Nothing verifies that a sanitized lane actually compiled with CBM_SANITIZED_BUILD (UBSan and trap-UBSan have no probe)

[moffermann]

Summary

Nothing verifies that a sanitized lane actually compiled with CBM_SANITIZED_BUILD defined. #1615 gave
CBM_SANITIZED one spelling and a compiler-probe backstop, but the backstop only covers the three
sanitizers that announce themselves. For UBSan and trap-UBSan there is no probe and never can be, so the
build system is the sole source of truth — and a silent regression there is invisible: the suite still
passes, just against native timing budgets on an instrumented binary.

That is not hypothetical. It is exactly what happened twice:

• trap-UBSan, recorded in the comment above SANITIZED_DEFINE (Makefile.cbm:88-96): the leg "once
  ran NATIVE timing budgets on an instrumented binary".
• TSan, fixed in 0a163d4f: CFLAGS_TSAN never included SANITIZED_DEFINE, so every sanitized
  budget compiled to its native value. It surfaced as subprocess_run_spawn_failure failing on a PR
  whose diff could not possibly have caused it — twice on one SHA — and cost two more PRs (fix(config): self-heal an orphan managed marker on removal (#1558) #1590, fix(subprocess+tsan): widen the spawn-retry window where it was silently disabled #1595)
  to chase down.

Opening this per CONTRIBUTING.md ("Open an issue first — always"; build-system changes need prior
design discussion). Happy to implement once there is a decision on shape — nothing is written yet.

Why the existing mechanisms do not cover it

mechanism	covers	does not cover
SANITIZED_DEFINE (Makefile.cbm:93-96)	any lane keyed off $(SANITIZE)	a lane with its own sanitize variable — precisely the TSan gap
probes in sanitized.h (#1615)	ASan, TSan, MSan	UBSan, trap-UBSan — no macro, no __has_feature bit exists
the test suite passing	nothing here	a native budget on an instrumented binary is looser to violate, not tighter; it mostly still passes

The third row is the important one. The failure mode is a test that passes for months and then flakes on
a loaded runner, attributed to whatever PR happened to be in flight.

It is not only about budgets any more

scripts/msan.sh:120-129 reasons from this macro while ruling out a cause:

CBM_THREAD_STACK_MB at 256 MiB and at 1024 MiB (the cap). The knob is verified compiled in
(CBM_SANITIZED_BUILD is defined for this lane) and the floor is applied in cbm_thread_create [...]
The fault address did not move by a single byte between 256 MiB and 1024 MiB — a 4x stack increase
changing nothing is what rules out "the stack is merely too small".

If the define were ever absent on that lane, cbm_thread_stack_floor would compile to return requested
(src/foundation/compat_thread.c:33-48), the 4x increase would have changed nothing because the knob was
inert, and a documented "do not repeat" conclusion would be wrong. The premise is asserted in a comment;
nothing checks it.

Proposed shape

Three small pieces, and no changes under .github/workflows/ at all — which is the point of this shape.

1. The binary reports what it was compiled with. A --build-config early-exit in
tests/test_main.c, next to the existing --version (line ~796) and --list-suites (line ~861),
printing one line of key=value:

$ build/c/test-runner --build-config
sanitized=1 test_seams=1

sanitized comes straight from CBM_SANITIZED, so it answers the only question that matters: what did
the compiler actually receive, not what the Makefile intended to send.

2. scripts/test.sh asserts it. This is the single choke point — every sanitized lane in
_test.yml already goes through it:

leg	_test.yml	invocation	expected
test-unix	:93	scripts/test.sh CC=… CXX=… (default ASan+UBSan)	sanitized=1
test-diag	:221	scripts/test.sh … SANITIZE="-fsanitize=address,undefined …"	sanitized=1
test-lsan-macos	:308	scripts/test.sh … (ASan+LSan)	sanitized=1
test-tsan	:340	scripts/test.sh --tsan …	sanitized=1
Windows CLANGARM64	test.sh:173-177	trap-UBSan injected by the script itself	sanitized=1
explicit plain build	—	scripts/test.sh SANITIZE=	sanitized=0

test.sh already knows the answer: it tracks SANITIZE_GIVEN (:147), the --tsan mode (:98, :179-185)
and the ARM64 trap-UBSan default (:173-177). So it can compute the expectation from the flags it is about
to pass to make, run --build-config on the binary it just built, and fail loudly on mismatch, before the
suite runs:

ERROR: build config mismatch — this lane builds instrumented but the binary reports sanitized=0.
       Every sanitized-budget branch compiled to its NATIVE value.
       Check that this lane's flag set carries $(SANITIZED_DEFINE) or -DCBM_SANITIZED_BUILD=1.

Note the TSan mode must check $BUILD_DIR/test-runner-tsan, not test-runner.

3. scripts/msan.sh gets the same one-liner. It is the only lane that bypasses test.sh
(_test.yml:284 → Docker → msan.sh, running build/msan/test-runner). One check next to the build at
msan.sh:66-68 — and it would turn the comment quoted above into something enforced.

What this would have caught, and how fast

The TSan gap would have failed test-tsan in seconds, at the build step, with a message naming the cause
— instead of surfacing as an unrelated-looking flaky spawn test across three PRs.

Limits, stated up front

• It proves the define reached the compiler, not that the sanitizer is instrumenting. That was never
  the failure mode, and -fsanitize= not working would fail far louder.
• It cannot cover a future lane that routes around both scripts. Partial mitigation: the fix(build): give sanitized-build detection one spelling and a backstop #1615 probes
  still self-heal ASan/TSan/MSan, so a new lane would have to be UBSan-only to slip through.
• It adds one flag to a test-only binary. No production surface.
• test-windows (_test.yml:398) runs unsanitized, so it asserts sanitized=0 — which is also worth
  having: it catches the inverse mistake of leaking the define into a native lane.

Alternatives considered

1. #error in sanitized.h when a probe fires without the define. Rejected in fix(build): give sanitized-build detection one spelling and a backstop #1615, for reasons
   that still hold: it breaks the binary instead of correcting it, and breaks an out-of-tree
   make CFLAGS_EXTRA=-fsanitize=address. It is also blind to UBSan, the case this issue is really about.
2. Env var (CBM_EXPECT_SANITIZED=1) plus an in-suite test. A lane that forgets the variable skips
   the check silently — the same silence that caused the problem. Inverting the default (fail when unset)
   breaks every ad-hoc local run.
3. Grep Makefile.cbm in CI for the define on each flag set. Verifies the intent, not the binary, and
   would have missed nothing only by accident — a flag set can also be assembled in a script.
4. A make print-flags target compared in CI. Same weakness: it reports what make would pass, not
   what the compiler received, and it needs the CI workflow edits this shape avoids.

Scope

Three files: tests/test_main.c, scripts/test.sh, scripts/msan.sh. Roughly 40-60 lines including the
error text. One issue, one PR, per CONTRIBUTING.md.

Found while doing #1615 and #1618; not blocking either.

[DeusData]

This is the right generalisation, and it comes from a real miss on our side rather than a hypothetical.

#1595 widened the spawn-retry budget for sanitized builds and it did nothing on the TSan leg, because CBM_SANITIZED_BUILD was never defined there — SANITIZED_DEFINE keys off $(SANITIZE), and TSan uses its own TSAN_SANITIZE. 169 thread-instrumented compile lines, not one carrying the define. The fix was a one-line Makefile change; the expensive part was that nothing could tell us the budget was inert, so the leg kept failing on a PR whose diff made causation impossible.

Your #1615 then found a fourth spelling I had missed entirely (tests/test_cs_lsp_bench.c checking ASan only, so TSan and MSan measured instrumented parses against a native 200 ms budget). Two independent instances of the same class inside a week is the argument for asserting it rather than remembering it.

A runtime or build-time assertion that a lane which is instrumented also knows it is instrumented would have caught both at the moment they were introduced instead of after a release cycle. The Makefile comment already warns about exactly this for trap-UBSan — the lesson was written down and then not enforced, which is precisely the gap a check closes.

Fair warning on timing: our CI is badly backed up tonight behind a v0.10.5 that fixes a batch of install and startup blockers, so this will get proper attention after that lands. Thank you for filing it as an issue with the analysis rather than just patching around it — and for the three accurate reports on my code this week.