Hi! I am currently working on a project involving `crosvm`. To allow for rootless networking plumbing I am relying on `passt` (ver 2026_10_02.cba3570). I have found that prolonged (>5 seconds, max. 20 seconds) `https` transfers crash with `crosvm` claiming that the backend device (`passt`) was exiting early. In the instant the program crashes I find this in my dmesg: ``` [Sun Nov 8 21:26:32 2026] audit: type=1326 audit(1794169582.685:13): auid=1005 uid=1005 gid=998 ses=1 subj=unconfined pid=15127 comm="passt.avx2" exe="/gnu/store/cjdgl8d27afhnw34x2z3pwmaxa4sw4jg-passt-2026_10_02.cba3570/bin/passt.avx2" sig=31 arch=c000003e syscall=318 compat=0 ip=0x7f87c63b8735 code=0x80000000 ``` Debugging further my friend KI/AI, they said this expected under the seccomp confinement (no get_random, no brk). Rebuilding with `EXTRA_SYSCALLS=getrandom brk`, shows: ``` 60.4309: Flow 2 (TCP connection): STALLED dropped passt.avx2: tcp_vu.c:294: tcp_vu_sock_recv: Assertion `cnt < ARRAY_SIZE(iov_msg) - j' failed. Bad system call ``` The log: ``` ... spamy output: 342.5609: Flow 2 (TCP connection): timer expires in 1.000s 342.5609: Flow 2 (TCP connection): STALLED 342.5612: Flow 2 (TCP connection): ACK_FROM_TAP_DUE dropped 342.5612: Flow 2 (TCP connection): STALLED dropped 342.5645: Flow 2 (TCP connection): ACK_FROM_TAP_DUE 342.5646: Flow 2 (TCP connection): timer expires in 1.000s 342.5646: Flow 2 (TCP connection): STALLED 342.5647: Flow 2 (TCP connection): ACK_FROM_TAP_DUE dropped 342.5647: Flow 2 (TCP connection): STALLED dropped ... lots more of the same spammy output... 343.0239: Flow 2 (TCP connection): timer expires in 1.000s 343.0249: Flow 2 (TCP connection): timer expires in 1.000s 343.0250: Flow 2 (TCP connection): timer expires in 1.000s 343.0252: Flow 2 (TCP connection): STALLED 343.0252: Flow 2 (TCP connection): ACK_FROM_TAP_DUE dropped 343.0252: Flow 2 (TCP connection): STALLED dropped 343.0257: Flow 2 (TCP connection): ACK_FROM_TAP_DUE 343.0257: Flow 2 (TCP connection): timer expires in 1.000s 343.0258: Flow 2 (TCP connection): STALLED 343.0259: Flow 2 (TCP connection): STALLED dropped 343.0260: Flow 2 (TCP connection): timer expires in 1.000s 343.0261: Flow 2 (TCP connection): ACK_FROM_TAP_DUE dropped passt.avx2: tcp_vu.c:294: tcp_vu_sock_recv: Assertion `cnt < ARRAY_SIZE(iov_msg) - j' failed. Bad system call ``` Working diagnosis: 1. passt's tcp_vu.c assert in tcp_vu_sock_recv() compares a signed cnt against a size_t capacity, so a negative error return trips the assert and kills passt — which crosvm then reports as the backend disconnecting and stops the VM — while the underlying iov_tail_clone() failure never gets printed; 2. seccomp profiles omit getrandom/brk/gettid, so the process dies under SECCOMP_RET_KILL with no message at all. This was all I had time for today, hope this helps! Kind regards, John