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tests/TC159-special-setattr-combinations.md

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+# Test Case: Special Vnode Combined Setattr Rejection
+
+**Test ID**: TC159-special-setattr-combinations
+**Category**: Read-Only Vnode Operations
+**Priority**: Critical
+**Regression**: A special-vnode size field caused early success and hid other requested mutations
+
+## Objective
+
+Verify that size-only setattr remains an ignored no-op for special vnodes, while
+a single `VOP_SETATTR` containing size plus mode, ownership, timestamps, or all
+three classes returns `EROFS`.
+
+## Fixture and Probe
+
+Use `special-setattr.erofs` and its FIFO `/special.fifo`. Build the kernel-side
+probe against the same FreeBSD 15 source tree:
+
+```sh
+mkdir setattr-probe
+cp tests/final_review_setattr_probe.c setattr-probe/
+cp tests/final_review_setattr_probe.mk setattr-probe/Makefile
+make -C setattr-probe SYSDIR=/path/to/freebsd/sys
+```
+
+The probe performs one locked vnode lookup and one `VOP_SETATTR` per sysctl
+trigger, avoiding userspace syscalls that split attributes across operations.
+
+## Procedure
+
+1. Mount the fixture read-only and record mode, uid, gid, atime, and mtime.
+2. Load `erofs_setattr_probe.ko` and set
+   `debug.erofs_setattr_probe.path` to the FIFO.
+3. Trigger operation 1 (size only); require result 0.
+4. Trigger operations 2 through 5 (size+mode, size+owner, size+times, all);
+   require result 30 (`EROFS`) after each trigger.
+5. Require the metadata snapshot to remain unchanged, then unload the probe,
+   unmount, detach, and unload EROFS.
+
+## Expected Results
+
+- Size-only special-vnode setattr returns 0.
+- Every combined mutation returns `EROFS`, never false success.
+- No mode, ownership, or timestamp changes are observable.