unknown type name 'mbedtls_ecp_restart_ctx'

[jwhui]

Description

• Type: Bug
• Priority: Minor

mbed TLS build:
Version: 2.14.0
Configuration: MBEDTLS_ECP_ALT defined and MBEDTLS_ECP_RESTARTABLE not defined

Expected behavior
Compiles without error.

Actual behavior
error: unknown type name 'mbedtls_ecp_restart_ctx'

Steps to reproduce

1. Define MBEDTLS_ECP_ALT and do not define MBEDTLS_ECP_RESTARTABLE in config.
2. Compile library/ecdh.c.

I could simply add typedef void mbedtls_ecp_restart_ctx; to my config to fix the above issue, but is that expected?

[RonEld]

@jwhui Thank you for reporting this issue!
I believe this issue is very similar to #2163 (but not the same), but for the alternative ECP ( as opposed to the alternative ecdsa and acdh functions in that issue)
Obviously, we wouldn't expect a build failure.
As you can see in ecp.h , your fix suggestion is as expected, in case MBEDTLS_ECP_RESTARTABLE is not defined. However, since you have an alternative implementation of of ECP. all your structs defeinitions should be in your ecp_alt.h file.

You should add typedef void mbedtls_ecp_restart_ctx; to ecp_alt.h and not to your config file.
I don't think this is a bug, since it is a matter of alternative implemetnation, and an alternative implementation can define the structs however it fits the driver.
I would like to hear the opinion on this matter from my colleague @mpg

I hope this answers your question

[jwhui]

I find it somewhat odd that ecp_alt.h needs to include a definition for mbedtls_ecp_restart_ctx even when MBEDTLS_ECP_RESTARTABLE is not defined. However, is that what's intended?

[RonEld]

As you can see from my previous comment, mbedtls_ecp_restart_ctx should be defined even if MBEDTLS_ECP_RESTARTABLE is not defined, as it is used in other API, such as ecdh and ecdsa. ecp_alt.h is used to define your alternative implementation structures. Since it is not predictable how every platform will need to define their contexts.
Having said that, I agree that in matters of mbedtls_ecp_restart_ctx, when MBEDTLS_ECP_RESTARTABLE is not defined, it is perhaps not the best design, and we may consider moving the definition referenced earlier, to a location independent on MBEDTLS_ECP_ALT.

[mpg]

Hi @jwhui and thanks for your report! Sorry for replying so late. After analyzing the issue, I fully agree with my colleague @RonEld:

• currently you have to add something like typedef void mbedtls_ecp_restart_ctx; somewhere and the ideal place is your ecp_alt.h
• however, we acknowledge that's a bit of an annoyance, and I can confirm that's one we didn't fully anticipate when designing restartable ECC
• so I think we'll consider removing the requirement for you to define that type in your ecp_alt.h, probably by moving its definition out of the the MBEDTLS_ECP_ALT guards as @RonEld suggests, as it looks like the simplest way to solve the issue.

[mpg]

I'm relabeling as a bug, as I think that's a regression: existing ecp_alt code that was working and correct with Mbed TLS 2.13.0 will no longer build with Mbed TLS 2.14.0 (until it's modified to add the type definition). This is a break of our compatibility promises, it should not have happened, and should be fixed.

[RonEld]

@mpg I am not entirely convinced this is a bug.
One can have an alternative implementation for the restartable feature as well, in which case, mbedtls_ecp_restart_ctx will be defined differently.

[mpg]

@RonEld I think we ruled out alternative implementations of the restartable feature: https://github.com/ARMmbed/mbedtls/blob/development/include/mbedtls/check_config.h#L111 - when we did that, we should have moved the type out of the ALT guard, as it's not ALTable.

[RonEld]

@mpg thank for the reminder! ( I believe I even made that change:) )

[ciarmcom]

ARM Internal Ref: IOTSSL-2675

[jwhui]

@mpg @RonEld, thanks for looking into this!

[simonbutcher]

PR's #2334 and #2336 have been merged so this issue can now be closed.