Re: [PATCH 5/5] date: avoid overflow in approxidate relative-time multiplication
Hi Johannes
On 14/10/2026 07:09, Johannes Schindelin via GitGitGadget wrote:
Show 8 quoted lines
> 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.
Show 5 quoted lines
> 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
Show 56 quoted lines
> 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;
> }