EMUFI v1: An ElectroMagnetic Upset Fault Injector for Linux Filesystem Resilience Testing

Technical Report v1. We present EMUFI, an electromagnetic fault injection operator for the empirical validation of filesystem resilience claims under multi-bit upset (MBU) regimes. EMUFI is the direct successor of RadFI, refocused along three axes: (i) the threat model is restated from radiation-only single-event upsets to full electromagnetic perturbation; (ii) the perturbation operator is extended from single-bit-flip per bio to multi-bit upset events with a DRAM-organization-aware stride model (stride = 8 bits, calibrated to the DDR4 x8 device organization standard); and (iii) the per-event upset width is sampled from a class-conditional Weibull distribution calibrated to the LET ladder of canonical SEE rate references (JEDEC JEP89A). The reference implementation is a 1.0 kLoC Linux kernel module (emufi.ko) using kprobe-based runtime instrumentation on submit_bh (filesystem layer) and submit_bio_noacct (block layer). The full operator state, including a 4096-event forensic ring buffer, is exposed through debugfs for deterministic, replay-bound experimentation. Three empirical results are reported: (1) in-vivo causality and stride exactness on btrfs (7 events / 33 flips, 100% XOR consistency, 100% stride exactness); (2) userspace Weibull sampler validation at N=10^5 per LET class with maximum 0.14% deviation, within binomial noise; (3) a single-cell ext4 case study where 153 single-bit flips at LOW LET drive a 1 GiB-workload volume to a state where the standard kernel mount path declares group descriptors corrupted. The reference implementation source code is held in a private repository (roastercode/emufi) pending public release.