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Document how to interpret negative reports

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The abi_check script has common false positives. Document the intent of each
family of checks and typical cases of false positives that can be overridden.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
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Gilles Peskine 2022-06-20 18:51:44 +02:00
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scripts/abi_check.py
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@ -3,6 +3,8 @@
for backward incompatibilities between two different Git revisions within
an Mbed TLS repository. It must be run from the root of a Git working tree.
### How the script works ###
For the source (API) and runtime (ABI) interface compatibility, this script
is a small wrapper around the abi-compliance-checker and abi-dumper tools,
applying them to compare the header and library files.
@ -19,6 +21,66 @@ at a configurable location, or are given as a brief list of problems.
Returns 0 on success, 1 on non-compliance, and 2 if there is an error
while running the script.
### How to interpret non-compliance ###
This script has relatively common false positives. In many scenarios, it only
reports a pass if there is a strict textual match between the old version and
the new version, and it reports problems where there is a sufficient semantic
match but not a textual match. This section lists some common false positives.
This is not an exhaustive list: in the end what matters is whether we are
breaking a backward compatibility goal.
**API**: the goal is that if an application works with the old version of the
library, it can be recompiled against the new version and will still work.
This is normally validated by comparing the declarations in `include/*/*.h`.
A failure is a declaration that has disappeared or that now has a different
type.
* It's ok to change or remove macros and functions that are documented as
for internal use only or as experimental.
* It's ok to rename function or macro parameters as long as the semantics
has not changed.
* It's ok to change or remove structure fields that are documented as
private.
* It's ok to add fields to a structure that already had private fields
or was documented as extensible.
**ABI**: the goal is that if an application was built against the old version
of the library, the same binary will work when linked against the new version.
This is normally validated by comparing the symbols exported by `libmbed*.so`.
A failure is a symbol that is no longer exported by the same library or that
now has a different type.
* All ABI changes are acceptable if the library version is bumped
(see `scripts/bump_version.sh`).
* ABI changes that concern functions which are declared only inside the
library directory, and not in `include/*/*.h`, are acceptable only if
the function was only ever used inside the same library (libmbedcrypto,
libmbedx509, libmbedtls). As a counter example, if the old version
of libmbedtls calls mbedtls_foo() from libmbedcrypto, and the new version
of libmbedcrypto no longer has a compatible mbedtls_foo(), this does
require a version bump for libmbedcrypto.
**Storage format**: the goal is to check that persistent keys stored by the
old version can be read by the new version. This is normally validated by
comparing the `*read*` test cases in `test_suite*storage_format*.data`.
A failure is a storage read test case that is no longer present with the same
function name and parameter list.
* It's ok if the same test data is present, but its presentation has changed,
for example if a test function is renamed or has different parameters.
* It's ok if redundant tests are removed.
**Generated test coverage**: the goal is to check that automatically
generated tests have as much coverage as before. This is normally validated
by comparing the test cases that are automatically generated by a script.
A failure is a generated test case that is no longer present with the same
function name and parameter list.
* It's ok if the same test data is present, but its presentation has changed,
for example if a test function is renamed or has different parameters.
* It's ok if redundant tests are removed.
"""
# Copyright The Mbed TLS Contributors