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powerpc: Remove broken GregorianDay()

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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 <[email protected]>
Cc: Andrew Morton <[email protected]> # as an example of what UBSan finds.
Cc: Alessandro Zummo <[email protected]>
Cc: Alexandre Belloni <[email protected]>
Cc: [email protected]
Signed-off-by: Daniel Axtens <[email protected]>
Acked-by: Alexandre Belloni <[email protected]>
Signed-off-by: Michael Ellerman <[email protected]>
This commit is contained in:
Daniel Axtens 2015-12-15 18:09:14 +11:00 • committed by Michael Ellerman
commit 00b912b0c8
5 changed files with 5 additions and 39 deletions
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2
drivers/rtc/rtc-opal.c
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@ -40,7 +40,7 @@ static void opal_to_tm(u32 y_m_d, u64 h_m_s_ms, struct rtc_time *tm)
tm->tm_min = bcd2bin((h_m_s_ms >> 48) & 0xff);
tm->tm_sec = bcd2bin((h_m_s_ms >> 40) & 0xff);
GregorianDay(tm);
tm->tm_wday = -1;
}
static void tm_to_opal(struct rtc_time *tm, u32 *y_m_d, u64 *h_m_s_ms)