The size of the seedfile used by the entropy module when
MBEDTLS_ENTROPY_NV_SEED is enabled is 32 byte when
MBEDTLS_ENTROPY_FORCE_SHA256 is enabled or MBEDTLS_SHA512_C is
disabled, and 64 bytes otherwise. A larger seedfile is ok on
entry (the code just grabs the first N bytes), but a smaller seedfile
is not ok. Therefore, if you run a component with a 32-byte seedfile
and then a component with a 64-byte seedfile, the second component
fails in the unit tests (up to test_suite_entropy which erases the
seedfile and creates a fresh one).
This is ok up to now because we only enable MBEDTLS_ENTROPY_NV_SEED
together with MBEDTLS_ENTROPY_FORCE_SHA256. But it prevents enabling
MBEDTLS_ENTROPY_NV_SEED without MBEDTLS_ENTROPY_FORCE_SHA256.
To fix this, unconditionally create a seedfile before each component.
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