Re: [PATCH 5/5] date: avoid overflow in approxidate relative-time multiplication
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From: Phillip Wood <hidden>
Date: 2026-10-15 09:28:18
Hi Johannes On 14/10/2026 07:09, Johannes Schindelin via GitGitGadget wrote:
From: Johannes Schindelin <redacted> 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
quoted hunk ↗ jump to hunk
Pointed out by Coverity. Assisted-by: Claude Opus 4.6 and GPT-6.1 Sol Signed-off-by: Johannes Schindelin <redacted> --- 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; }