Disclaimer: It took me a solid 4-5 hours of looking for an answer and after figuring it out I decided to post it here for people in the same place.
OP-TEE is quite a good environment to develop TAs and CAs, however, there is no straightforward method of acquiring a datetime formatted properly. There is no struct tm either. Therefore, it made me wonder how do I get a datetime format in OP-TEE TAs?
What I spent a long time trying was to utilize the already supported mbedTLS libraries which, for a newcomer, would seem like they do support getting datetime format. After all, they do have gmtime which is supposed to return this value.
However, unfortunately, the gmtime and relevant functions have no implementation for the platform OP-TEE on ARMv8. That's a pretty tough luck.
So how do you get UTC time in an OP-TEE TA?
All OP-TEE development for ARMv8 is done using C. However, it lacks major libc support. Practically, it has very little libraries (e.g. string.h) which are skimmed down versions from the original libc corresponding libraries.
With that, the provided <time.h> in the OP-TEE contains nothing but a typedef for time_t and that's it.
The problem can be broken down to two sections:
This is an interesting problem, and while the straight forward solution is to simply do this:
TEE_Time tt;
TEE_GetREETime(&tt);
This solution may not be satisfactory for many people who would not want to rely on REE's (Rich Execution Environment, AKA the vulnerable environment) count of epochs. This can be problematic for security sensitive operations where you need time to be legit and with no room for REE to modify it to perform a certain attack.
In the case described above, you will have to obtain the epochs from a hardware clock which will depend on the hardware board you are developing the TA on. You can even retrieve it from location devices which also return UTC time in NMEA statements. While it may not be 100% precise to the second, it may just be enough. If you need very high precision, you will need to get it from a hardware device securely.
Either way, you have to figure out how to get the epochs on your own. This answer focuses on the second part: getting the datetime.
gmtime which does not exist in the OP-TEE. It has no implementation. And you need a minimalist implementation to keep things simple.Luckily, I was able to find this answer. Which links to newlib libraries that develop for Free BSD which are ideal for embedded systems. Hence why it's useful here!
I was able to put it together from their implementation and you can use it here:
gmtime_r.h:
#include <inttypes.h>
#define SECSPERMIN 60L
#define MINSPERHOUR 60L
#define HOURSPERDAY 24L
#define SECSPERHOUR (SECSPERMIN * MINSPERHOUR)
#define SECSPERDAY (SECSPERHOUR * HOURSPERDAY)
#define DAYSPERWEEK 7
#define MONSPERYEAR 12
#define YEAR_BASE 1900
#define EPOCH_YEAR 1970
#define EPOCH_WDAY 4
#define EPOCH_YEARS_SINCE_LEAP 2
#define EPOCH_YEARS_SINCE_CENTURY 70
#define EPOCH_YEARS_SINCE_LEAP_CENTURY 370
#define isleap(y) ((((y) % 4) == 0 && ((y) % 100) != 0) || ((y) % 400) == 0)
typedef int64_t time_t;
struct tm
{
int tm_sec;
int tm_min;
int tm_hour;
int tm_mday;
int tm_mon;
int tm_year;
int tm_wday;
int tm_yday;
int tm_isdst;
};
struct tm* gmtime_r (time_t tim_p, struct tm* res);
gmtime_r.c:
#include "gmtime_r.h"
#define EPOCH_ADJUSTMENT_DAYS 719468L
/* year to which the adjustment was made */
#define ADJUSTED_EPOCH_YEAR 0
/* 1st March of year 0 is Wednesday */
#define ADJUSTED_EPOCH_WDAY 3
/* there are 97 leap years in 400-year periods. ((400 - 97) * 365 + 97 * 366) */
#define DAYS_PER_ERA 146097L
/* there are 24 leap years in 100-year periods. ((100 - 24) * 365 + 24 * 366) */
#define DAYS_PER_CENTURY 36524L
/* there is one leap year every 4 years */
#define DAYS_PER_4_YEARS (3 * 365 + 366)
/* number of days in a non-leap year */
#define DAYS_PER_YEAR 365
/* number of days in January */
#define DAYS_IN_JANUARY 31
/* number of days in non-leap February */
#define DAYS_IN_FEBRUARY 28
/* number of years per era */
#define YEARS_PER_ERA 400
struct tm* gmtime_r (time_t tim_p, struct tm* res)
{
time_t days, rem;
const time_t lcltime = tim_p;
int era, weekday, year;
unsigned erayear, yearday, month, day;
unsigned long eraday;
days = lcltime / SECSPERDAY + EPOCH_ADJUSTMENT_DAYS;
rem = lcltime % SECSPERDAY;
if (rem < 0)
{
rem += SECSPERDAY;
--days;
}
/* compute hour, min, and sec */
res->tm_hour = (int) (rem / SECSPERHOUR);
rem %= SECSPERHOUR;
res->tm_min = (int) (rem / SECSPERMIN);
res->tm_sec = (int) (rem % SECSPERMIN);
/* compute day of week */
if ((weekday = ((ADJUSTED_EPOCH_WDAY + days) % DAYSPERWEEK)) < 0)
weekday += DAYSPERWEEK;
res->tm_wday = weekday;
/* compute year, month, day & day of year */
/* for description of this algorithm see
* http://howardhinnant.github.io/date_algorithms.html#civil_from_days */
era = (days >= 0 ? days : days - (DAYS_PER_ERA - 1)) / DAYS_PER_ERA;
eraday = days - era * DAYS_PER_ERA; /* [0, 146096] */
erayear = (eraday - eraday / (DAYS_PER_4_YEARS - 1) + eraday / DAYS_PER_CENTURY -
eraday / (DAYS_PER_ERA - 1)) / 365; /* [0, 399] */
yearday = eraday - (DAYS_PER_YEAR * erayear + erayear / 4 - erayear / 100); /* [0, 365] */
month = (5 * yearday + 2) / 153; /* [0, 11] */
day = yearday - (153 * month + 2) / 5 + 1; /* [1, 31] */
month += month < 10 ? 2 : -10;
year = ADJUSTED_EPOCH_YEAR + erayear + era * YEARS_PER_ERA + (month <= 1);
res->tm_yday = yearday >= DAYS_PER_YEAR - DAYS_IN_JANUARY - DAYS_IN_FEBRUARY ?
yearday - (DAYS_PER_YEAR - DAYS_IN_JANUARY - DAYS_IN_FEBRUARY) :
yearday + DAYS_IN_JANUARY + DAYS_IN_FEBRUARY + isleap(erayear);
res->tm_year = year - YEAR_BASE;
res->tm_mon = month;
res->tm_mday = day;
res->tm_isdst = 0;
return (res);
}
You can put both of these files in your TA folder, and make sure you add gmtime.c in the sources list in the sub.mk. And finally, in the TA itself you can use it:
TEE_Time tt;
TEE_GetREETime(&tt);
struct tm *lt, temp;
lt = gmtime((time_t)tt.seconds, &temp);
DMSG("%4d-%2d-%2d %2d:%2d:%2d", lt->tm_year + 1900, lt->tm_mon + 1, lt->tm_mday, lt->tm_hour, lt->tm_min, lt->tm_sec);
This will print the time and date in a proper format.
Currently, I have not yet ported the implementation of strftime, but soon I will do that as well which will auto-format the tm struct and add the 1900 to tm_year and 1 to tm_mon.
In the meantime, I hope this finds someone in need.