[mbed-tls] Re: MBEDTLS_PLATFORM_TIME_TYPE_MACRO for custom mbedtls_time_t

-- Best regards, Gilles Peskine On 29/07/2026 17:42, Huber, Ralf wrote: > > Hello Gilles, > > Thank you for your reply. I can try submitting a patch, but I have > never worked with Github so far, so I first need to figure out how > that works 😃 > > After digging a bit deeper, I have some concerns about the types that > can reasonably be used for MBEDTLS_PLATFORM_TIME_TYPE_MACRO. I’m > hesitant to submit a patch unless I understand its effects. > > From x509.c: > > int mbedtls_x509_time_gmtime(mbedtls_time_t tt, mbedtls_x509_time *now) > > { > > struct tm tm; > > if (mbedtls_platform_gmtime_r(&tt, &tm) == NULL) { > > return -1; > > } > > now->year = tm.tm_year + 1900; > > now->mon  = tm.tm_mon  + 1; > > now->day  = tm.tm_mday; > > now->hour = tm.tm_hour; > > now->min  = tm.tm_min; > > now->sec  = tm.tm_sec; > > return 0; > > } > > static int x509_get_current_time(mbedtls_x509_time *now) > > { > > return mbedtls_x509_time_gmtime(mbedtls_time(NULL), now); > > } > > x509_get_current_time() passes the output of the custom mbedtls_time() > directly to mbedtls_x509_time_gmtime(), where it is immediately passed > to mbedtls_platform_gmtime_r(). > > For this particular piece of code, MBEDTLS_PLATFORM_TIME_TYPE_MACRO > could be anything (basic type, structure, function pointer, etc.), as > long as the custom mbedtls_platform_gmtime_r() can process the return > value from the custom mbedtls_time(). > > However, if the return value of mbedtls_time() is used somewhere in > Mbed TLS without passing it through mbedtls_platform_gmtime_r(), then > the caller likely expects that mbedtls_time() behaves like the time() > function of <time.h>. MBEDTLS_PLATFORM_TIME_TYPE_MACRO must then have > the same properties as time_t (as defined in the C standard), or at > least it must have those properties that Mbed TLS relies on. > > To add a compile-time check for suitable configurations of > MBEDTLS_PLATFORM_TIME_TYPE_MACRO, I would need to know what properties > Mbed TLS expects from it, and then somehow check if the custom > MBEDTLS_PLATFORM_TIME_TYPE_MACRO type has these properties. > > We only use a subset of Mbed TLS in our application. Using uint32_t > for MBEDTLS_PLATFORM_TIME_TYPE_MACRO might work for us, but maybe > there are other features in Mbed TLS where it would fail. I haven’t > searched through the whole Mbed TLS code. > > Is it certain that the only thing that is ever done with the return > value of mbedtls_time(), is that it is being passed to > mbedtls_platform_gmtime_r()? I.e. the only purpose of > MBEDTLS_PLATFORM_TIME_TYPE_MACRO is to align the return type of > mbedtls_time() with the type of the first parameter of > mbedtls_platform_gmtime_r()? > > Best regards, > > Ralf > > > *KION Supply Chain Solutions* > > *Von:*Gilles Peskine gilles.peskine@arm.com > *Gesendet:* Montag, 20. Juli 2026 18:51 > *An:* Huber, Ralf ralf.huber@kiongroup.com; > mbed-tls@lists.trustedfirmware.org > *Betreff:* Re: [mbed-tls] MBEDTLS_PLATFORM_TIME_TYPE_MACRO for custom > mbedtls_time_t > > *WARNING: This email originated outside of the company. DO NOT CLICK > links or attachments or enter any information into forms unless you > trust the sender and know the content is safe.* > > Hello, > > I agree that this looks like a mistake. The various platform > interfaces are subtly different, and I suspect that this was inspired > by a similar check for a different interface that doesn't apply here. > This is not tested (I think we have no tests for custom platform types). > > Can you please submit a patch? > > -- > Best regards, > > Gilles Peskine > TF-PSA-Crypto and Mbed TLS developer > > On 18/07/2026 15:15, Huber, Ralf via mbed-tls wrote: > > Dear Mbed TLS contributors, > > We are using mbedtls v4 for X.509 certificate verification on a 32 > bit microcontroller. The device has an RTC chip and we use its > time for mbedtls by defining MBEDTLS_PLATFORM_TIME_ALT. This is > working so far. > > The compiler provides <time.h>, with time_t being a signed 32 bit > type. I would like to stop mbedtls from using the date and time > functions of <time.h> and change mbedtls_time_t to _unsigned_ 32 > bit to avoid the year 2038 problem (when signed 32 bit time_t will > overflow). To do this, I defined > > #define MBEDTLS_PLATFORM_TIME_TYPE_MACRO unsigned long > > I also changed the custom time function > (MBEDTLS_PLATFORM_TIME_ALT) to return unsigned values and I > defined MBEDTLS_PLATFORM_GMTIME_R_ALT and provided a custom > mbedtls_platform_gmtime_r() that accepts unsigned values. > > When I build mbedtls with these settings, I get an error from > tf_psa_crypto_check_config.h: > > #if defined(MBEDTLS_PLATFORM_TIME_TYPE_MACRO) &&\ > > ( defined(MBEDTLS_PLATFORM_STD_TIME) ||\ > > defined(MBEDTLS_PLATFORM_TIME_ALT) ) > > #error "MBEDTLS_PLATFORM_TIME_TYPE_MACRO and > MBEDTLS_PLATFORM_STD_TIME/MBEDTLS_PLATFORM_TIME_ALT cannot be > defined simultaneously" > > #endif > > I wonder if this check is actually necessary or maybe a mistake? > Actually,  I expected the opposite, i.e. > > #if defined(MBEDTLS_PLATFORM_TIME_TYPE_MACRO) &&\ > > !( defined(MBEDTLS_PLATFORM_STD_TIME) ||\ > > defined(MBEDTLS_PLATFORM_TIME_ALT) ) > > #error "Must provide alternate time() function compatible with > type defined by MBEDTLS_PLATFORM_TIME_TYPE_MACRO" > > #endif > > Could somebody please explain why MBEDTLS_PLATFORM_TIME_TYPE_MACRO > can’t be used with MBEDTLS_PLATFORM_TIME_ALT? Can/Should I comment > out the #error? I tried that and the library compiles without > errors, and the software using the library also compiles without > errors, but I have not yet checked, if it actually works. > > Best regards, > > Ralf Huber > > KION Supply Chain Solutions > > Linde Material Handling GmbH, > Sitz der Gesellschaft: Aschaffenburg, Registergericht: > Aschaffenburg HRB9963, Ust-IdNr. DE814809128, Geschäftsführung: > Andreas Krinninger (Vorsitzender), Dr. Karoline Jung-Senssfelder, > Ulrike Just, Dr. Frank Schepp, Vorsitzende des Aufsichtsrats: > Valeria Gargiulo > > > > Linde Material Handling GmbH, > Sitz der Gesellschaft: Aschaffenburg, Registergericht: Aschaffenburg > HRB9963, Ust-IdNr. DE814809128, Geschäftsführung: Andreas Krinninger > (Vorsitzender), Dr. Karoline Jung-Senssfelder, Ulrike Just, Dr. Frank > Schepp, Vorsitzende des Aufsichtsrats: Valeria Gargiulo >