From ae6023081a82909127f365efa2ee550abe94b54e Mon Sep 17 00:00:00 2001
From: Gabriel Kaszewski <gabrielkaszewski@gmail.com>
Date: Sun, 18 Oct 2026 00:22:37 +0200
Subject: [PATCH] feat(kernel): add Delegate syscall for capability delegation

---
 crates/kernel/src/lib.rs              | 15 ++++++++
 crates/kernel/src/syscall.rs          | 44 +++++++++++++++++++++
 crates/kernel/src/tests/capability.rs | 55 +++++++++++++++++++++++++++
 crates/kernel/src/tests/mod.rs        |  1 +
 4 files changed, 115 insertions(+)
 create mode 100644 crates/kernel/src/tests/capability.rs

diff --git a/crates/kernel/src/lib.rs b/crates/kernel/src/lib.rs
index 707377c..76237b6 100644
--- a/crates/kernel/src/lib.rs
+++ b/crates/kernel/src/lib.rs
@@ -193,4 +193,19 @@ impl<P: Platform> Kernel<P> {
         }
         self.scheduler.remove(tid);
     }
+
+    #[cfg(test)]
+    pub fn create_endpoint_for_test(&mut self) -> types::id::EndpointId {
+        self.endpoints.create().expect("create endpoint")
+    }
+
+    #[cfg(test)]
+    pub fn grant_cap_for_test(
+        &mut self,
+        pid: ProcessId,
+        resource: Resource,
+        permissions: PermissionSet,
+    ) -> CapSlot {
+        self.grant_cap(pid, resource, permissions).expect("grant cap")
+    }
 }
diff --git a/crates/kernel/src/syscall.rs b/crates/kernel/src/syscall.rs
index 678bf6d..08bd83b 100644
--- a/crates/kernel/src/syscall.rs
+++ b/crates/kernel/src/syscall.rs
@@ -21,6 +21,9 @@ impl<P: Platform> Kernel<P> {
             Syscall::SpawnThread { entry, stack } => self.handle_spawn_thread(pid, entry, stack),
             Syscall::CreateProcess { create_cap, image } => self.handle_create_process(pid, create_cap, image),
             Syscall::Kill { process } => self.handle_kill(pid, process),
+            Syscall::Delegate { slot, to_endpoint, permissions } => {
+                self.handle_delegate(pid, slot, to_endpoint, permissions)
+            }
             _ => SyscallOutcome::Complete(SyscallResult::Error(
                 domain::error::KernelError::WouldBlock,
             )),
@@ -321,4 +324,45 @@ impl<P: Platform> Kernel<P> {
         self.process_memory[target_pid.raw() as usize] = None;
         SyscallOutcome::Complete(SyscallResult::Ok)
     }
+
+    fn handle_delegate(
+        &mut self,
+        caller_pid: types::id::ProcessId,
+        slot: types::bounded::CapSlot,
+        to_endpoint: types::bounded::CapSlot,
+        restrict: types::permission::PermissionSet,
+    ) -> SyscallOutcome {
+        let (resource, permissions) = match self.check_cap(
+            caller_pid,
+            slot,
+            types::permission::Permission::Delegate,
+        ) {
+            Ok(cap) => (cap.resource().clone(), *cap.permissions()),
+            Err(e) => return SyscallOutcome::Complete(SyscallResult::Error(e)),
+        };
+
+        let restricted = permissions.restrict_to(&restrict);
+
+        let target_pid = match self.check_cap(
+            caller_pid,
+            to_endpoint,
+            types::permission::Permission::Execute,
+        ) {
+            Ok(cap) => match cap.resource() {
+                domain::resource::Resource::Process(pid) => *pid,
+                _ => return SyscallOutcome::Complete(SyscallResult::Error(
+                    domain::error::KernelError::PermissionDenied {
+                        slot: to_endpoint,
+                        needed: types::permission::Permission::Execute,
+                    },
+                )),
+            },
+            Err(e) => return SyscallOutcome::Complete(SyscallResult::Error(e)),
+        };
+
+        match self.grant_cap(target_pid, resource, restricted) {
+            Ok(_) => SyscallOutcome::Complete(SyscallResult::Ok),
+            Err(e) => SyscallOutcome::Complete(SyscallResult::Error(e)),
+        }
+    }
 }
diff --git a/crates/kernel/src/tests/capability.rs b/crates/kernel/src/tests/capability.rs
new file mode 100644
index 0000000..397bb8a
--- /dev/null
+++ b/crates/kernel/src/tests/capability.rs
@@ -0,0 +1,55 @@
+use super::*;
+use crate::{SyscallOutcome, SyscallResult};
+use domain::syscall::Syscall;
+use types::bounded::CapSlot;
+use types::permission::PermissionSet;
+use types::address::DataRef;
+
+#[test]
+fn delegate_without_delegate_permission_fails() {
+    let (mut k, _) = kernel_with_process();
+    // slot 0 has ProcessCreate cap — no Delegate permission
+    let outcome = k.handle_syscall(Syscall::Delegate {
+        slot: CapSlot::new(0).unwrap(),
+        to_endpoint: CapSlot::new(1).unwrap(),
+        permissions: PermissionSet::empty(),
+    });
+    assert!(matches!(
+        outcome,
+        SyscallOutcome::Complete(SyscallResult::Error(_))
+    ));
+}
+
+#[test]
+fn delegate_with_valid_cap_succeeds() {
+    let (mut k, pid) = kernel_with_process();
+
+    // Create a second process to delegate to
+    let target_slot = match k.handle_syscall(Syscall::CreateProcess {
+        create_cap: CapSlot::new(0).unwrap(),
+        image: DataRef::empty(),
+    }) {
+        SyscallOutcome::Complete(SyscallResult::Slot(s)) => s,
+        _ => panic!("expected slot"),
+    };
+
+    // Grant a delegatable cap to the caller (endpoint cap with Delegate perm)
+    let ep_id = k.create_endpoint_for_test();
+    let source_slot = k.grant_cap_for_test(
+        pid,
+        domain::resource::Resource::Endpoint(ep_id),
+        PermissionSet::empty()
+            .with(types::permission::Permission::Read)
+            .with(types::permission::Permission::Delegate),
+    );
+
+    let outcome = k.handle_syscall(Syscall::Delegate {
+        slot: source_slot,
+        to_endpoint: target_slot, // using process cap as target identifier
+        permissions: PermissionSet::empty().with(types::permission::Permission::Read),
+    });
+    assert!(matches!(
+        outcome,
+        SyscallOutcome::Complete(SyscallResult::Ok)
+    ));
+}
diff --git a/crates/kernel/src/tests/mod.rs b/crates/kernel/src/tests/mod.rs
index e0a1f1a..e7030e4 100644
--- a/crates/kernel/src/tests/mod.rs
+++ b/crates/kernel/src/tests/mod.rs
@@ -1,3 +1,4 @@
+mod capability;
 mod exit;
 mod lifecycle;
 mod memory;