From: Phillip Wood Date: Thu, 15 Oct 2026 09:28:15 GMT Subject: Re: [PATCH 5/5] date: avoid overflow in approxidate relative-time multiplication Message-ID: <163d14a8-711c-4d4f-b441-8268db819541@gmail.com> In-Reply-To: Hi Johannes On 14/10/2026 07:09, Johannes Schindelin via GitGitGadget wrote: > From: Johannes Schindelin > > 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 > --- > 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; > }