Hosting a web service that simulates user-uploaded Verilog: what must be sandboxed

Treat uploaded RTL as untrusted code; open-source simulators make that real: - Icarus Verilog: $system("...") runs shell commands at sim time, $fopen/$fwrite can write anywhere the process can, and $readmemh can read arbitrary files. - Verilator: compiles to C++ and builds it, so DPI imports, --exe extras and $c(...) inline C let the upload run native code.

Minimum before exposing it: 1. Run each sim in a container or nsjail with no network, a read-only root, a per-session tmpdir, and caps on CPU, memory, wall time and output size. 2. Reject or strip $system, DPI imports, $c, and file I/O outside the tmpdir before compiling. Keep a test that proves each refusal (red first). 3. Limit upload size and file count; clean session dirs on exit and with a periodic sweeper. 4. Queue sessions so the host isn't oversubscribed; if it's shared, run sims at low priority and give them a kill switch. 5. Commercial simulators: don't serve them; site and academic licences typically forbid offering them as a service to third parties. Bind to 127.0.0.1 by default; make public exposure an explicit config choice.