fix: add missing syscalls to GPU device seccomp allowlist

The allowlist was derived from crosvm's gpu_common + gpu_device seccomp
policies, but those are applied after process startup. systemd applies
the filter before exec, so process lifecycle (execve, wait4, arch_prctl,
set_tid_address), capability management (capget, capset), and socket
server (bind, listen, accept4, socketpair) syscalls are also needed.

Also create a shader cache directory at /run/vmsilo/<name>/gpu/cache and
set __GL_SHADER_DISK_CACHE_PATH so the GPU device backend doesn't fail
trying to create /home for shader cache in the sandboxed mount namespace.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docs/superpowers/specs/2026-03-18-cloud-hypervisor-design.md

@@ -164,24 +164,26 @@ RestrictNamespaces = yes
 **Seccomp allowlist** (from crosvm `gpu_common.policy` + `gpu_device.policy`):
 
 ```
-access brk clock_gettime clock_nanosleep clone clone3 close connect
-dup dup2 epoll_create1 epoll_ctl epoll_pwait epoll_wait eventfd2
-exit exit_group fallocate fcntl flock fstat fstatfs ftruncate futex
-getcwd getdents getdents64 getegid geteuid getgid getpid getrandom
-getsockopt gettid gettimeofday getuid inotify_add_watch inotify_init1
-inotify_rm_watch io_uring_enter io_uring_register io_uring_setup ioctl
-kcmp kill lseek lstat madvise membarrier memfd_create mkdir mknodat
-mmap mprotect mremap munmap nanosleep newfstatat open openat pipe2
-poll ppoll prctl pread64 prlimit64 read readlink readlinkat readv
-recvfrom recvmsg rename restart_syscall rseq rt_sigaction rt_sigprocmask
-rt_sigreturn sched_get_priority_max sched_get_priority_min
-sched_getaffinity sched_setaffinity sched_setscheduler sched_yield
-sendmmsg sendmsg sendto set_robust_list setpriority setsockopt shutdown
-sigaltstack socket stat statfs statx sysinfo tgkill uname unlink
-unlinkat userfaultfd write writev
+accept4 access arch_prctl bind brk capget capset clock_gettime
+clock_nanosleep clone clone3 close connect dup dup2 epoll_create1
+epoll_ctl epoll_pwait epoll_wait eventfd2 execve exit exit_group
+fallocate fcntl flock fstat fstatfs ftruncate futex getcwd getdents
+getdents64 getegid geteuid getgid getpgrp getpid getppid getrandom
+getresgid getresuid getsockopt gettid gettimeofday getuid
+inotify_add_watch inotify_init1 inotify_rm_watch io_uring_enter
+io_uring_register io_uring_setup ioctl kcmp kill listen lseek lstat
+madvise membarrier memfd_create mkdir mknodat mmap mprotect mremap
+munmap nanosleep newfstatat open openat pipe2 poll ppoll prctl pread64
+prlimit64 read readlink readlinkat readv recvfrom recvmsg rename
+restart_syscall rseq rt_sigaction rt_sigprocmask rt_sigreturn
+sched_get_priority_max sched_get_priority_min sched_getaffinity
+sched_setaffinity sched_setscheduler sched_yield sendmmsg sendmsg
+sendto set_robust_list set_tid_address setpriority setsockopt shutdown
+sigaltstack socket socketpair stat statfs statx sysinfo tgkill uname
+unlink unlinkat userfaultfd wait4 write writev
 ```
 
-Note: systemd cannot do argument-level filtering like crosvm's seccomp policies (e.g., restricting `socket()` to AF_UNIX, `clone()` to CLONE_THREAD, `mmap` to specific PROT flags). The syscall set is identical but without argument restrictions.
+Note: This allowlist is a superset of crosvm's `gpu_common.policy` + `gpu_device.policy` — it additionally includes process lifecycle syscalls (`execve`, `wait4`, `arch_prctl`, `set_tid_address`), capability management (`capget`, `capset`), and socket server syscalls (`bind`, `listen`, `accept4`) that crosvm's own seccomp doesn't need (crosvm applies its policy after startup; systemd applies it before). systemd also cannot do argument-level filtering like crosvm's seccomp policies (e.g., restricting `socket()` to AF_UNIX, `clone()` to CLONE_THREAD).
 
 **Service dependencies:**
 - `After=vmsilo-<vmname>-wayland-seccontext.service`

modules/services.nix

@@ -58,8 +58,13 @@ let
       vm.gpu;
 
   gpuSyscallAllowlist = [
+    "accept4"
     "access"
+    "arch_prctl"
+    "bind"
     "brk"
+    "capget"
+    "capset"
     "clock_gettime"
     "clock_nanosleep"
     "clone"
@@ -73,6 +78,7 @@ let
     "epoll_pwait"
     "epoll_wait"
     "eventfd2"
+    "execve"
     "exit"
     "exit_group"
     "fallocate"
@@ -88,8 +94,12 @@ let
     "getegid"
     "geteuid"
     "getgid"
+    "getpgrp"
     "getpid"
+    "getppid"
     "getrandom"
+    "getresgid"
+    "getresuid"
     "getsockopt"
     "gettid"
     "gettimeofday"
@@ -103,6 +113,7 @@ let
     "ioctl"
     "kcmp"
     "kill"
+    "listen"
     "lseek"
     "lstat"
     "madvise"
@@ -146,11 +157,13 @@ let
     "sendmsg"
     "sendto"
     "set_robust_list"
+    "set_tid_address"
     "setpriority"
     "setsockopt"
     "shutdown"
     "sigaltstack"
     "socket"
+    "socketpair"
     "stat"
     "statfs"
     "statx"
@@ -160,6 +173,7 @@ let
     "unlink"
     "unlinkat"
     "userfaultfd"
+    "wait4"
     "write"
     "writev"
   ];
@@ -252,6 +266,7 @@ in
               ${pkgs.coreutils}/bin/install -d -m 0755 -o ${toString userUid} -g ${toString userGid} \
                 /run/vmsilo/${vm.name} \
                 /run/vmsilo/${vm.name}/gpu \
+                /run/vmsilo/${vm.name}/gpu/shader-cache \
                 /run/vmsilo/${vm.name}/sound
               ${pkgs.coreutils}/bin/install -d -m 0755 \
                 /run/vmsilo/${vm.name}/virtiofs
@@ -571,6 +586,7 @@ in
 
               environment = {
                 LD_LIBRARY_PATH = "${pkgs.vulkan-loader}/lib";
+                __GL_SHADER_DISK_CACHE_PATH = "/run/vmsilo/${vm.name}/gpu/shader-cache";
               };
 
               serviceConfig = {