Subject: Re: [PATCH] alias: detect loops in mixed execution mode

From: Ævar Arnfjörð Bjarmason <[email-masked]>
Date: Fri, 19 Oct 2018 10:28:22 +0200

On Thu, Oct 18 2018, Ævar Arnfjörð Bjarmason wrote:

> +static void init_cmd_history(struct strbuf *env, struct string_list *cmd_list)
> +{
> +	const char *old = getenv(COMMAND_HISTORY_ENVIRONMENT);
> +	struct strbuf **cmd_history, **ptr;
> +
> +	if (!old || !*old)
> +		return;
> +
> +	strbuf_addstr(env, old);
> +	strbuf_rtrim(env);
> +
> +	cmd_history = strbuf_split_buf(old, strlen(old), ' ', 0);
> +	for (ptr = cmd_history; *ptr; ptr++) {
> +		strbuf_rtrim(*ptr);
> +		string_list_append(cmd_list, (*ptr)->buf);
> +	}
> +	strbuf_list_free(cmd_history);
> +}
> +
> +static void add_cmd_history(struct strbuf *env, struct string_list *cmd_list,
> +			    const char *cmd)
> +{
> +	string_list_append(cmd_list, cmd);
> +	if (env->len)
> +		strbuf_addch(env, ' ');
> +	strbuf_addstr(env, cmd);
> +	setenv(COMMAND_HISTORY_ENVIRONMENT, env->buf, 1);
> +}
> +
>  static int run_argv(int *argcp, const char ***argv)
>  {
>  	int done_alias = 0;
> -	struct string_list cmd_list = STRING_LIST_INIT_NODUP;
> +	struct string_list cmd_list = STRING_LIST_INIT_DUP;
>  	struct string_list_item *seen;
> +	struct strbuf env = STRBUF_INIT;
>
> +	init_cmd_history(&env, &cmd_list);
>  	while (1) {
>  		/*
>  		 * If we tried alias and futzed with our environment,
> @@ -711,7 +742,7 @@ static int run_argv(int *argcp, const char ***argv)
>  			      " not terminate:%s"), cmd_list.items[0].string, sb.buf);
>  		}
>
> -		string_list_append(&cmd_list, *argv[0]);
> +		add_cmd_history(&env, &cmd_list, *argv[0]);
>
>  		/*
>  		 * It could be an alias -- this works around the insanity

Just to sanity check an assumption of mine: One thing I didn't do is use
sq_quote_buf() and sq_dequote_to_argv() like we do for
CONFIG_DATA_ENVIRONMENT. This is because in the case of config we need
to deal with:

    $ git config alias.cfgdump
    !env
    $ git -c x.y=z -c "foo.bar='baz'" cfgdump|grep baz
    GIT_CONFIG_PARAMETERS='x.y=z' 'foo.bar='\''baz'\'''

But in this case I don't see how a command-name would ever contain
whitespace. So we skip quoting and just delimit by space.

There's also nothing stopping you from doing e.g.:

    $ GIT_COMMAND_HISTORY='foo bar' ~/g/git/git --exec-path=$PWD one
    fatal: alias loop detected: expansion of 'foo' does not terminate:
      foo
      bar
      one
      two <==
      three
      four
      five ==>

Or even confuse the code by adding a whitespace at the beginning:

    $ GIT_COMMAND_HISTORY=' foo bar' ~/g/git/git --exec-path=$PWD one
    fatal: alias loop detected: expansion of '' does not terminate:

      foo
      bar
      one
      two <==
      three
      four
      five ==>

I thought none of this was worth dealing with. Worst case someone's
screwing with this, but I don't see how it would happen accidentally,
and even then we detect the infinite loop and just degrade to confusing
error messages because you decided to screw with git's GIT_* env vars.