This commit is the first in a series demonstrating how code-size
can be reduced by hardcoding parts of the SSL configuration at
compile-time, focusing on the example of the configuration of
the ExtendedMasterSecret extension.
The flexibility of an SSL configuration defined a runtime vs.
compile-time is necessary for the use of Mbed TLS as a
dynamically linked library, but is undesirable in constrained
environments because it introduces the following overhead:
- Definition of SSL configuration API (code-size overhead)
(and on the application-side: The API needs to be called)
- Additional fields in the SSL configuration (RAM overhead,
and potentially code-size overhead if structures grow
beyond immediate-offset bounds).
- Dereferencing is needed to obtain configuration settings.
- Code contains branches and potentially additional structure
fields to distinguish between different configurations.
Considering the example of the ExtendedMasterSecret extension,
this instantiates as follows:
- mbedtls_ssl_conf_extended_master_secret() and
mbedtls_ssl_conf_extended_master_secret_enforced()
are introduced to configure the ExtendedMasterSecret extension.
- mbedtls_ssl_config contains bitflags `extended_ms` and
`enforce_extended_master_secret` reflecting the runtime
configuration of the ExtendedMasterSecret extension.
- Whenever we need to access these fields, we need a chain
of dereferences `ssl->conf->extended_ms`.
- Determining whether Client/Server should write the
ExtendedMasterSecret extension needs a branch
depending on `extended_ms`, and the state of the
ExtendedMasterSecret negotiation needs to be stored in a new
handshake-local variable mbedtls_ssl_handshake_params::extended_ms.
Finally (that's the point of ExtendedMasterSecret) key derivation
depends on this handshake-local state of ExtendedMasterSecret.
All this is unnecessary if it is known at compile-time that the
ExtendedMasterSecret extension is used and enforced:
- No API calls are necessary because the configuration is fixed
at compile-time.
- No SSL config fields are necessary because there are corresponding
compile-time constants instead.
- Accordingly, no dereferences for field accesses are necessary,
and these accesses can instead be replaced by the corresponding
compile-time constants.
- Branches can be eliminated at compile-time because the compiler
knows the configuration. Also, specifically for the ExtendedMasterSecret
extension, the field `extended_ms` in the handshake structure
is unnecessary, because we can fail immediately during the Hello-
stage of the handshake if the ExtendedMasterSecret extension
is not negotiated; accordingly, the non-ExtendedMS code-path
can be eliminated from the key derivation logic.
A way needs to be found to allow fixing parts of the SSL configuration
at compile-time which removes this overhead in case it is used,
while at the same time maintaining readability and backwards
compatibility.
This commit proposes the following approach:
From the user perspective, for aspect of the SSL configuration
mbedtls_ssl_config that should be configurable at compile-time,
introduce a compile-time option MBEDTLS_SSL_CONF_FIELD_NAME.
If this option is not defined, the field is kept and configurable
at runtime as usual. If the option is defined, the field is logically
forced to the value of the option at compile time.
Internally, read-access to fields in the SSL configuration which are
configurable at compile-time gets replaced by new `static inline` getter
functions which evaluate to the corresponding field access or to the
constant MBEDTLS_SSL_CONF_FIELD_NAME, depending on whether the latter
is defined or not.
Write-access to fields which are configurable at compile-time needs
to be removed: Specifically, the corresponding API itself either
needs to be removed or replaced by a stub function without effect.
This commit takes the latter approach, which has the benefit of
not requiring any change on the example applications, but introducing
the risk of mismatching API calls and compile-time configuration,
in case a user doesn't correctly keep track of which parts of the
configuration have been fixed at compile-time, and which haven't.
Write-access for the purpose of setting defaults is simply omitted.
A positive option looks better, but comes with the following compatibility
issue: people using a custom config.h that is not based on the default
config.h and need TLS support would need to manually change their config in
order to still get TLS.
Work around that by making the public option negative. Internally the positive
option is used, though.
In the future (when preparing the next major version), we might want to switch
back to a positive option as this would be more consistent with other options
we have.
For now the option has no effect.
Adapted existing example config files. The fact that I needed to do this
highlights that this is a slightly incompatible change: existing users need to
update their existing custom configs (if standalone as opposed to based on the
default config) in order to still get the same behaviour.
The alternative would be to have a negative config option (eg NO_TLS or
DTLS_ONLY) but this doesn't fit as nicely with the existing options, so
hopefully the minor incompatibility is acceptable.
I don't think it's worth adding a new component to all.sh:
- builds with both DTLS and TLS are done in the default (and full) config
- TLS-only builds are done with eg config-suite-b.h in test-ref-configs
- a DTLS-only build is done with config-thread.h in test-ref-configs
- builds with none of them (and SSL_TLS_C enabled) are forbidden
This commit adds a minimal test configuration `baremetal.h` to the `configs`
folder supporting ECDHE-ECDSA-AES-CCM-8 with Secp256R1 and SHA-256 only.
The configuration lacks some options which are currently needed to
successfully build and run the example applications `ssl_client2`
and `ssl_server2`, such as `MBEDTLS_NET_C`. To still allow testing
a configuration close to `baremetal.h`, the commit also adds
`baremetal_test.h`, containing minimal amendments to `baremetal.h`
that allow building and running `ssl_client2` and `ssl_server2`.
This is useful for testing interop with GnuTLS, which sends records larger
than 512 bytes.
This change is triggered by the addition of CCM interop testing with GnuTLS a
few commits ago.
Conflict: configs/config-picocoin.h was both edited and removed.
Resolution: removed, since this is the whole point of PR #1280 and the
changes in development are no longer relevant.
Enable the MBEDTLS_AES_ROM_TABLES option in the
configs/config-no-entropy.h to place AES lookup tables in ROM. This
saves considerable RAM space, a resource that is very limited in small
devices that use this configuration.
target_config.h is no longer needed for target/platform configurations so
this change removes it from the default and platform configurations for mbed
builds.
* development: (73 commits)
Bump yotta dependencies version
Fix typo in documentation
Corrected misleading fn description in ssl_cache.h
Corrected URL/reference to MPI library
Fix yotta dependencies
Fix minor spelling mistake in programs/pkey/gen_key.c
Bump version to 2.1.2
Fix CVE number in ChangeLog
Add 'inline' workaround where needed
Fix references to non-standard SIZE_T_MAX
Fix yotta version dependencies again
Upgrade yotta dependency versions
Fix compile error in net.c with musl libc
Add missing warning in doc
Remove inline workaround when not useful
Fix macroization of inline in C++
Changed attribution for Guido Vranken
Merge of IOTSSL-476 - Random malloc in pem_read()
Fix for IOTSSL-473 Double free error
Fix potential overflow in CertificateRequest
...
Conflicts:
include/mbedtls/ssl_internal.h
library/ssl_cli.c
- in the future thread might need more than just EC J-PAKE
- use the same format as the other mini configurations (no doxygen doc, only
showing what is enabled)
