refactor: rewrite exec command with temporary TTY support

- Change exec to return int instead of noreturn void
- Introduce exec_container_option struct for exec configuration
- Add temporary PTY and console socket support (to be rewritten)
- Add container monitor for signal and I/O forwarding
- Use fork to manage exec process lifecycle

NOTE: current TTY implementation is temporary and has limitations

Signed-off-by: ComixHe <ComixHe1895@outlook.com>

Author: ComixHe

src/linyaps_box/app.cpp

@@ -49,8 +49,7 @@ try {
                                           return 0;
                                       },
                                        [](const command::exec_options &options) -> int {
-                                           command::exec(options);
-                                           __builtin_unreachable();
+                                           return command::exec(options);
                                        },
                                        [](const command::kill_options &options) {
                                            command::kill(options);

src/linyaps_box/command/exec.cpp

@@ -8,7 +8,7 @@
 #include "linyaps_box/runtime.h"
 #include "linyaps_box/status_directory.h"
 
-void linyaps_box::command::exec(const struct exec_options &options)
+auto linyaps_box::command::exec(const struct exec_options &options) -> int
 {
     std::unique_ptr<status_directory> dir =
             std::make_unique<impl::status_directory>(options.global_.get().root);
@@ -20,12 +20,17 @@ void linyaps_box::command::exec(const struct exec_options &options)
         throw std::runtime_error("container not found");
     }
 
-    config::process_t proc;
-    proc.cwd = options.cwd.value_or("/");
-    proc.args = options.command;
-    proc.terminal = isatty(STDIN_FILENO) == 1 && isatty(STDOUT_FILENO) == 1;
-    proc.no_new_privileges = options.no_new_privs;
-    proc.env = options.envs.value_or(std::vector<std::string>{});
+    exec_container_option option;
+    option.proc.cwd = options.cwd.value_or("/");
+    option.proc.args = options.command;
+    option.proc.terminal = options.tty;
+    option.proc.no_new_privileges = options.no_new_privs;
+    option.proc.env = options.envs.value_or(std::vector<std::string>{});
+    option.preserve_fds = options.preserve_fds;
+
+    if (option.proc.terminal && options.console_socket) {
+        option.console_socket = unixSocketClient::connect(options.console_socket.value());
+    }
 
 #ifdef LINYAPS_BOX_ENABLE_CAP
     if (options.caps) {
@@ -45,14 +50,14 @@ void linyaps_box::command::exec(const struct exec_options &options)
                     });
         };
 
-        transform_cap(proc.capabilities.effective);
-        transform_cap(proc.capabilities.ambient);
-        transform_cap(proc.capabilities.bounding);
-        transform_cap(proc.capabilities.permitted);
+        transform_cap(option.proc.capabilities.effective);
+        transform_cap(option.proc.capabilities.ambient);
+        transform_cap(option.proc.capabilities.bounding);
+        transform_cap(option.proc.capabilities.permitted);
     }
 #endif
 
     // TODO: support exec fully
 
-    container->second.exec(proc);
+    return container->second.exec(std::move(option));
 }

src/linyaps_box/command/exec.h

@@ -8,6 +8,6 @@
 
 namespace linyaps_box::command {
 
-[[noreturn]] void exec(const exec_options &options);
+[[nodiscard]] auto exec(const exec_options &options) -> int;
 
 } // namespace linyaps_box::command

src/linyaps_box/command/options.cpp

@@ -96,24 +96,27 @@ linyaps_box::command::options linyaps_box::command::parse(int argc, char *argv[]
                          "for example `1000` for UID=1000 "
                          "or `1000:1000` for UID=1000 and GID=1000")
             ->type_name("UID[:GID]");
-    cmd_exec->add_option("--cwd", exec_opt.cwd, "Current working directory.");
+    cmd_exec->add_option("--cwd", exec_opt.cwd, "Current working directory.")->type_name("PATH");
     cmd_exec->add_option("--env", exec_opt.envs, "Environment variables to set")
-            ->multi_option_policy(CLI::MultiOptionPolicy::TakeAll)
-            ->check(
-                    [](const std::string &str) {
-                        if (str.find('=') == std::string::npos) {
-                            return "invalid argument, env must be in the format of KEY=VALUE";
-                        }
-                        return "";
-                    },
-                    "env_check");
+            ->type_name("ENV")
+            ->take_all()
+            ->check([](const std::string &str) {
+                if (str.find('=') == std::string::npos) {
+                    return "invalid argument, env must be in the format of KEY=VALUE";
+                }
+                return "";
+            });
     cmd_exec->add_option("--console-socket",
                          exec_opt.console_socket,
                          "Path to an unix socket that will receive the master end of the console's "
                          "pseudoterminal")
             ->type_name("SOCKET")
             ->check(socket_check);
-    cmd_exec->add_flag("--tty", exec_opt.tty, "Allocate a pseudo-TTY")->default_val(false);
+    cmd_exec->add_flag("-t,--tty", exec_opt.tty, "Allocate a pseudo-TTY")->take_last();
+    cmd_exec->add_option("--preserve-fds",
+                         exec_opt.preserve_fds,
+                         "Pass N additional file descriptors to the container")
+            ->type_name("N");
     // TODO: enable capabilities and no_new_privs support after rewrite exec,
     //      cmd_exec->add_option("-c,--cap", options.exec.caps, "Set capabilities")
     //              ->check(

src/linyaps_box/command/options.h

@@ -36,13 +36,13 @@ struct list_options
 struct exec_options
 {
     explicit exec_options(global_options &global)
-        : no_new_privs(false)
-        , global_(global)
+        : global_(global)
     {
     }
 
-    bool no_new_privs;
-    bool tty;
+    bool no_new_privs{ false };
+    bool tty{ false };
+    int preserve_fds{ 0 };
     std::reference_wrapper<global_options> global_;
     std::vector<std::string> command;
     std::string user;
@@ -64,8 +64,8 @@ struct run_options
     std::string ID;
     std::string bundle;
     std::string config;
-    std::string console_socket;
-    int preserve_fds{};
+    std::optional<std::string> console_socket;
+    int preserve_fds{ 0 };
 };
 
 struct kill_options

src/linyaps_box/command/run.cpp

@@ -22,8 +22,8 @@ auto linyaps_box::command::run(const struct run_options &options) -> int
     run_container_options_t run_options;
     run_options.preserve_fds = options.preserve_fds;
 
-    if (container.get_config().process.terminal && !options.console_socket.empty()) {
-        run_options.console_socket = unixSocketClient::connect(options.console_socket);
+    if (container.get_config().process.terminal && options.console_socket) {
+        run_options.console_socket = unixSocketClient::connect(options.console_socket.value());
     }
 
     return container.run(std::move(run_options));

src/linyaps_box/container_ref.cpp

@@ -4,7 +4,14 @@
 
 #include "linyaps_box/container_ref.h"
 
+#include "linyaps_box/container_monitor.h"
+#include "linyaps_box/terminal.h"
+#include "linyaps_box/utils/file.h"
 #include "linyaps_box/utils/log.h"
+#include "linyaps_box/utils/process.h"
+#include "linyaps_box/utils/session.h"
+#include "linyaps_box/utils/socket.h"
+#include "linyaps_box/utils/terminal.h"
 
 #include <csignal> // IWYU pragma: keep
 #include <utility>
@@ -38,73 +45,110 @@ void linyaps_box::container_ref::kill(int signal) const
     throw std::system_error(errno, std::system_category(), std::move(ss).str());
 }
 
-void linyaps_box::container_ref::exec(const linyaps_box::config::process_t &process)
+auto linyaps_box::container_ref::exec(exec_container_option option) -> int
 {
     auto target = std::to_string(this->status().PID);
 
-    std::vector<const char *> argv{
-        "nsenter",
-        "--target",
-        target.c_str(),
-        "--user",
-        "--mount",
-        "--pid",
-        "--no-fork",
-        // FIXME:
-        // Old nsenter command do not support --wdns,
-        // so we have to implement nsenter by ourself in the future.
-        "--preserve-credentials",
-    };
+    // TODO: support detach later
+    utils::prctl(PR_SET_CHILD_SUBREAPER, 1, 0, 0, 0);
 
-    for (const auto &arg : process.args) {
-        argv.push_back(arg.c_str());
+    std::optional<unixSocketClient> recv_socketpair;
+    if (option.proc.terminal && !option.console_socket) {
+        auto [socket1, socket2] = utils::socketpair(AF_UNIX, SOCK_SEQPACKET | SOCK_CLOEXEC, 0);
+        option.console_socket = unixSocketClient{ std::move(socket1) };
+        recv_socketpair = unixSocketClient{ std::move(socket2) };
     }
-    argv.push_back(nullptr);
 
-    std::vector<const char *> c_env;
-    c_env.reserve(process.env.size());
-    for (const auto &env : process.env) {
-        c_env.push_back(env.c_str());
+    auto child = fork();
+    if (child < 0) {
+        throw std::system_error(errno, std::system_category(), "fork");
     }
-    c_env.push_back(nullptr);
-
-    LINYAPS_BOX_DEBUG() << [&argv]() -> std::string {
-        auto result = std::accumulate(argv.cbegin(),
-                                      argv.cend() - 1,
-                                      std::string{ "args:[" },
-                                      [](std::string init, const std::string &val) {
-                                          init += val;
-                                          init.push_back(' ');
-                                          return init;
-                                      });
-        result.push_back(']');
-        result.insert(0, "execvp nsenter with arguments: ");
-        return result;
-    }();
 
-    // FIXME:
-    // We only handle the command arguments for now
-    // here are some other fields in process we need to consider:
-    // terminal
-    // console.height
-    // console.width
-    // cwd
-    // env
-    // rlimits
-    // apparmor_profile
-    // capabilities
-    // no_new_privileges
-    // oom_score_adj
-
-    ::execvpe("nsenter", const_cast<char **>(argv.data()), const_cast<char **>(c_env.data()));
+    if (child == 0) {
+        // TODO: create terminal after rewrite exec. it should be created in the container namespace
+        if (option.console_socket) {
+            utils::setsid();
+            auto [master, slave] = create_pty_pair();
+
+            slave.setup_stdio();
+            // TODO: use fchown after we implement exec option `--user`
+            slave.set_size({});
+
+            option.console_socket->send_fd(std::move(master).take());
+            option.console_socket.reset();
+        }
+
+        std::vector<const char *> argv{
+            "nsenter",
+            "--target",
+            target.c_str(),
+            "--user",
+            "--mount",
+            "--pid",
+            // FIXME:
+            // Old nsenter command do not support --wdns,
+            // so we have to implement nsenter by ourself in the future.
+            "--preserve-credentials",
+        };
+
+        for (const auto &arg : option.proc.args) {
+            argv.push_back(arg.c_str());
+        }
+        argv.push_back(nullptr);
+
+        std::vector<const char *> c_env;
+        c_env.reserve(option.proc.env.size());
+        for (const auto &env : option.proc.env) {
+            c_env.push_back(env.c_str());
+        }
+        c_env.push_back(nullptr);
 
-    std::stringstream ss;
-    ss << "execvp nsenter with arguments:";
-    for (const auto &arg : argv) {
-        ss << " " << arg;
+        // FIXME:
+        // We only handle the command arguments for now
+        // here are some other fields in process we need to consider:
+        // terminal
+        // console.height
+        // console.width
+        // cwd
+        // env
+        // rlimits
+        // apparmor_profile
+        // capabilities
+        // no_new_privileges
+        // oom_score_adj
+
+        ::execvpe("nsenter", const_cast<char **>(argv.data()), const_cast<char **>(c_env.data()));
+        _exit(EXIT_FAILURE);
     }
 
-    throw std::system_error(errno, std::system_category(), std::move(ss).str());
+    auto in = utils::file_descriptor{ utils::fileno(stdin), false };
+    auto out = utils::file_descriptor{ utils::fileno(stdout), false };
+
+    container_monitor monitor{ child };
+
+    [&recv_socketpair, &monitor, &in, &out]() {
+        if (!recv_socketpair) {
+            return;
+        }
+
+        LINYAPS_BOX_DEBUG() << "Container requires a terminal";
+
+        std::string payload;
+        auto master = terminal_master{ recv_socketpair->recv_fd(payload) };
+
+        recv_socketpair->release();
+
+        in.set_nonblock(true);
+        out.set_nonblock(true);
+
+        monitor.enable_io_forwarding(std::move(master), in, out);
+    }();
+
+    if (!monitor.enable_signal_forwarding()) {
+        return 0;
+    }
+
+    return monitor.wait_container_exit();
 }
 
 const linyaps_box::status_directory &linyaps_box::container_ref::status_dir() const

src/linyaps_box/container_ref.h

@@ -7,9 +7,17 @@
 #include "linyaps_box/config.h"
 #include "linyaps_box/container_status.h"
 #include "linyaps_box/status_directory.h"
+#include "linyaps_box/unixsocket.h"
 
 namespace linyaps_box {
 
+struct exec_container_option
+{
+    int preserve_fds;
+    config::process_t proc;
+    std::optional<unixSocketClient> console_socket;
+};
+
 class container_ref
 {
 public:
@@ -23,7 +31,7 @@ class container_ref
 
     [[nodiscard]] auto status() const -> container_status_t;
     void kill(int signal) const;
-    [[noreturn]] void exec(const config::process_t &process);
+    [[nodiscard]] auto exec(exec_container_option option) -> int;
 
 protected:
     [[nodiscard]] auto status_dir() const -> const status_directory &;

src/linyaps_box/status_directory.h

@@ -10,6 +10,7 @@
 #include <vector>
 
 namespace linyaps_box {
+// TODO: place container status into a directory
 class status_directory : public virtual interface
 {
 protected: