Re: [PATCH 5/5] date: avoid overflow in approxidate relative-time multiplication

[Junio C Hamano <gitster@pobox.com>]

"Johannes Schindelin via GitGitGadget" <gitgitgadget@gmail.com>
writes:

> On platforms where `time_t` is unsigned (and where the pre-processor
> allows us to determine this fact), reject counts that are too large and
> report a parse error. Narrowing them first could otherwise produce
> negative or truncated values that become bogus offsets. Leave signed
> `time_t` count parsing unchanged.

Does this mean that "git log -p" in a project that were made on
reasonably sized platforms and then cloned to my box may cause the
history traversal to stop with an error when a commit has author or
committer timestamp that would not fit the time_t on my box?

It would have been horrible regression than showing the entire
history with patches with bogus timestamp if it were the case.

But I do not think that is what happens.  approxidate_careful() is
only used to parse input by local user (e.g., "git commit
--date="...", "git log @{...}"), and the changes in this patch would
not affect behaviours of other uses of timestamps, so we are safe.

> Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
> ---
>  date.c | 26 ++++++++++++++++++++++++--
>  1 file changed, 24 insertions(+), 2 deletions(-)
>
> diff --git a/date.c b/date.c
> index 014065b419..0d65804d2f 100644
> --- a/date.c
> +++ b/date.c
> @@ -1298,7 +1298,18 @@ static const char *approxidate_alpha(const char *date, struct tm *tm, struct tm
>  	while (tl->type) {
>  		size_t len = strlen(tl->type);
>  		if (match_string(date, tl->type) >= len-1) {
> -			update_tm(tm, now, tl->length * *num);
> +			time_t length = tl->length, num_t = *num;
> +			time_t seconds;
> +#ifndef is_unsigned_type
> +			seconds = length * num_t;
> +#else
> +			time_t max_val = is_unsigned_type(length)
> +				? maximum_unsigned_value_of_type(length)
> +				: maximum_signed_value_of_type(length);
> +			seconds = mult_overflows(length, num_t)
> +				? max_val : length * num_t;
> +#endif
> +			update_tm(tm, now, seconds);
>  			*num = 0;
>  			*touched = 1;
>  			return end;
> @@ -1368,8 +1379,15 @@ static const char *approxidate_digit(const char *date, struct tm *tm, int *num,
>  	}
>  
>  	/* Accept zero-padding only for small numbers ("Dec 02", never "Dec 0002") */
> -	if (date[0] != '0' || end - date <= 2)
> +	if (date[0] != '0' || end - date <= 2) {
> +#ifdef is_unsigned_type
> +		if (is_unsigned_type((time_t)0) &&
> +		    (number > INT_MAX ||
> +		     number > maximum_unsigned_value_of_type((time_t)0)))
> +			return NULL;
> +#endif
>  		*num = number;
> +	}
>  	return end;
>  }
>  
> @@ -1398,6 +1416,10 @@ static timestamp_t approxidate_str(const char *date,
>  		if (isdigit(c)) {
>  			pending_number(&tm, &number);
>  			date = approxidate_digit(date-1, &tm, &number, time_sec);
> +			if (!date) {
> +				*error_ret = 1;
> +				return 0;
> +			}
>  			touched = 1;
>  			continue;
>  		}