[common] Always spawn login command as the super user

This allows the app to work in a jailed state (after the user has signed
the app and the linked libraries with TrollStore's method).

The technique is borrowed from TrollStore's documentation:

- <https://github.com/opa334/TrollStore/blob/264a9402abe30be7156c7caf51b4361ca95b7b2f/README.md#root-helpers>

App/entitlements.plist

@@ -8,6 +8,8 @@
 	<true/>
 	<key>com.apple.private.security.no-container</key>
 	<true/>
+	<key>com.apple.private.persona-mgmt</key>
+	<true/>
 	<key>com.apple.security.iokit-user-client-class</key>
 	<array>
 		<string>IOUserClient</string>

Common/Controllers/SubProcess.swift

@@ -170,6 +170,19 @@ class SubProcess {
 		posix_spawn_file_actions_adddup2(&actions, fds.replica, STDERR_FILENO)
 		defer { posix_spawn_file_actions_destroy(&actions) }
 
+		#if targetEnvironment(simulator) || targetEnvironment(macCatalyst)
+		let attrp = nil
+		#else
+		// Spawn as the super user even in a jailed state, where the rootfs has the nosuid option set.
+		var attr: posix_spawnattr_t!
+		posix_spawnattr_init(&attr)
+		posix_spawnattr_set_persona_np(&attr, 99, POSIX_SPAWN_PERSONA_FLAGS_OVERRIDE)
+		posix_spawnattr_set_persona_uid_np(&attr, 0)
+		posix_spawnattr_set_persona_gid_np(&attr, 0)
+		defer { posix_spawnattr_destroy(&attr) }
+		let attrp = UnsafeMutablePointer(&attr)
+		#endif
+
 		// TODO: At some point, come up with some way to keep track of working directory changes.
 		// When opening a new tab, we can switch straight to the previous tab’s working directory.
 		let argv: [UnsafeMutablePointer<CChar>?]
@@ -189,7 +202,7 @@ class SubProcess {
 		}
 
 		var pid = pid_t()
-		let result = ie_posix_spawn(&pid, Self.login, &actions, nil, argv, envp)
+		let result = ie_posix_spawn(&pid, Self.login, &actions, attrp, argv, envp)
 		close(fds.replica)
 		if result != 0 {
 			// Fork failed.

Common/Supporting Files/NewTermCommon.h

@@ -20,4 +20,11 @@ static inline int ie_posix_spawn(pid_t *pid, const char *path, const posix_spawn
 #else
 extern int ie_getpwuid_r(uid_t uid, struct passwd *pw, char *buf, size_t buflen, struct passwd **pwretp);
 extern int ie_posix_spawn(pid_t *pid, const char *path, const posix_spawn_file_actions_t *file_actions, const posix_spawnattr_t *attrp, char *const argv[], char *const envp[]);
+
+#define POSIX_SPAWN_PERSONA_FLAGS_OVERRIDE ((uint32_t) 1)
+
+// https://github.com/apple-oss-distributions/xnu/blob/1031c584a5e37aff177559b9f69dbd3c8c3fd30a/libsyscall/wrappers/spawn/spawn_private.h#L87-L89
+extern int posix_spawnattr_set_persona_np(const posix_spawnattr_t *attr, uid_t persona_id, uint32_t flags);
+extern int posix_spawnattr_set_persona_uid_np(const posix_spawnattr_t *attr, uid_t uid);
+extern int posix_spawnattr_set_persona_gid_np(const posix_spawnattr_t *attr, gid_t gid);
 #endif