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tests/TC161-build-nm-failure.md

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+# Test Case: Build Undefined-Symbol Tool Failure
+
+**Test ID**: TC161-build-nm-failure
+**Category**: Build Qualification
+**Priority**: Critical
+**Regression**: `nm -u | awk` could pass when `nm` itself failed
+
+## Objective
+
+Verify that `build.sh` treats `nm` failure as fatal independently of the awk
+`bcmp` check, safely removes its temporary output, preserves prior module output,
+and still builds both supported configurations normally.
+
+## Procedure
+
+1. On FreeBSD 15, run normal exact-source builds with `WITH_ZSTDIO=0` and 1.
+   Require both SUCCESS lines and save each module hash.
+2. Create a private executable `nm` shim containing:
+
+   ```sh
+   #!/bin/sh
+   exit 73
+   ```
+
+3. Put only that shim directory before the normal system paths and run
+   `WITH_ZSTDIO=0 ./build.sh`, capturing stdout/stderr and exit status.
+4. Require nonzero status, `ERROR: nm failed while checking erofs.ko`, no SUCCESS
+   line, unchanged hash for the previously published `build/erofs.ko`, and no
+   `build/obj/nm-undef.*` file.
+5. Run one final normal build to prove the trap and shim did not contaminate the
+   environment.
+
+## Expected Results
+
+- The shimmed build fails after module linkage and before publication.
+- Awk cannot convert an `nm` execution failure into PASS.
+- Temporary output is removed on failure and normal dual-configuration builds
+  remain successful.