Build fails if gmtime_s() or gmtime_r() are not available

[RonEld]

Description

• Type: Bug
• Priority: Major

---

Bug

OS
Mbed OS
mbed TLS build:
Version: 2.12.0
Compiler and options (if you used a pre-built binary, please indicate how you obtained it): ARM Compiler
Additional environment information:

Expected behavior
build to succeed
Actual behavior
build failure:

"/tmp/H7cWpV", line 124 (column 3): Warning: L6312W: Empty Execution region description for region RW_IRAM1
Error: L6218E: Undefined symbol gmtime_r (referred from BUILD/K64F/ARM/mbed-os/features/mbedtls/src/x509.o).
Finished: 0 information, 1 warning and 1 error messages.
[ERROR] "/tmp/H7cWpV", line 124 (column 3): Warning: L6312W: Empty Execution region description for region RW_IRAM1
Error: L6218E: Undefined symbol gmtime_r (referred from BUILD/K64F/ARM/mbed-os/features/mbedtls/src/x509.o).

Steps to reproduce
build an mbed OS application, with Mbed TLS 2.12.0, using mbed-cli

This is from change introduced in 512b4ee

[ciarmcom]

ARM Internal Ref: IOTSSL-2436

[gilles-peskine-arm]

That's not a problem with armcc, it's a problem with libc compatibility. If the toolchain makes a difference, it's because it's picking up a different libc. gmtime_r is a POSIX function, not a standard C function. It's legitimate for a libc to have gmtime but not gmtime_r. We should use gmtime_r where it's present and gmtime otherwise.

[gilles-peskine-arm]

The documentation for MBEDTLS_HAVE_TIME_DATE states:

System has time.h and time(), gmtime() and the clock is correct.

#1198 added the requirement that the platform must have gmtime_r or gmtime_s, but didn't change the documentation. I do think we should support MBEDTLS_HAVE_TIME_DATE on platforms with gmtime only. Mbed OS is a pretty compelling use case.

[andresag01]

To expand on the information above:

• This problem occurs with Mbed TLS version 2.12.0 and the current Mbed OS master (1023280)
• As @gilles-peskine-arm, this is a problem with the libc used to build Mbed OS and Mbed TLS as gmtime_r is a POSIX function
• I have tested and can confirm that the problem happens with the Mbed OS cli toolchain options uARM and ARM. I suspect the problem also happens with IAR and ARMC6, but I have not tested this.
• From the Mbed TLS side, the issue was introduced in gmtime_r and mbedtls_time on Windows #1198, and from the Mbed OS side the issue was introduced in Integrate mbed OS RTC with mbed TLS ARMmbed/mbed-os#4846 which enables MBEDTLS_HAVE_TIME_DATE

@gilles-peskine-arm: I agree that the problem can be solved by detecting whether the libc is POSIX (by checking _POSIX_VERSION for example) and then using the appropriate function. However, as pointed out in #1198, the issue is that gmtime() is not thread safe. Do you think we should provide some sort of _ALT option, perhaps something else, to allow platforms like Mbed OS to deal with this sort of problem safely?

[simonbutcher]

There's actually an underlying problem here that armcc doesn't actually support gmtime() either! It actually always returns NULL.

The fix in #1927 will doubtless help someone, but it doesn't help Mbed OS. The PR in #1927 will now allow the code to build - but it still won't yield a useful date and time nor do the right certificate date checks.

Therefore, I've changed the title to 'Build fails if gmtime_s() or gmtime_r() are not available' and created a new bug of #1975 - 'Arm C Compiler doesn't support gmtime()'.

[andresag01]

@sbutcher-arm: Thanks for the information regarding the state of gmtime() for armcc.

While the PR #1927 does not fully resolve the armcc situation for Mbed OS, I do think that it is a first step towards fixing the issue for Mbed OS. This is because #1927 allows us to configure an implementation for gmtime() at compile time. So in the case of Mbed OS, we could detect the supported toolchains/libcs that do not have a gmtime() and automatically configure a suitable implementation.

[simonbutcher]

The Arm C Compiler does not support gmtime() or gmtime_r(). It's not intended that the Mbed TLS project should provide implementations for these functions when the platform doesn't so on platforms where it's not available, we now have in PR #1927 which has now been merged support to allow 3rd parties to provide their own implementation.

In addition, on Mbed OS, date/time support is now disabled by default with Mbed OS PR #7868, so the build is no longer broken. Therefore this issue can now be closed.