Create Pass Through Approximation

Skill: Create PassThrough Approximation

Write passThrough propagation rules for external library methods

Inputs

From the caller; if omitted, fall back to the default. Ask only when a required input is missing and has no sensible default

• Methods to model <methods> — the target method(s) and what each propagates, from the tracking file's methods (all type: passthrough)
• Tracking file <tracking-file> — the passThrough approximation unit. Default: .opentaint/tracking/approximations/<name>.yaml
• Config output <config-file> — where to write the passThrough approximation. Default: .opentaint/pass-through/<name>.yaml
• Test model <test-model> (optional) — any compiled model to dry-run the config against for a load/parse check. Default: .opentaint/project if it exists, else any .opentaint/test-compiled/* model

Workflow

1. Write the passThrough config

Write passThrough: copies into <config-file>. When an object carries taint between calls — a setter stores it and a getter returns it later, or a builder holds it — route through a virtual slot, an access path [<base>, .<DeclaringClass>#<slot>#java.lang.Object]:

• the slot name is nominal — the engine never resolves it, so it need not be a real field
• type it java.lang.Object — a concrete type can fail the read-out type-check and drop the taint
• the writer and reader must name the identical Class#slot#java.lang.Object triple, or the taint drops

Getter / setter pair — the writer stores into the slot, the getter reads the same slot back to result:

passThrough:
- function: org.springframework.http.HttpEntity#setBody
  copy:
  - from: arg(0)
    to:
    - this
    - .org.springframework.http.HttpEntity#Body#java.lang.Object
- function: org.springframework.http.HttpEntity#getBody
  copy:
  - from:
    - this
    - .org.springframework.http.HttpEntity#Body#java.lang.Object
    to: result

Several writers sharing one slot — any of them taints the object, the reader pulls it back:

passThrough:
- function: org.apache.tools.ant.types.FileSet#setDir
  copy:
  - from: arg(0)
    to:
    - this
    - .org.apache.tools.ant.types.FileSet#path#java.lang.Object
- function: org.apache.tools.ant.types.FileSet#setFile
  copy:
  - from: arg(0)
    to:
    - this
    - .org.apache.tools.ant.types.FileSet#path#java.lang.Object

Cross-type builder — when a builder method consumes an argument and returns a different type, carry the taint along both the chained receiver (for further calls on this) and the returned object, slot included. Four copies: arg → returned-value slot, arg → builder slot, whole builder → returned value, builder slot → returned-value slot:

passThrough:
- function: org.springframework.ldap.query.LdapQueryBuilder#filter
  copy:
  - from: arg(0)
    to:
    - result
    - .org.springframework.ldap.query.LdapQuery#filter#java.lang.Object
  - from: arg(0)
    to:
    - this
    - .org.springframework.ldap.query.LdapQueryBuilder#filter#java.lang.Object
  - from: this
    to: result
  - from:
    - this
    - .org.springframework.ldap.query.LdapQueryBuilder#filter#java.lang.Object
    to:
    - result
    - .org.springframework.ldap.query.LdapQuery#filter#java.lang.Object

Builder terminal — a no-arg build() / toX() that returns a new object carrying what the builder accumulated; no argument is involved, so copy each slot from this to the matching slot on result (the setters that filled the builder slot are separate rules of their own):

passThrough:
- function: com.google.common.collect.ImmutableMap$Builder#build
  copy:
  - from:
    - this
    - .java.util.Map#MapKey#java.lang.Object
    to:
    - result
    - .java.util.Map#MapKey#java.lang.Object
  - from:
    - this
    - .java.util.Map#MapValue#java.lang.Object
    to:
    - result
    - .java.util.Map#MapValue#java.lang.Object

Conditional propagation — gate a rule with a condition (the copy still routes through a slot):

passThrough:
- function: com.example.lib.Parser#parse
  condition:
    typeIs: java.lang.String
    pos: arg(0)
  copy:
  - from: arg(0)
    to:
    - this
    - .com.example.lib.Parser#parsed#java.lang.Object

Full config — every function in one top-level passThrough: list (quote [*] — unquoted it parses as a YAML alias):

passThrough:
- function: org.springframework.beans.MutablePropertyValues#add
  copy:
  - from: arg(1)
    to:
    - this
    - .org.springframework.beans.PropertyValue#Value#java.lang.Object
- function: org.springframework.beans.PropertyValue#getValue
  overrides: false
  copy:
  - from:
    - this
    - .org.springframework.beans.PropertyValue#Value#java.lang.Object
    to: result
- function: org.springframework.beans.PropertyValues#getPropertyValues
  copy:
  - from:
    - this
    - .java.lang.Iterable#Element#java.lang.Object
    to:
    - result
    - '[*]'

2. Optional — dry-run the config for load errors

There's no dedicated load-check command. ONLY when invoked standalone — never under the appsec-agent orchestrator, whose subagents must not run opentaint scan (the orchestrator's scan phase verifies the config instead): if a compiled <test-model> is present you can catch YAML load/parse errors early by running a quick scan with the config applied (won't verify propagation — there's no matching flow — only that the config loads):

opentaint scan --project-model <test-model> \
  -o .opentaint/test-results/<name>/passthrough-loadcheck.sarif \
  --ruleset builtin \
  --passthrough-approximations <config-file>

A config error aborts the scan with the parse/load message — fix the YAML and re-run. Nice-to-have, not required; skip it when no model is around

3. Verification is the scan

There's no test project for passThrough. The main scan applies <config-file> and the scan agent reports back. You're re-invoked to fix the config when that scan shows:

• a method you modeled still in dropped-external-methods.yaml → the function matcher didn't match (check package, class, name, overrides), or the from/to doesn't land on the tainted position
• the flow still doesn't surface though the method is no longer dropped → most often a broken channel: the writer and reader name different Class#slot#java.lang.Object triples, or the slot isn't typed java.lang.Object
• a config load / parse error → fix the YAML (an unknown condition key, a bad position, or a 2-part field modifier all fail to load)

Never invoke or grep the analyzer JAR — its internals aren't a stable API; for built-in rules use opentaint health --rules, for everything else the CLI

4. When the config won't converge

After ~2 fix re-invocations without a clearer cause — matcher fields and from/to checked, writer/reader slots confirmed identical, the modeled method no longer in dropped-external-methods.yaml, but the scan still doesn't surface the flow — don't keep guessing at the copy. Report non-convergence to the caller: a passThrough can't express this method's propagation, so the fix is a dataflow approximation for it (a custom dataflow overrides the passThrough). The orchestrator re-plans the method as a dataflow unit and removes this passThrough config before the dataflow one is tested

Output

• The passThrough config at <config-file>
• Tracking updated: written + artifact (per Tracking)
• Report the config path and the methods modeled

Tracking

In <tracking-file>, once the config is written:

artifact: .opentaint/pass-through/<name>.yaml
stages:
  written: done

Do not touch other stages or fields

Reference

Position bases

• this, result, arg(0), arg(1), …
• any(<classifier>) — expands to every argument matching the classifier (a cartesian product across positions, bound consistently), not a single argument. Rare — prefer an explicit arg(N)

Access-path modifiers (list form [<base>, <modifier>])

• .<DeclaringClass>#<slot>#<fieldType> — a field or virtual slot; type it java.lang.Object. The slot name is arbitrary (a descriptive name, or the conventional <rule-storage> for a generic carrier)
• [*] — array element (no leading dot). For java.util collections this does not carry element taint; route it through the conventional .java.lang.Iterable#Element#java.lang.Object slot instead (as the built-in List/Collection models do)

Function matching

• Simple: package.Class#method
• Complex: {package, class, name} — for one hard-to-name function, not for matching many at once (see Gotchas)

Overrides

• overrides: true (default): applies to the class and all subclasses
• overrides: false: exact class only

Conditions (the only keys that load from YAML)

• take a pos: <position>: typeIs, constantMatches, constantEq, tainted
• take the position directly, no pos: field: isConstant, isNull — adding pos: fails to load
• nest other conditions: anyOf, allOf, not
• constantGt / constantLt load but crash the scan when actually evaluated against a constant (their string-typed bound fails an engine type-check) — avoid until fixed

Gotchas

• The # comments in the examples here are for you — don't copy them into the config you write; keep produced YAML comment-free
• The approximation merges with built-ins at the rule level — a provided rule overrides a built-in only if it matches one. Don't redefine a method already in approximated-external-methods.yaml unless debug-rule shows the built-in isn't propagating taint here, then override deliberately
• A wrong argument position copies the wrong value — point from/to at the tainted one
• In doubt about how a method moves taint — which argument or field reaches the result — read the library's source rather than guessing
• Model one function per rule — don't use a regex/wildcard pattern: matcher (e.g. name: get.*, class: .*) or arg(*) to cover many functions at once; it over-models, copying taint through methods you never vetted and manufacturing false positives. Write an explicit function: per method