Merge branch 'jn/shell-disable-interactive'

When the interactive access to git-shell is not enabled, we issue a
message meant to help the system admininstrator to enable it. Add
an explicit way to help the end users who connect to the service by
issuing custom messages to refuse such an access.

* jn/shell-disable-interactive:
  shell: new no-interactive-login command to print a custom message
  shell doc: emphasize purpose and security model

Documentation/git-shell.txt

@@ -9,25 +9,81 @@ git-shell - Restricted login shell for Git-only SSH access
 SYNOPSIS
 --------
 [verse]
-'git shell' [-c <command> <argument>]
+'chsh' -s $(command -v git-shell) <user>
+'git clone' <user>`@localhost:/path/to/repo.git`
+'ssh' <user>`@localhost`
 
 DESCRIPTION
 -----------
 
-A login shell for SSH accounts to provide restricted Git access. When
-'-c' is given, the program executes <command> non-interactively;
-<command> can be one of 'git receive-pack', 'git upload-pack', 'git
-upload-archive', 'cvs server', or a command in COMMAND_DIR. The shell
-is started in interactive mode when no arguments are given; in this
-case, COMMAND_DIR must exist, and any of the executables in it can be
-invoked.
+This is a login shell for SSH accounts to provide restricted Git access.
+It permits execution only of server-side Git commands implementing the
+pull/push functionality, plus custom commands present in a subdirectory
+named `git-shell-commands` in the user's home directory.
 
-'cvs server' is a special command which executes git-cvsserver.
+COMMANDS
+--------
 
-COMMAND_DIR is the path "$HOME/git-shell-commands". The user must have
-read and execute permissions to the directory in order to execute the
-programs in it. The programs are executed with a cwd of $HOME, and
-<argument> is parsed as a command-line string.
+'git shell' accepts the following commands after the '-c' option:
+
+'git receive-pack <argument>'::
+'git upload-pack <argument>'::
+'git upload-archive <argument>'::
+	Call the corresponding server-side command to support
+	the client's 'git push', 'git fetch', or 'git archive --remote'
+	request.
+'cvs server'::
+	Imitate a CVS server.  See linkgit:git-cvsserver[1].
+
+If a `~/git-shell-commands` directory is present, 'git shell' will
+also handle other, custom commands by running
+"`git-shell-commands/<command> <arguments>`" from the user's home
+directory.
+
+INTERACTIVE USE
+---------------
+
+By default, the commands above can be executed only with the '-c'
+option; the shell is not interactive.
+
+If a `~/git-shell-commands` directory is present, 'git shell'
+can also be run interactively (with no arguments).  If a `help`
+command is present in the `git-shell-commands` directory, it is
+run to provide the user with an overview of allowed actions.  Then a
+"git> " prompt is presented at which one can enter any of the
+commands from the `git-shell-commands` directory, or `exit` to close
+the connection.
+
+Generally this mode is used as an administrative interface to allow
+users to list repositories they have access to, create, delete, or
+rename repositories, or change repository descriptions and
+permissions.
+
+If a `no-interactive-login` command exists, then it is run and the
+interactive shell is aborted.
+
+EXAMPLE
+-------
+
+To disable interactive logins, displaying a greeting instead:
++
+----------------
+$ chsh -s /usr/bin/git-shell
+$ mkdir $HOME/git-shell-commands
+$ cat >$HOME/git-shell-commands/no-interactive-login <<\EOF
+#!/bin/sh
+printf '%s\n' "Hi $USER! You've successfully authenticated, but I do not"
+printf '%s\n' "provide interactive shell access."
+exit 128
+EOF
+$ chmod +x $HOME/git-shell-commands/no-interactive-login
+----------------
+
+SEE ALSO
+--------
+ssh(1),
+linkgit:git-daemon[1],
+contrib/git-shell-commands/README
 
 GIT
 ---

shell.c

@@ -6,6 +6,7 @@
 
 #define COMMAND_DIR "git-shell-commands"
 #define HELP_COMMAND COMMAND_DIR "/help"
+#define NOLOGIN_COMMAND COMMAND_DIR "/no-interactive-login"
 
 static int do_generic_cmd(const char *me, char *arg)
 {
@@ -65,6 +66,18 @@ static void run_shell(void)
 {
 	int done = 0;
 	static const char *help_argv[] = { HELP_COMMAND, NULL };
+
+	if (!access(NOLOGIN_COMMAND, F_OK)) {
+		/* Interactive login disabled. */
+		const char *argv[] = { NOLOGIN_COMMAND, NULL };
+		int status;
+
+		status = run_command_v_opt(argv, 0);
+		if (status < 0)
+			exit(127);
+		exit(status);
+	}
+
 	/* Print help if enabled */
 	run_command_v_opt(help_argv, RUN_SILENT_EXEC_FAILURE);