Seccomp

Linux / 19 Sep 2026 / 5 min read

seccomp User Notifications Move Selected System Calls to a Supervisor

seccomp User Notifications Move Selected System Calls to a Supervisor A seccomp filter can do more than allow a system call or reject it in the kernel. When a filter returns SECCOMP_RET_USER_NOTIF, Linux can suspend the calling thread and deliver a description of that attempted system call to a user-space supervisor. The mechanism creates an interposition boundary around selected calls. It is useful when a less-privileged process needs an operation mediated by another process, such as a container manager handling a call that the container cannot perform directly. The boundary is deliberately narrower than a general security-policy engine: notification state can race with mutable target memory, and the kernel documentation warns against treating the supervisor’s inspection as an authorization primitive.

Linux seccomp

Linux / 18 Sep 2026 / 5 min read

Seccomp User Notifications Delegate Selected System Calls to a Supervisor

A seccomp filter can stop a selected system call before execution and turn it into a request on a listener file descriptor. The calling thread remains blocked while a userspace supervisor examines the notification and returns a result. This creates a mediation boundary that is narrower than tracing every system call and more dynamic than encoding every decision directly in classic BPF. The mechanism is SECCOMP_RET_USER_NOTIF. A filter returns that action for operations that require external mediation. A filter installed with SECCOMP_FILTER_FLAG_NEW_LISTENER yields a listener file descriptor, and a supervisor uses seccomp notification ioctls on that descriptor.

Linux Seccomp

Cybersecurity / 18 Sep 2026 / 7 min read

Seccomp User Notification Moves Selected System Calls Behind a Supervisor Decision

A sandboxed process may need an operation that cannot be represented safely as a permanent seccomp allow rule. The operation can depend on runtime policy, external state, or a resource that only a more privileged component should inspect. Allowing the system call unconditionally widens the sandbox, while rejecting it removes required functionality. Linux seccomp user notification provides a mediation point for this case. A seccomp filter can return SECCOMP_RET_USER_NOTIF for selected calls. The kernel then blocks the triggering task and emits a notification through a listener file descriptor. A supervisor reads that notification and sends a response that determines the immediate disposition of the intercepted call.

Cybersecurity Linux Security

Cybersecurity / 18 Sep 2026 / 5 min read

Seccomp User Notification Delegates Syscall Execution Across a Privilege Boundary

A confined process can reach a syscall that the kernel would reject under its current credentials, while a separate supervisor has enough privilege to perform an equivalent operation safely on its behalf. Linux seccomp user notification creates a mediation channel for that arrangement: a filter can stop the calling thread, emit a notification to a listener, and wait for a userspace response. That mechanism is more precise than treating the supervisor as a general syscall proxy. The notification carries register-level syscall data and an identifier tied to the pending request. The supervisor can synthesize a return value, inject a file descriptor, or in selected cases tell the kernel to continue the original syscall. Each option places the trust boundary in a different location.

Cybersecurity Linux Security

Software Engineering / 18 Sep 2026 / 6 min read

seccomp User Notification Delegates Selected Syscalls to a Supervisor

seccomp User Notification Delegates Selected Syscalls to a Supervisor A seccomp filter can stop a selected system call before the kernel executes it and emit a notification to a user-space supervisor instead. The target thread remains blocked while the supervisor receives the event and returns a disposition. This behavior turns a filter result into a controlled handoff across the kernel/user-space boundary. The mechanism is SECCOMP_RET_USER_NOTIF. It differs from ordinary seccomp actions because the BPF filter does not finish the decision by itself. A listener file descriptor becomes the coordination point for notification receipt, response delivery, and optional file-descriptor injection.

Linux seccomp

Cybersecurity / 17 Sep 2026 / 6 min read

Seccomp User Notification Moves Selected Syscall Decisions to a Supervisor

A confined process issues a system call that its ordinary seccomp policy cannot safely reduce to a static allow-or-deny decision. The arguments may refer to mutable process memory, or the operation may need privileged work performed outside the confined process. Returning a fixed errno is too restrictive, while permitting the call directly gives the target more authority than the deployment intends. Linux seccomp user notification creates a mediation path for this case. A filter can return SECCOMP_RET_USER_NOTIF, causing the kernel to block the triggering task and emit a request on a listener file descriptor. A userspace supervisor receives the request and later supplies a result. This mechanism changes where a selected syscall decision is made, but it does not turn seccomp into a general reference monitor without additional policy and race controls.

Cybersecurity Linux Security

Software Engineering / 17 Sep 2026 / 6 min read

Seccomp User Notification Delegates Selected System Calls to a Supervisor

A system call selected by a seccomp filter can stop before kernel execution and appear as a request on a notification file descriptor. With SECCOMP_RET_USER_NOTIF, Linux turns that call into a coordination point between the blocked target and a userspace supervisor. The supervisor can emulate a result, inject a file descriptor for suitable operations, or permit the kernel to continue the original call. This mechanism is deliberately narrower than a general userspace security policy engine. Its strongest boundary is the kernel-mediated suspension and response protocol. Data reached through target-memory pointers can still change around a supervisor’s inspection, and a response that continues the original call re-enters ordinary kernel execution with that race still relevant.

Cybersecurity Linux Security

Cybersecurity / 17 Sep 2026 / 7 min read

Seccomp Filters Reduce Syscall Surface Without Forming a Complete Sandbox

Seccomp Filters Reduce Syscall Surface Without Forming a Complete Sandbox A service can run with a short seccomp allowlist and still retain broad authority through file descriptors, filesystem permissions, network endpoints, and credentials. The filter may sharply reduce the kernel interfaces reachable through system calls, yet the process can remain capable of damaging actions through operations that are explicitly allowed. This is the central boundary of seccomp: it filters syscall attempts; it does not define the full security policy of a process.

Cybersecurity Linux Security

Software Engineering / 17 Sep 2026 / 6 min read

Linux seccomp Notification Splits Syscall Entry From Supervisor Response

A seccomp filter can stop a task at syscall entry and turn that event into a message for another process. With SECCOMP_RET_USER_NOTIF, the kernel does not immediately execute the selected syscall. It creates a notification for a listener, blocks the calling task, and waits for a response that can supply a return value, inject a file descriptor, or permit the syscall to continue. That boundary is narrower than general syscall emulation. The notification carries syscall metadata and register argument values, while memory referenced by pointer arguments remains in the target process. The target can also disappear or have its notification invalidated while a supervisor is making a decision. Those properties make identity, memory ownership, and response timing part of the interface contract.

Software Engineering Linux