We gratefully accept bug reports and contributions from the community. All PRs are reviewed by the project team / community, and may need some modifications to be accepted.
More details on all of these points may be found in the sections below.
The project aims to minimise the impact on users upgrading to newer versions of the library and it should not be necessary for a user to make any changes to their own code to work with a newer version of the library. Unless the user has made an active decision to use newer features, a newer generation of the library or a change has been necessary due to a security issue or other significant software defect, no modifications to their own code should be necessary. To achieve this, API compatibility is maintained between different versions of Mbed TLS on the main development branch and in LTS (Long Term Support) branches, as described in BRANCHES.md.
To minimise such disruption to users, where a change to the interface is required, all changes to the ABI or API, even on the main development branch where new features are added, need to be justifiable by either being a significant enhancement, new feature or bug fix which is best resolved by an interface change. If there is an API change, the contribution, if accepted, will be merged only when there will be a major release.
Where changes to an existing interface are necessary, functions in the public interface which need to be changed, are marked as ‘deprecated’. This is done with the preprocessor symbols MBEDTLS_DEPRECATED_WARNING and MBEDTLS_DEPRECATED_REMOVED. Then, a new function with a new name but similar if not identical behaviour to the original function containing the necessary changes should be created alongside the existing deprecated function.
When a build is made with the deprecation preprocessor symbols defined, a compiler warning will be generated to warn a user that the function will be removed at some point in the future, notifying users that they should change from the older deprecated function to the newer function at their own convenience.
Therefore, no changes are permitted to the definition of functions in the public interface which will change the API. Instead the interface can only be changed by its extension. As described above, if a function needs to be changed, a new function needs to be created alongside it, with a new name, and whatever change is necessary, such as a new parameter or the addition of a return value.
Periodically, the library will remove deprecated functions from the library which will be a breaking change in the API, but such changes will be made only in a planned, structured way that gives sufficient notice to users of the library.
Mbed TLS maintains several LTS (Long Term Support) branches, which are maintained continuously for a given period. The LTS branches are provided to allow users of the library to have a maintained, stable version of the library which contains only security fixes and fixes for other defects, without encountering additional features or API extensions which may introduce issues or change the code size or RAM usage, which can be significant considerations on some platforms. To allow users to take advantage of the LTS branches, these branches maintain backwards compatibility for both the public API and ABI.
When backporting to these branches please observe the following rules:
It would be highly appreciated if contributions are backported to LTS branches in addition to the development branch by contributors.
The list of maintained branches can be found in the Current Branches section of BRANCHES.md.
As mentioned, tests that show the correctness of the feature or bug fix should be added to the pull request, if no such tests exist.
Mbed TLS includes a comprehensive set of test suites in the tests/ directory that are dynamically generated to produce the actual test source files (e.g. test_suite_rsa.c). These files are generated from a function file (e.g. suites/test_suite_rsa.function) and a data file (e.g. suites/test_suite_rsa.data). The function file contains the test functions. The data file contains the test cases, specified as parameters that will be passed to the test function.
A Knowledge Base article describing how to add additional tests is available on the Mbed TLS website.
A test script tests/scripts/basic-build-test.sh is available to show test coverage of the library. New code contributions should provide a similar level of code coverage to that which already exists for the library.
Sample applications, if needed, should be modified as well.
Once a PR has been made, the Continuous Integration (CI) tests are triggered and run. You should follow the result of the CI tests, and fix failures.
It is advised to enable the githooks scripts prior to pushing your changes, for catching some of the issues as early as possible.
Mbed TLS is well documented, but if you think documentation is needed, speak out!
All new files should include the Apache-2.0 standard license header where possible. For licensing details, please see the License section of the README.
The copyright on contributions is retained by the original authors of the code. Where possible for new files, this should be noted in a comment at the top of the file in the form: “Copyright The Mbed TLS Contributors”.
When contributing code to us, the committer and all authors are required to make the submission under the terms of the Developer Certificate of Origin, confirming that the code submitted can (legally) become part of the project, and be subject to the same Apache 2.0 license. This is done by including the standard Git Signed-off-by: line in every commit message. If more than one person contributed to the commit, they should also add their own Signed-off-by: line.