysis tracks whether a variable must carry a constant value at a given point in the program. Semgrep then performs constant folding when matching literal patterns. Semgrep can track Boolean, numeric, and string constants.

Semgrep AppSec Platform supports interprocedural (cross function), interfile (cross file) constant propagation. Semgrep Community Edition (CE) supports intrafile (single file) constant propagation.

For example:

metavariable-comparison​

Using constant propagation, the metavariable-comparison operator works with any constant variable, instead of just literals.

For example:

Mutable objects​

In general, Semgrep assumes that constant objects are immutable and won't be modified by function calls. This may lead to false positives, especially in languages where strings are mutable such as C and Ruby.

The only exceptions are method calls whose returning value is ignored. In these cases, Semgrep assumes that the method call may be mutating the callee object. This helps reducing false positives in Ruby. For example:

If constant propagation doesn't seem to work, consider whether the constant may be unexpectedly mutable. For example, given the following rule designed to taint the REGEX class variable:

rules:
  - id: redos-detection
    message: Potential ReDoS vulnerability detected with $REGEX
    severity: ERROR
    languages:
      - java
    mode: taint
    options:
      symbolic_propagation: true
    pattern-sources:
      - patterns:
          - pattern: $REDOS
          - metavariable-analysis:
              analyzer: redos
              metavariable: $REDOS
    pattern-sinks:
      - pattern: Pattern.compile(...)

Semgrep fails to match its use in Test2 when presented with the following code:

import java.util.regex.Pattern;

public String REGEX = "(a+)+$";

public class Test2 {
    public static void main(String[] args) {
        Pattern pattern = Pattern.compile(REGEX);
    }
}

However, if you change the variable from public to private, Semgrep does return a match:

import java.util.regex.Pattern;

private String REGEX = "(a+)+$";

public class Test2 {
    public static void main(String[] args) {
        Pattern pattern = Pattern.compile(REGEX);
    }
}

Because REGEX is public in the first code snippet, Semgrep doesn't propagate its value to other classes on the assumption that it could have mutated. However, in the second example, Semgrep understands that REGEX is private and is only assigned to once. Therefore, Semgrep assumes it to be immutable.

The rule would also work with:

...
public final String REGEX = "(a+)+$";
...

Disable constant propagation​

You can disable constant propagation in a per-rule basis using rule options: by setting constant_propagation: false.

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