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[PATCH] rtc: fix rtc_time64_to_tm calculation

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  [PATCH] rtc: fix rtc_time64_to_tm calculation Andrzej Hajda <a.hajda@samsung.com> - 2016-01-07 13:20 +0100
    Re: [PATCH] rtc: fix rtc_time64_to_tm calculation Alexandre Belloni <alexandre.belloni@free-electrons.com> - 2016-01-07 18:30 +0100

#1303533 — [PATCH] rtc: fix rtc_time64_to_tm calculation

Type of local variable days has been changed recently to unsigned,
but it can take negative values. As a result it works incorrectly for some
arguments. The patch fixes it.

The problem has been detected using proposed semantic patch
scripts/coccinelle/tests/unsigned_lesser_than_zero.cocci [1].

[1]: http://permalink.gmane.org/gmane.linux.kernel/2120705

Fixes: cab572b82c4b ('rtc: fix overflow and incorrect calculation in rtc_time64_to_tm')
Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
---
 drivers/rtc/rtc-lib.c | 13 ++++++++-----
 1 file changed, 8 insertions(+), 5 deletions(-)

diff --git a/drivers/rtc/rtc-lib.c b/drivers/rtc/rtc-lib.c
index cf2b23c..6323d13 100644
--- a/drivers/rtc/rtc-lib.c
+++ b/drivers/rtc/rtc-lib.c
@@ -54,7 +54,7 @@ void rtc_time64_to_tm(time64_t time, struct rtc_time *tm)
 {
 	unsigned int month, year;
 	int secs;
-	unsigned long days;
+	unsigned long days, leaps;
 
 	/*
 	 * time must be positive
@@ -66,13 +66,16 @@ void rtc_time64_to_tm(time64_t time, struct rtc_time *tm)
 	tm->tm_wday = (days + 4) % 7;
 
 	year = 1970 + days / 365;
-	days -= (year - 1970) * 365
-		+ LEAPS_THRU_END_OF(year - 1)
-		- LEAPS_THRU_END_OF(1970 - 1);
-	if (days < 0) {
+	days -= (year - 1970) * 365;
+	leaps = LEAPS_THRU_END_OF(year - 1) - LEAPS_THRU_END_OF(1970 - 1);
+
+	while (days < leaps) {
 		year -= 1;
 		days += 365 + is_leap_year(year);
 	}
+
+	days -= leaps;
+
 	tm->tm_year = year - 1900;
 	tm->tm_yday = days + 1;
 
-- 
1.9.1

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#1303748

On 07/01/2016 at 13:11:50 +0100, Andrzej Hajda wrote :
> Type of local variable days has been changed recently to unsigned,
> but it can take negative values. As a result it works incorrectly for some
> arguments. The patch fixes it.
> 

Yeah, I was not planning to send that patch finally because it had more
issues than expected. What you did seems good, I'll have a closer look
when I'm back from vacations. My current internet access is too limited
to be able to pull/push anything with git.

> The problem has been detected using proposed semantic patch
> scripts/coccinelle/tests/unsigned_lesser_than_zero.cocci [1].
> 
> [1]: http://permalink.gmane.org/gmane.linux.kernel/2120705
> 
> Fixes: cab572b82c4b ('rtc: fix overflow and incorrect calculation in rtc_time64_to_tm')
> Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
> ---
>  drivers/rtc/rtc-lib.c | 13 ++++++++-----
>  1 file changed, 8 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/rtc/rtc-lib.c b/drivers/rtc/rtc-lib.c
> index cf2b23c..6323d13 100644
> --- a/drivers/rtc/rtc-lib.c
> +++ b/drivers/rtc/rtc-lib.c
> @@ -54,7 +54,7 @@ void rtc_time64_to_tm(time64_t time, struct rtc_time *tm)
>  {
>  	unsigned int month, year;
>  	int secs;
> -	unsigned long days;
> +	unsigned long days, leaps;
>  
>  	/*
>  	 * time must be positive
> @@ -66,13 +66,16 @@ void rtc_time64_to_tm(time64_t time, struct rtc_time *tm)
>  	tm->tm_wday = (days + 4) % 7;
>  
>  	year = 1970 + days / 365;
> -	days -= (year - 1970) * 365
> -		+ LEAPS_THRU_END_OF(year - 1)
> -		- LEAPS_THRU_END_OF(1970 - 1);
> -	if (days < 0) {
> +	days -= (year - 1970) * 365;
> +	leaps = LEAPS_THRU_END_OF(year - 1) - LEAPS_THRU_END_OF(1970 - 1);
> +
> +	while (days < leaps) {
>  		year -= 1;
>  		days += 365 + is_leap_year(year);
>  	}
> +
> +	days -= leaps;
> +
>  	tm->tm_year = year - 1900;
>  	tm->tm_yday = days + 1;
>  
> -- 
> 1.9.1
> 

-- 
Alexandre Belloni, Free Electrons
Embedded Linux, Kernel and Android engineering
http://free-electrons.com
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