Re: [PATCH v2 5/5] pretty: add support for separator option in %(trailers)

From: Anders Waldenborg <anders@0x63.nu>
To: Junio C Hamano <gitster@pobox.com>
Cc: git@vger.kernel.org, Jeff King <peff@peff.net>,
	Olga Telezhnaya <olyatelezhnaya@gmail.com>
Subject: Re: [PATCH v2 5/5] pretty: add support for separator option in %(trailers)
Date: Mon, 05 Nov 2018 19:24:14 +0100	[thread overview]
Message-ID: <871s7zl6xp.fsf@0x63.nu> (raw)
In-Reply-To: <xmqqpnvkjmtu.fsf@gitster-ct.c.googlers.com>

Junio C Hamano writes:
> Anders Waldenborg <anders@0x63.nu> writes:
>
>> @@ -1352,6 +1353,17 @@ static size_t format_commit_one(struct strbuf *sb, /* in UTF-8 */
>>  						arg++;
>>
>>  					opts.only_trailers = 1;
>> +				} else if (skip_prefix(arg, "separator=", &arg)) {
>> +					size_t seplen = strcspn(arg, ",)");
>> +					strbuf_reset(&sepbuf);
>> +					char *fmt = xstrndup(arg, seplen);
>> +					strbuf_expand(&sepbuf, fmt, format_fundamental, NULL);
>
> This somehow feels akin to using end-user supplied param to printf(3)
> as its format argument e.g.
>
> 	int main(int ac, char *av) {
> 		printf(av[1]);
> 		return 0;
> 	}
>
> which is not a good idea.  Is there a mechanism with which we can
> ensure that the separator=<what> specification will never come from
> potentially malicious sources (e.g. not used to show things on webpage
> allowing random folks who access he site to supply custom format)?

I can't see a case where this could add anything that isn't already
possible.

AFAICU strbuf_expand doesn't suffer from the worst things that printf(3)
suffers from wrt untrusted format string (i.e no printf style %n which
can write to memory, and no vaargs on stack which allows leaking random
stuff).

The separator option is part of the full format string. If a malicious
user can specify that, they can't really do anything new, as the
separator only can expand %n and %xNN, which they already can do in the
full string.

But maybe I'm missing something?