Subject: Re: [PATCH v4] line-range: fix infinite loop bug with '$' regex

From: Ævar Arnfjörð Bjarmason <[email-masked]>
Date: Mon, 19 Dec 2022 23:55:41 +0100

On Mon, Dec 19 2022, Lars Kellogg-Stedman wrote:

> When the -L argument to "git log" is passed the zero-width regular
> expression "$" (as in "-L :$:line-range.c"), this results in an
> infinite loop in find_funcname_matching_regexp().
>
> Modify find_funcname_matching_regexp to correctly match the entire line
> instead of the zero-width match at eol and update the loop condition to
> prevent an infinite loop in the event of other undiscovered corner cases.
>
> The primary change is that we pre-decrement the beginning-of-line marker
> ('bol') before comparing it to '\n'. In the case of '$', where we match the
> '\n' at the end of the line and start the loop with bol == eol, this
> ensures that bol will find the beginning of the line on which the match
> occurred.
>
> Originally reported in <https://stackoverflow.com/q/74690545/147356>.
>
> Signed-off-by: Lars Kellogg-Stedman <[email-masked]>
> ---
>  line-range.c        |  7 ++++---
>  t/t4211-line-log.sh | 22 ++++++++++++++++++++++
>  2 files changed, 26 insertions(+), 3 deletions(-)
>
> diff --git a/line-range.c b/line-range.c
> index 955a8a9535..47bf0d6f1a 100644
> --- a/line-range.c
> +++ b/line-range.c
> @@ -135,7 +135,7 @@ static const char *find_funcname_matching_regexp(xdemitconf_t *xecfg, const char
>  {
>  	int reg_error;
>  	regmatch_t match[1];
> -	while (1) {
> +	while (*start) {
>  		const char *bol, *eol;
>  		reg_error = regexec(regexp, start, 1, match, 0);
>  		if (reg_error == REG_NOMATCH)
> @@ -148,8 +148,8 @@ static const char *find_funcname_matching_regexp(xdemitconf_t *xecfg, const char
>  		/* determine extent of line matched */
>  		bol = start+match[0].rm_so;
>  		eol = start+match[0].rm_eo;
> -		while (bol > start && *bol != '\n')
> -			bol--;
> +		while (bol > start && *--bol != '\n')
> +			; /* nothing */
>  		if (*bol == '\n')
>  			bol++;
>  		while (*eol && *eol != '\n')
> @@ -161,6 +161,7 @@ static const char *find_funcname_matching_regexp(xdemitconf_t *xecfg, const char
>  			return bol;
>  		start = eol;
>  	}
> +	return NULL;
>  }
>  
>  static const char *parse_range_funcname(
> diff --git a/t/t4211-line-log.sh b/t/t4211-line-log.sh
> index ac9e4d0928..c6540e822f 100755
> --- a/t/t4211-line-log.sh
> +++ b/t/t4211-line-log.sh
> @@ -315,4 +315,26 @@ test_expect_success 'line-log with --before' '
>  	test_cmp expect actual
>  '
>  
> +test_expect_success 'setup tests for zero-width regular expressions' '
> +	cat >expect <<-EOF
> +	Modify func1() in file.c
> +	Add func1() and func2() in file.c
> +	EOF
> +'
> +
> +test_expect_success 'zero-width regex $ matches any function name' '
> +	git log --format="%s" --no-patch "-L:$:file.c" >actual &&
> +	test_cmp expect actual
> +'
> +
> +test_expect_success 'zero-width regex ^ matches any function name' '
> +	git log --format="%s" --no-patch "-L:^:file.c" >actual &&
> +	test_cmp expect actual
> +'
> +
> +test_expect_success 'zero-width regex .* matches any function name' '
> +	git log --format="%s" --no-patch "-L:.*:file.c" >actual &&
> +	test_cmp expect actual
> +'
> +

Untested, but as most of this is repeated & would fail if the "setup"
test is skipped, a one-off function would be better, e.g.:
	
	test_zerowidth_regex () {
		local rx="$1" &&
		
		test_expect_success "zero-width regex '$rx' matches any function name" '
			cat >expect <<-\EOF &&
			Modify func1() in file.c
			Add func1() and func2() in file.c
			EOF
			git log --format="%s" --no-patch -L:"$rx":file.c >actual &&
			test_cmp expect actual
		'
	}
	
	test_zerowidth_regex '$'
	test_zerowidth_regex '^'
	test_zerowidth_regex '.*'

The change of direction for the fix itself looks good to me (re my
earlier feedback on a previous round), i.e. that we should fix our own
code, not forbid '$' in regexes.