tree 25a35dac0cdb04b6cc263703ffdb877404a674f6
parent 473f98f2e013f116c0c48d02feff23b12e3e9733
author Hanno Becker <hanno.becker@arm.com> 1561542782 +0100
committer Hanno Becker <hanno.becker@arm.com> 1562581073 +0100

Introduce framework for macro-based definitions of ciphersuites

This commit is a step towards the goal of allowing to hardcode the choice
of a single ciphersuite at compile-time. The hoped for benefit of this is
that whereever a ciphersuite attribute is queried and checked against a
compile-time constant, the check can be recognized as either true or false
at compile-time, hence leading to a code-size reduction.

For this to work, the ciphersuite attribute getter functions
mbedtls_ssl_suite_get_xxx() will be modified to return something
the compiler can recognize as a compile-time constant. In particular,
in order to avoid relying on constant propagation abilities of the
compiler, these functions should ideally return constant symbols
(instead of, say, fields in a globally const structure instance).

This puts us in the following situation: On the one hand, there's the
array of ciphersuite information structures defining the attribute of
those ciphersuites the stack knows about. On the other hand, we need
direct access to those fields through constant symbols in the getter
functions.

In order to avoid any duplication of information, this commit exemplifies
how ciphersuites can be conveniently defined on the basis of macro
definitions, and how the corresponding instances of the ciphersuite
information structure can be auto-generated from this.

In the approach, to add support for a ciphersuite with official name
NAME (such as TLS_ECDHE_ECDSA_WITH_AES_128_CCM_8), the following macro
constants need to be defined in ssl_ciphersuites.h:

  MBEDTLS_SUITE__ NAME __ID
  MBEDTLS_SUITE__ NAME __NAME
  MBEDTLS_SUITE__ NAME __CIPHER
  MBEDTLS_SUITE__ NAME __MAC
  ...

To make check-names.sh happy, one also needs a dummy macro

  MBEDTLS_SUITE__ NAME()

These ciphersuite attribute values can then be queried via

  MBEDTLS_SSL_SUITE_ID( NAME_MACRO )
  ...

where NAME_MACRO can be any macro expanding to a defined NAME.

Further, a convenience macro

  MBEDTLS_SSL_SUITE_INFO( NAME_MACRO )

is provided that again takes a macro NAME_MACRO expanding to a
defined NAME, and itself expands to an instance of
mbedtls_ssl_ciphersuite_info_t using the macro attributes
defined for NAME. This macro is then used in ssl_ciphersuites.c
when defining the array of known ciphersuite information structures,
(a) without duplicating the information, and (b) with increased
readability, because there's only one line for each ciphersuite.