LibSSH+SSHServer: Provide a properly set environment to children

Before this patch, exec commands were inheriting the environment of the
SSHServer instance while interactive shell spawned with an almost empty
env. Now, both paths start with the same basic environment and also
include modifications requested by the ssh client.

This should improve compatibility in a lot of situations but most
visibly it makes syntax highlighting and autocompletion work in
interactive shells.

The list of default environment variables is heavily inspired by what
OpenSSH is doing. Compared to what it produces on my system, we only
lack XDG_*, SSH_* and DBUS_SESSION_BUS_ADDRESS variables. As noted in
the FIXME the value of PATH also differs.

Userland/Libraries/LibSSH/Session.h

@@ -83,6 +83,7 @@ struct Session : public RefCounted<Session> {
     bool has_streaming_coroutine { false };
 
     Optional<PtyInfo> pty_info {};
+    Vector<ByteString> env {};
     Variant<Empty, ExecData, SFTP::Server, Shell> system {};
 
     bool is_closed { false };

Userland/Services/SSHServer/SSHClient.cpp

@@ -13,12 +13,14 @@
 #include <AK/Random.h>
 #include <AK/TemporaryChange.h>
 #include <LibCore/Account.h>
+#include <LibCore/Environment.h>
 #include <LibCore/EventLoop.h>
 #include <LibCore/Process.h>
 #include <LibCore/Socket.h>
 #include <LibCore/System.h>
 #include <LibCrypto/Curves/Ed25519.h>
 #include <LibCrypto/Curves/X25519.h>
+#include <LibFileSystem/FileSystem.h>
 #include <LibSSH/DataTypes.h>
 #include <LibSSH/IdentificationString.h>
 #include <Services/SSHServer/ServerConfiguration.h>
@@ -610,7 +612,7 @@ ErrorOr<void> SSHClient::handle_channel_request(GenericMessage& message)
     dbgln_if(SSH_DEBUG, "CHANNEL_REQUEST id({}): {:s}", session.local_channel_id, request_type.bytes());
 
     if (request_type == "env"sv.bytes() && !want_reply) {
-        dbgln("FIXME: Ignored channel request: {:s}", request_type.bytes());
+        TRY(handle_channel_env(session, message, want_reply));
         return {};
     }
 
@@ -708,6 +710,47 @@ Coroutine<void> SSHClient::async_stream_data_to_subsystem(NonnullRefPtr<Session>
     co_return;
 }
 
+// 6.4.  Environment Variable Passing
+// https://datatracker.ietf.org/doc/html/rfc4254#section-6.4
+ErrorOr<void> SSHClient::handle_channel_env(NonnullRefPtr<Session> const& session, GenericMessage& message, bool want_reply)
+{
+    auto variable_name = TRY(decode_string(message.payload));
+    auto variable_value = TRY(decode_string(message.payload));
+
+    session->env.append(ByteString::formatted("{:s}={:s}"sv, variable_name.bytes(), variable_value.bytes()));
+
+    if (want_reply)
+        TRY(send_channel_success_message(session));
+
+    return {};
+}
+
+static ErrorOr<Vector<ByteString>> make_env(Vector<ByteString> const& client_environment)
+{
+    Vector<ByteString> env;
+    env.extend(client_environment);
+
+    auto account = TRY(Core::Account::self(Core::Account::Read::PasswdOnly));
+
+    auto user = ByteString::formatted("USER={}", account.username());
+    env.append(user);
+
+    auto logname = ByteString::formatted("LOGNAME={}", account.username());
+    env.append(logname);
+
+    auto home = ByteString::formatted("HOME={}", account.home_directory());
+    env.append(home);
+
+    auto shell = ByteString::formatted("SHELL={}", account.shell());
+    env.append(shell);
+
+    // FIXME: DEFAULT_PATH is made for Serenity this won't work perfectly on other systems.
+    auto path = ByteString::formatted("PATH={}", DEFAULT_PATH);
+    env.append(path);
+
+    return env;
+}
+
 // 6.5.  Starting a Shell or a Command
 // https://datatracker.ietf.org/doc/html/rfc4254#section-6.5
 ErrorOr<void> SSHClient::handle_channel_exec(NonnullRefPtr<Session> const& session, GenericMessage& message)
@@ -728,6 +771,7 @@ ErrorOr<void> SSHClient::handle_channel_exec(NonnullRefPtr<Session> const& sessi
     auto child = TRY(Core::Process::spawn({
         .executable = shell,
         .arguments = args,
+        .environment = TRY(make_env(session->env)),
         .file_actions = {
             Core::FileAction::DuplicateFile { stdin_fds[0], STDIN_FILENO },
             Core::FileAction::DuplicateFile { stdout_fds[1], STDOUT_FILENO },
@@ -783,13 +827,15 @@ ErrorOr<void> SSHClient::handle_channel_shell(NonnullRefPtr<Session> const& sess
         args.append(shell);
         args.append("--login"sv);
 
-        Vector<StringView> env;
+        auto env = TRY(make_env(session->env));
         auto term = ByteString::formatted("TERM={}", pty_info.term_environment_variable);
         env.append(term);
-        auto home = ByteString::formatted("HOME={}", account.home_directory());
-        env.append(home);
 
-        TRY(Core::System::exec(shell, args, Core::System::SearchInPath::No, env));
+        Vector<StringView> env_view;
+        for (auto const& var : env)
+            env_view.append(var);
+
+        TRY(Core::System::exec(shell, args, Core::System::SearchInPath::No, env_view));
         VERIFY_NOT_REACHED();
     }
 

Userland/Services/SSHServer/SSHClient.h

@@ -86,6 +86,7 @@ private:
     ErrorOr<void> handle_channel_subsystem(Session&, GenericMessage&);
     ErrorOr<void> handle_channel_pseudo_terminal(Session&, GenericMessage&);
     ErrorOr<void> handle_channel_window_change(Session&, GenericMessage&);
+    ErrorOr<void> handle_channel_env(NonnullRefPtr<Session> const&, GenericMessage&, bool want_reply);
     ErrorOr<void> handle_channel_exec(NonnullRefPtr<Session> const&, GenericMessage&);
     ErrorOr<void> handle_channel_shell(NonnullRefPtr<Session> const&);
     ErrorOr<void> send_channel_success_message(Session const&);