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GregorianDay() is supposed to calculate the day of the week (tm->tm_wday) for a given day/month/year. In that calcuation it indexed into an array called MonthOffset using tm->tm_mon-1. However tm_mon is zero-based, not one-based, so this is off-by-one. It also means that every January, GregoiranDay() will access element -1 of the MonthOffset array. It also doesn't appear to be a correct algorithm either: see in contrast kernel/time/timeconv.c's time_to_tm function. It's been broken forever, which suggests no-one in userland uses this. It looks like no-one in the kernel uses tm->tm_wday either (see e.g. drivers/rtc/rtc-ds1305.c:319). tm->tm_wday is conventionally set to -1 when not available in hardware so we can simply set it to -1 and drop the function. (There are over a dozen other drivers in drivers/rtc that do this.) Found using UBSAN. Cc: Andrey Ryabinin <aryabinin@virtuozzo.com> Cc: Andrew Morton <akpm@linux-foundation.org> # as an example of what UBSan finds. Cc: Alessandro Zummo <a.zummo@towertech.it> Cc: Alexandre Belloni <alexandre.belloni@free-electrons.com> Cc: rtc-linux@googlegroups.com Signed-off-by: Daniel Axtens <dja@axtens.net> Acked-by: Alexandre Belloni <alexandre.belloni@free-electrons.com> Signed-off-by: Michael Ellerman <mpe@ellerman.id.au> |
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