feat(model): a bound on the data lowers the rung, it does not clear it

contributor-scan 2.12. c/injection and c/memory carried zero sanitizers,
so no C flow could ever be discharged: every `argv -> strcpy` entailed
however carefully it was bounded.

Which sinks can be discharged this way is read from the sink fact's own
CWE rather than declared: CWE-121 is a stack buffer overflow, so
gets/sprintf/strcat/strcpy qualify. It is the only overflow CWE in the
fact tables and it is C-only, so no Python or JavaScript sink is touched
and the pair corpus is unaffected -- gates byte-identical.

Not a sanitizer list, because `strlen` appears in the guarded and the
unguarded case alike; the discharge is a guard, not a call.

**The first implementation required the bound to DOMINATE the sink, and it
found nothing.** libpng guards `strcpy(outname+len, ".png")` with
`(len = strlen(inname)) > 250`, but that check sits on an else-if chain
whose sibling branch skips it entirely, and the protection reaches the
sink through an `error` flag: `++error` in the guarded branch, `if
(!error)` around the copy. Joern's dominator set for that sink has 89
calls and correctly excludes the bound; `controlledBy` returns none
either. Verified against the CPG before the rule was changed rather than
after it failed.

So the bound is searched in the method, and the consequence is
corroborated rather than silence. That is the honest rung: a bound on this
variable means the graph has NOT shown the copy is unguarded, and it has
not shown it is guarded either -- off-by-one being how bounds fail. The
judge is asked. Proving the flag-mediated case needs path-sensitive
reasoning this arbiter does not do.

  libpng entailed   3 -> 1

The survivor is also a false positive: `strcpy(output + len, ".icc")` in
iccfrompng.c, where `output = malloc(len + 5)` makes it fit exactly. That
is a different discharge shape -- allocated to size, not bounds-checked --
and it is recorded as 2.13 rather than chased. Both shapes were found on
one repository, and extending a rule until that repository is clean is the
overfitting Req 8 exists to prevent; 2.13 requires a second C checkout
with a genuine CWE-121 so the change can be measured for what it silences
as well as what it clears.

Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>
Claude-Session: https://claude.ai/code/session_01T7XD7x5KonMdmGRkp63EQG

src/openultrasast/model/taint.py

@@ -40,6 +40,7 @@ def request_params(
         "file": file,
         "parameterSources": "true" if parameter_sources else "false",
         "callDepth": str(call_depth),
+        "boundedSinks": spec.bounded_sinks,
     }
 
 
@@ -76,10 +77,18 @@ def verdict(
     # still vulnerable on the strength of a flow that names no attacker input.
     modelled = [flow for flow in flows if _from_modelled_source(flow, spec)]
     considered = modelled or flows
-    unsanitized = [flow for flow in considered if not flow["sanitized"]]
+    # A flow the graph cannot see a discharge for is the finding; a sanitized or BOUNDED one is a question.
+    # libpng bounds `strcpy(outname+len, ".png")` with `(len = strlen(inname)) > 250` against a char[256],
+    # and entailing it is a claim the code contradicts. But the bound reaches the sink through an `error`
+    # flag rather than by dominating it, so the graph has not shown the copy is guarded either -- and a
+    # bound is no proof of safety in any case, off-by-one being the classic way they fail. Corroborated is
+    # the honest rung: the model cannot assert this, so the judge is asked instead of the site being
+    # silenced.
+    open_flows = [flow for flow in considered if not flow["sanitized"] and not flow["bounded"]]
     # Deterministic by construction: a total order, then take the first. Nothing depends on engine ordering.
-    chosen = min(unsanitized or considered, key=lambda flow: _rank(flow, spec))
-    rung = Rung.ENTAILED if not chosen["sanitized"] else Rung.CORROBORATED
+    chosen = min(open_flows or considered, key=lambda flow: _rank(flow, spec))
+    settled = bool(chosen["sanitized"]) or bool(chosen["bounded"])
+    rung = Rung.CORROBORATED if settled else Rung.ENTAILED
     return Verdict(rung=rung, family=spec.family, witness=_witness(chosen), location=_location(chosen))
 
 
@@ -102,6 +111,8 @@ def _flows(rows: object, *, function: str) -> list[dict[str, object]]:
                 "sinkLine": str(row.get("sinkLine", "")),
                 "sinkMethod": str(row.get("sinkMethod", "")),
                 "sinkFile": str(row.get("sinkFile", "")),
+                "bounded": bool(row.get("bounded", False)),
+                "bound": str(row.get("bound", "")),
                 "source": str(row.get("source", "")),
                 "sanitized": bool(row.get("sanitized", False)),
                 "length": _as_int(row.get("length")),
@@ -172,7 +183,9 @@ def _witness(flow: Mapping[str, object]) -> str:
     line = flow.get("sinkLine") or "?"
     where = str(flow.get("sinkFile") or "")
     at = f" in {where}" if where else ""
-    return f"{flow['source']} -> {flow['sink']}{at} (line {line}, {flow['length']} steps)"
+    bound = str(flow.get("bound") or "")
+    governed = f", governed by `{bound}`" if flow.get("bounded") and bound else ""
+    return f"{flow['source']} -> {flow['sink']}{at} (line {line}, {flow['length']} steps{governed})"
 
 
 def _location(flow: Mapping[str, object]) -> str: