diff --git a/dmoj/cptbox/_cptbox.pyx b/dmoj/cptbox/_cptbox.pyx
index cf36c9136..837ea34d0 100644
--- a/dmoj/cptbox/_cptbox.pyx
+++ b/dmoj/cptbox/_cptbox.pyx
@@ -120,7 +120,7 @@ cdef extern from 'helper.h' nogil:
         int stderr_
         bool use_seccomp
         int abi_for_seccomp
-        bint *seccomp_whitelist
+        int *seccomp_handlers
 
     void cptbox_closefrom(int lowfd)
     int cptbox_child_run(child_config *)
@@ -474,14 +474,14 @@ cdef class Process:
     cpdef _cpu_time_exceeded(self):
         pass
 
-    cpdef _get_seccomp_whitelist(self):
-        raise NotImplementedError()
+    cpdef _get_seccomp_handlers(self):
+        return [-1] * MAX_SYSCALL
 
     cpdef _spawn(self, file, args, env=(), chdir=''):
         cdef child_config config
         config.argv = NULL
         config.envp = NULL
-        config.seccomp_whitelist = NULL
+        config.seccomp_handlers = NULL
 
         try:
             config.address_space = self._child_address
@@ -500,20 +500,22 @@ cdef class Process:
 
             config.use_seccomp = self._use_seccomp()
             if config.use_seccomp:
-                whitelist = self._get_seccomp_whitelist()
-                assert len(whitelist) == MAX_SYSCALL
-                config.seccomp_whitelist = <bint*>malloc(sizeof(bint) * MAX_SYSCALL)
-                if not config.seccomp_whitelist:
+                handlers = self._get_seccomp_handlers()
+                assert len(handlers) == MAX_SYSCALL
+
+                config.seccomp_handlers = <int*>malloc(sizeof(int) * MAX_SYSCALL)
+                if not config.seccomp_handlers:
                     PyErr_NoMemory()
+
                 for i in range(MAX_SYSCALL):
-                    config.seccomp_whitelist[i] = whitelist[i]
+                    config.seccomp_handlers[i] = handlers[i]
 
             if self.process.spawn(pt_child, &config):
                 raise RuntimeError('failed to spawn child')
         finally:
             free(config.argv)
             free(config.envp)
-            free(config.seccomp_whitelist)
+            free(config.seccomp_handlers)
 
     cpdef _monitor(self):
         cdef int exitcode
diff --git a/dmoj/cptbox/handlers.py b/dmoj/cptbox/handlers.py
index 25fad47e2..34971f7b2 100644
--- a/dmoj/cptbox/handlers.py
+++ b/dmoj/cptbox/handlers.py
@@ -1,25 +1,29 @@
 import errno
 
+from dmoj.cptbox._cptbox import Debugger
+
 DISALLOW = 0
 ALLOW = 1
 _CALLBACK = 2
 STDOUTERR = 3
 
 
-def errno_handler(name, code):
-    def handler(debugger):
+class ErrnoHandlerCallback:
+    errno: int
+    error_name: str
+
+    def __init__(self, error_name: str, errno: int) -> None:
+        self.errno = errno
+        self.error_name = error_name
+
+    def __call__(self, debugger: Debugger) -> bool:
         def on_return():
-            debugger.errno = code
+            debugger.errno = self.errno
 
         debugger.syscall = -1
         debugger.on_return(on_return)
         return True
 
-    handler.error_name = name
-    handler.errno = code
-    return handler
-
 
-for err in dir(errno):
-    if err[0] == 'E':
-        globals()['ACCESS_%s' % err] = errno_handler(err, getattr(errno, err))
+for code, name in errno.errorcode.items():
+    globals()[f'ACCESS_{name}'] = ErrnoHandlerCallback(name, code)
diff --git a/dmoj/cptbox/handlers.pyi b/dmoj/cptbox/handlers.pyi
index a9536dd20..61f168630 100644
--- a/dmoj/cptbox/handlers.pyi
+++ b/dmoj/cptbox/handlers.pyi
@@ -1,13 +1,19 @@
-from typing import Any
+from dmoj.cptbox._cptbox import Debugger
 
 ALLOW: int
 DISALLOW: int
 _CALLBACK: int
 STDOUTERR: int
-ACCESS_EACCES: Any
-ACCESS_EAGAIN: Any
-ACCESS_EFAULT: Any
-ACCESS_EINVAL: Any
-ACCESS_ENOENT: Any
-ACCESS_EPERM: Any
-ACCESS_ENAMETOOLONG: Any
+
+class ErrnoHandlerCallback:
+    errno: int
+    error_name: str
+    def __call__(self, debugger: Debugger) -> bool: ...
+
+ACCESS_EACCES: ErrnoHandlerCallback
+ACCESS_EAGAIN: ErrnoHandlerCallback
+ACCESS_EFAULT: ErrnoHandlerCallback
+ACCESS_EINVAL: ErrnoHandlerCallback
+ACCESS_ENOENT: ErrnoHandlerCallback
+ACCESS_EPERM: ErrnoHandlerCallback
+ACCESS_ENAMETOOLONG: ErrnoHandlerCallback
diff --git a/dmoj/cptbox/helper.cpp b/dmoj/cptbox/helper.cpp
index 9e660cafa..7e49b0f21 100644
--- a/dmoj/cptbox/helper.cpp
+++ b/dmoj/cptbox/helper.cpp
@@ -93,9 +93,16 @@ int cptbox_child_run(const struct child_config *config) {
         }
 
         for (int syscall = 0; syscall < MAX_SYSCALL; syscall++) {
-            if (config->seccomp_whitelist[syscall]) {
+            int handler = config->seccomp_handlers[syscall];
+            if (handler == 0) {
                 if ((rc = seccomp_rule_add(ctx, SCMP_ACT_ALLOW, syscall, 0))) {
-                    fprintf(stderr, "seccomp_rule_add(..., %d): %s\n", syscall, strerror(-rc));
+                    fprintf(stderr, "seccomp_rule_add(..., SCMP_ACT_ALLOW, %d): %s\n", syscall, strerror(-rc));
+                    // This failure is not fatal, it'll just cause the syscall to trap anyway.
+                }
+            } else if (handler > 0) {
+                if ((rc = seccomp_rule_add(ctx, SCMP_ACT_ERRNO(handler), syscall, 0))) {
+                    fprintf(stderr, "seccomp_rule_add(..., SCMP_ACT_ERRNO(%d), %d): %s\n",
+                        handler, syscall, strerror(-rc));
                     // This failure is not fatal, it'll just cause the syscall to trap anyway.
                 }
             }
diff --git a/dmoj/cptbox/helper.h b/dmoj/cptbox/helper.h
index ff6219ff0..b8558e029 100644
--- a/dmoj/cptbox/helper.h
+++ b/dmoj/cptbox/helper.h
@@ -22,7 +22,7 @@ struct child_config {
     int stdout_;
     int stderr_;
     bool use_seccomp;
-    int *seccomp_whitelist;
+    int *seccomp_handlers;
 };
 
 void cptbox_closefrom(int lowfd);
diff --git a/dmoj/cptbox/isolate.py b/dmoj/cptbox/isolate.py
index f3cff3fd0..e1391542f 100644
--- a/dmoj/cptbox/isolate.py
+++ b/dmoj/cptbox/isolate.py
@@ -1,8 +1,9 @@
 import logging
 import os
 import sys
+from typing import Optional, Tuple
 
-from dmoj.cptbox._cptbox import AT_FDCWD, bsd_get_proc_cwd, bsd_get_proc_fdno
+from dmoj.cptbox._cptbox import AT_FDCWD, Debugger, bsd_get_proc_cwd, bsd_get_proc_fdno
 from dmoj.cptbox.filesystem_policies import FilesystemPolicy
 from dmoj.cptbox.handlers import (
     ACCESS_EACCES,
@@ -12,9 +13,10 @@
     ACCESS_ENOENT,
     ACCESS_EPERM,
     ALLOW,
+    ErrnoHandlerCallback,
 )
 from dmoj.cptbox.syscalls import *
-from dmoj.cptbox.tracer import MaxLengthExceeded
+from dmoj.cptbox.tracer import HandlerCallback, MaxLengthExceeded
 from dmoj.utils.unicode import utf8text
 
 log = logging.getLogger('dmoj.security')
@@ -194,7 +196,7 @@ def __init__(self, read_fs, write_fs=None, writable=(1, 2)):
     def _compile_fs_jail(self, fs):
         return FilesystemPolicy(fs or [])
 
-    def is_write_flags(self, open_flags):
+    def is_write_flags(self, open_flags: int) -> bool:
         for flag in open_write_flags:
             # Strict equality is necessary here, since e.g. O_TMPFILE has multiple bits set,
             # and O_DIRECTORY & O_TMPFILE > 0.
@@ -203,8 +205,8 @@ def is_write_flags(self, open_flags):
 
         return False
 
-    def check_file_access(self, syscall, argument, is_open=False):
-        def check(debugger):
+    def check_file_access(self, syscall, argument, is_open=False) -> HandlerCallback:
+        def check(debugger: Debugger) -> bool:
             file_ptr = getattr(debugger, 'uarg%d' % argument)
             try:
                 file = debugger.readstr(file_ptr)
@@ -224,8 +226,8 @@ def check(debugger):
 
         return check
 
-    def check_file_access_at(self, syscall, is_open=False):
-        def check(debugger):
+    def check_file_access_at(self, syscall, is_open=False) -> HandlerCallback:
+        def check(debugger: Debugger) -> bool:
             try:
                 file = debugger.readstr(debugger.uarg1)
             except MaxLengthExceeded as e:
@@ -244,7 +246,9 @@ def check(debugger):
 
         return check
 
-    def _file_access_check(self, rel_file, debugger, is_open, flag_reg=1, dirfd=AT_FDCWD):
+    def _file_access_check(
+        self, rel_file, debugger, is_open, flag_reg=1, dirfd=AT_FDCWD
+    ) -> Tuple[str, Optional[ErrnoHandlerCallback]]:
         # Either process called open(NULL, ...), or we failed to read the path
         # in cptbox.  Either way this call should not be allowed; if the path
         # was indeed NULL we can end the request before it gets to the kernel
@@ -304,7 +308,7 @@ def _file_access_check(self, rel_file, debugger, is_open, flag_reg=1, dirfd=AT_F
 
         return real, None
 
-    def get_full_path(self, debugger, file, dirfd=AT_FDCWD):
+    def get_full_path(self, debugger: Debugger, file: str, dirfd: int = AT_FDCWD) -> str:
         dirfd = (dirfd & 0x7FFFFFFF) - (dirfd & 0x80000000)
         if not file.startswith('/'):
             dir = self._getcwd_pid(debugger.pid) if dirfd == AT_FDCWD else self._getfd_pid(debugger.pid, dirfd)
@@ -312,15 +316,15 @@ def get_full_path(self, debugger, file, dirfd=AT_FDCWD):
         file = '/' + os.path.normpath(file).lstrip('/')
         return file
 
-    def do_kill(self, debugger):
+    def do_kill(self, debugger: Debugger) -> bool:
         # Allow tgkill to execute as long as the target thread group is the debugged process
         # libstdc++ seems to use this to signal itself, see <https://github.com/DMOJ/judge/issues/183>
         return True if debugger.uarg0 == debugger.pid else ACCESS_EPERM(debugger)
 
-    def do_prlimit(self, debugger):
+    def do_prlimit(self, debugger: Debugger) -> bool:
         return True if debugger.uarg0 in (0, debugger.pid) else ACCESS_EPERM(debugger)
 
-    def do_prctl(self, debugger):
+    def do_prctl(self, debugger: Debugger) -> bool:
         PR_GET_DUMPABLE = 3
         PR_SET_NAME = 15
         PR_GET_NAME = 16
diff --git a/dmoj/cptbox/tracer.py b/dmoj/cptbox/tracer.py
index 7e6e681c6..608e491d6 100644
--- a/dmoj/cptbox/tracer.py
+++ b/dmoj/cptbox/tracer.py
@@ -6,10 +6,10 @@
 import subprocess
 import sys
 import threading
-from typing import List, Optional
+from typing import Callable, List, Optional
 
 from dmoj.cptbox._cptbox import *
-from dmoj.cptbox.handlers import ALLOW, DISALLOW, _CALLBACK
+from dmoj.cptbox.handlers import ALLOW, DISALLOW, ErrnoHandlerCallback, _CALLBACK
 from dmoj.cptbox.syscalls import SYSCALL_COUNT, by_id, sys_exit, sys_exit_group, sys_getpid, translator
 from dmoj.utils.communicate import safe_communicate as _safe_communicate
 from dmoj.utils.os_ext import OOM_SCORE_ADJ_MAX, oom_score_adj
@@ -39,6 +39,8 @@
     PTBOX_ABI_FREEBSD_X64: 64,
 }
 
+HandlerCallback = Callable[[Debugger], bool]
+
 
 class MaxLengthExceeded(ValueError):
     pass
@@ -166,8 +168,8 @@ def __init__(
         if self._spawn_error:
             raise self._spawn_error
 
-    def _get_seccomp_whitelist(self):
-        whitelist = [False] * MAX_SYSCALL_NUMBER
+    def _get_seccomp_handlers(self):
+        handlers = [-1] * MAX_SYSCALL_NUMBER
         index = _SYSCALL_INDICIES[NATIVE_ABI]
         for i in range(SYSCALL_COUNT):
             # Ensure at least one syscall traps.
@@ -178,9 +180,11 @@ def _get_seccomp_whitelist(self):
             for call in translator[i][index]:
                 if call is None:
                     continue
-                if isinstance(handler, int):
-                    whitelist[call] = handler == ALLOW
-        return whitelist
+                if isinstance(handler, int) and handler == ALLOW:
+                    handlers[call] = 0
+                elif isinstance(handler, ErrnoHandlerCallback):
+                    handlers[call] = handler.errno
+        return handlers
 
     def wait(self):
         self._died.wait()
diff --git a/testsuite/helloworld/tests/sandbox_py3_mkdir/helloworld.py b/testsuite/helloworld/tests/sandbox_py3_mkdir/helloworld.py
new file mode 100644
index 000000000..4f6d56aa3
--- /dev/null
+++ b/testsuite/helloworld/tests/sandbox_py3_mkdir/helloworld.py
@@ -0,0 +1,2 @@
+import os
+os.mkdir('test')
diff --git a/testsuite/helloworld/tests/sandbox_py3_mkdir/test.yml b/testsuite/helloworld/tests/sandbox_py3_mkdir/test.yml
new file mode 100644
index 000000000..36d143c1e
--- /dev/null
+++ b/testsuite/helloworld/tests/sandbox_py3_mkdir/test.yml
@@ -0,0 +1,6 @@
+language: PY3
+time: 2
+memory: 65536
+source: helloworld.py
+expect: IR
+feedback: PermissionError