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

[Phillip Wood <phillip.wood123@gmail.com>]

Hi Johannes

On 14/10/2026 07:09, Johannes Schindelin via GitGitGadget wrote:
> From: Johannes Schindelin <johannes.schindelin@gmx.de>
> 
> The `approxidate_alpha()` function multiplies a time unit length (e.g.,
> 604800 for weeks) by a user-supplied number to compute a time offset in
> seconds. Both operands are `int`, and the product is passed to
> `update_tm()` as `time_t`. For inputs like "99999 weeks ago", the int
> multiplication overflows (604800 * 99999 is roughly 60 billion, far
> beyond `INT_MAX`), producing a wrapped value that results in a wrong
> date.

Signed integer overflow is undefined so we don't necessarily get a 
wrapped value - the whole program is invalid.

It looks like "length" is always positive, can the user supplied number 
ever be negative?

> Use the checked `time_t` multiplication so oversized products do not
> overflow or wrap around.

This is only true when compiling with gcc or clang though because 
mult_overflows() always returns 0 on other compilers, so we're still 
exposed to undefined behavior when compiling with a different compiler 
on platforms where time_t is signed. I wonder if it would be better to 
perform the multiplication on a known type and then check that the 
result can be losslessly converted to time_t

	intmax_t length tl->length, num_t = *num, result;

	if (signed_mult_overflows(length, num_t))
		// error
	result = length * num_t;
	if ((intmax_t)((time_t)result) != result ||
	    (result < 0 && (time_t)result > 0))
		// error

that would avoid overflow on all platforms, regardless of the compiler 
and regardless of the type of time_t. Having mult_overflows() only 
implemented for gcc seems like a footgun waiting to happen when we're 
typically checking for overflow to avoid some form of bad behavior.

> 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.

I'm a bit confused what this is trying to do - why can't we check this 
when time_t is signed?

	if ((timestamp_t)((int)number) != number)
		return NULL

	return *val = value;

Ensures "value" fits in "val" without truncation, we could also add a 
check that value is positive when time_t is unsigned if we wanted.

Thanks

Phillip
> Pointed out by Coverity.
> 
> Assisted-by: Claude Opus 4.6 and GPT-6.1 Sol
> 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;
>   		}