mbedtls_hardware_poll should have a public prototype

[gilles-peskine-arm]

In Mbed TLS 3, the function mbedtls_hardware_poll() is the main way to set up an entropy source on a platform. Integrators enable MBEDTLS_ENTROPY_HARDWARE_ALT and define a function which “must be called mbedtls_hardware_poll(), have the same prototype as declared in library/entropy_poll.h, and accept NULL as first argument”.

library/entropy_poll.h is not a public header, so there's no way for the compiler to validate that mbedtls_hardware_poll() has the correct type. Furthermore there's no way to tell how mbedtls_hardware_poll() should behave, or even what its prototype is, without looking at the source code of Mbed TLS.

The declaration and documentation of mbedtls_hardware_poll() should be in a public header, like we do for ALT interfaces.

This only affects Mbed TLS 3.6. In 2.28, entropy_poll.h was private. In TF-PSA-Crypto, this interface will change and we intend to declare the function in a public header.

Note: since no integrator has complained about this (to my knowledge), we are likely to treat this as a low-priority issue.