refactor: capabilities - add unified launch routine

programs/include/montauk/capabilities.h

@@ -42,23 +42,17 @@ namespace caps {
 
     inline uint64_t bit_for_name(const char* name) {
         if (name == nullptr || name[0] == '\0') return 0;
-        // "all" means "everything this launcher may pass on", which the
-        // caller-delegable clamp in for_binary() then narrows.  It excludes
-        // CAP_SYSTEM_IMAGE: authority to rewrite a program image is never
-        // something a wildcard should hand out, only an explicit name.
+        // "all" means "everything this launcher may pass on".
         if (montauk::streq(name, "all"))
             return montauk::abi::CAP_ALL & ~montauk::abi::CAP_SYSTEM_IMAGE;
         for (const auto& entry : NAMES) {
             if (montauk::streq(entry.name, name)) return entry.bit;
         }
-        // Unknown names are ignored rather than rejected.  Failing closed
-        // costs a program some authority; failing open would hand out
-        // authority nobody asked for.
+
         return 0;
     }
 
-    // Read an array-of-strings key into a capability mask.  A missing key is
-    // an empty mask, which is the correct default for an absent grant.
+    // Read an array-of-strings key into a capability mask.
     inline uint64_t mask_from_key(const montauk::toml::Doc& doc, const char* key) {
         montauk::toml::Value* arr = doc.get_array(key);
         if (arr == nullptr) return 0;
@@ -138,10 +132,6 @@ namespace caps {
     }
 
     // The calling process's own capability masks.
-    //
-    // There is no syscall to ask "what am I?", so this scans the process table
-    // for our own PID.  The buffer is heap-allocated because ProcInfo is large
-    // enough that MAX_SCAN_PROCS of them would be a ~29 KB stack frame.
     inline bool self(montauk::abi::SpawnCapabilities& out) {
         out = {0, 0, 0};
 
@@ -172,16 +162,11 @@ namespace caps {
     }
 
     // Build a spawn request for `binary_path`, clamped to what the caller may
-    // actually delegate.  The kernel enforces the same bound; clamping here
-    // means a launcher that holds less authority than the table declares
-    // degrades to a reduced grant instead of failing the spawn outright.
+    // actually delegate.
     inline montauk::abi::SpawnCapabilities for_binary(const char* binary_path,
                                                       uint64_t caller_delegable) {
         montauk::abi::SpawnCapabilities caps{0, 0, 0};
 
-        // A caller with nothing to delegate cannot produce a non-empty grant,
-        // so skip the file read entirely.  This is the common case: every
-        // unprivileged session, on every launch.
         if (caller_delegable == 0) return caps;
 
         if (!lookup(binary_path, caps)) return caps;
@@ -192,5 +177,38 @@ namespace caps {
         return caps;
     }
 
+    // The calling process's delegable set, cached.
+    inline uint64_t my_delegable() {
+        static uint64_t cached = 0;
+        static bool     loaded = false;
+        if (!loaded) {
+            cached = self_delegable();
+            loaded = true;
+        }
+        return cached;
+    }
+
+    struct LaunchOptions {
+        const char* args = nullptr;
+        // Owner identity for the child.  Requires CAP_USER_ADMIN and is
+        // reserved for session creators (init, login);
+        const char* user = nullptr;
+        // True for a console host that reads and writes the child's I/O
+        // through the childio calls rather than sharing its own.
+        bool redirected = false;
+    };
+
+    // Unified launch helper.
+    inline int launch(const char* path, const LaunchOptions& opt = {}) {
+        if (path == nullptr || path[0] == '\0') return -1;
+
+        if (opt.redirected && opt.user != nullptr) return -1;
+
+        montauk::abi::SpawnCapabilities caps = for_binary(path, my_delegable());
+        return opt.redirected
+            ? spawn_redir_with_caps(path, opt.args, caps)
+            : spawn_with_caps(path, opt.args, opt.user, caps);
+    }
+
 } // namespace caps
 } // namespace montauk