• Daniel Axtens's avatar
    powerpc: Remove broken GregorianDay() · 00b912b0
    Daniel Axtens authored
    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: default avatarDaniel Axtens <dja@axtens.net>
    Acked-by: default avatarAlexandre Belloni <alexandre.belloni@free-electrons.com>
    Signed-off-by: default avatarMichael Ellerman <mpe@ellerman.id.au>
    00b912b0
opal-rtc.c 2.11 KB