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Enhance precision and safety in time calculations #483

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Merged
merged 2 commits into from
Aug 31, 2024
Merged

Enhance precision and safety in time calculations #483

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100ddbd
Add mult_frac() to handle fractional multiplication safely
visitorckw Aug 30, 2024
85ab051
Update get_time_info() to use mult_frac() for precise time calculations
visitorckw Aug 30, 2024
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18 changes: 15 additions & 3 deletions src/utils.c
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Original file line number Diff line number Diff line change
Expand Up @@ -26,6 +26,19 @@

#define MAX_PATH_LEN 1024

/* Calculate "x * n / d" without unnecessary overflow or loss of precision.
*
* Reference:
* https://elixir.bootlin.com/linux/v6.10.7/source/include/linux/math.h#L121
*/
static inline uint64_t mult_frac(uint64_t x, uint64_t n, uint64_t d)
{
const uint64_t q = x / d;
const uint64_t r = x % d;

return q * n + r * n / d;
}

static void get_time_info(int32_t *tv_sec, int32_t *tv_nsec)
{
#if defined(HAVE_POSIX_TIMER)
Expand All @@ -40,14 +53,13 @@ static void get_time_info(int32_t *tv_sec, int32_t *tv_nsec)
*/
if (info.denom == 0)
(void) mach_timebase_info(&info);
/* Hope that the multiplication doesn't overflow. */
uint64_t nsecs = mach_absolute_time() * info.numer / info.denom;
uint64_t nsecs = mult_frac(mach_absolute_time(), info.numer, info.denom);
*tv_sec = nsecs / 1e9;
*tv_nsec = nsecs - (*tv_sec * 1e9);
#else /* low resolution timer */
clock_t t = clock();
*tv_sec = t / CLOCKS_PER_SEC;
*tv_nsec = (t % CLOCKS_PER_SEC) * (1e9 / CLOCKS_PER_SEC);
*tv_nsec = mult_frac(t % CLOCKS_PER_SEC, 1e9, CLOCKS_PER_SEC);
#endif
}

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