Re: [PATCH v2] line-range: Fix infinite loop bug with degenerate regex

From: Eric Sunshine <sunshine@sunshineco.com>
Date: Dec 7, 2022, 04:33 UTC
Message-ID: <CAPig+cTypsXUrzkdO7S3cGFRWdO5CiaTKU_uM7av=M3nGHQ0+w@mail.gmail.com>
In-Reply-To: <20221205193625.2424202-1-lars@oddbit.com>

On Mon, Dec 5, 2022 at 2:45 PM Lars Kellogg-Stedman <lars@oddbit.com> wrote:

> line-range: Fix infinite loop bug with degenerate regex

s/Fix/fix/

Show 6 quoted lines

> When the -L argument to "git log" is passed the degenerate regular
> expression "$" (as in "-L :$:line-range.c"), this results in an
> infinite loop in find_funcname_matching_regexp() (the function
> iterates through the file correctly, but when it reaches the end of
> the file it matches $ against the empty string, "", and at that points
> loops forever).

s/points/point/

Show 5 quoted lines

> Modify the loop condition from while (1) to while (*start) so that the
> loop exits when start is the empty string. In this case, "git log" exits
> with the error:
>
>     fatal: -L parameter '$' starting at line 1: no match

I've never really used `-L:funcname:`, so it took me by surprise that this would be reported as a fatal error. However, now that I've read through the code and put a bit of thought into degenerate cases, I can't come up with any better way to report it. Okay.

Show 21 quoted lines

> Originally reported in <https://stackoverflow.com/q/74690545/147356>.
>
> Signed-off-by: Lars Kellogg-Stedman <lars@oddbit.com>
> ---
> diff --git 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)
> @@ -161,6 +161,8 @@ static const char *find_funcname_matching_regexp(xdemitconf_t *xecfg, const char
>                         return bol;
>                 start = eol;
>         }
> +
> +    return NULL;
>  }

I'm not particularly familiar with this code (even though I touched it years ago), so I can't say whether this is the best fix, but it seems reasonable now that I've studied the code and put some thought into degenerate cases. I had wondered if it would be possible to catch these degenerate cases in some general way, but whatever ideas I had seemed too special-purpose; your solution here is straightforward and probably more robust than special-purpose code.

Style-wise, I'd probably drop the blank line before `return NULL`.

Please do re-roll once more with a test which verifies that this bug has been fixed. Here's a test you can add to the bottom of t/t4211-line-log.sh for that purpose (be sure to re-indent it with TABs which Gmail mushes out):

    test_expect_success 'degenerate -L:$: does not hang' '
        >content-immaterial &&
        git add content-immaterial &&
        git commit -m immaterial &&
        test_must_fail git log -L:$:content-immaterial
    '

And here's my sign-off which you can insert above your own sign-off if you include the above test.

    Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>