Checklist for making changes to libpldm

Philosophy and influences

• Good Practices in Library Design, Implementation, and Maintenance - Ulrich Drepper

• How Do I Make This Hard to Misuse? - Rusty Russell

• What If I Don't Actually Like My Users? - Rusty Russell

• Red flags that indicate questionable quality - Lennart Poettering

• Not sure if this is a gcc bug or some weird corner of UB or what... - Andrew Zonenberg

• The Good, the Bad, and the Weird - Trail of Bits Blog

• Logic for Programmers - Hillel Wayne

• Parse, don’t validate - Alexis King

References

• C17 draft standard

• SEI CERT C Coding Standard

• Common Weakness Enumeration (CWE) - Software Development

Definitions

• Error condition: An invalid state reached at runtime, caused either by resource exhaustion, or incorrect use of the library's public APIs and data types.

• Invariant: A condition in the library's implementation that must never evaluate false.

• Public API: Any definitions and declarations under include/libpldm.

• Wire format: Any message structure defined in the DMTF PLDM protocol specifications.

Elaborations

• Resource exhaustion is always an error condition and never an invariant violation.

• An invariant violation is always a programming failure of the library's implementation, and never the result of incorrect use of the library's public APIs (see error condition).

• Corollaries of the above two points:

  • Incorrect use of public API functions is always an error condition, and is dealt with by returning an error code.

  • Incorrect use of static functions in the library's implementation is an invariant violation which may be established using assert().

• assert() is the recommended way to demonstrate invariants are upheld.

Adding a new API

• [ ] My new public message codec functions take a struct representing the message as a parameter

  • Function prototypes must not decompose the message to individual parameters. This approach is not ergonomic and is difficult to make type-safe. This is especially true for message decoding functions which must use pointers for out-parameters, where it has often become ambiguous whether the underlying memory represents a single object or an array.

• [ ] My new public struct definitions are not marked __attribute__((packed))

• [ ] My new public struct definitions do not define a flexible array member, unless:

  • [ ] It's contained in an #ifndef __cplusplus macro guard, as flexible arrays are not specified by C++, and

  • [ ] I've implemented an accessor function so the array base pointer can be accessed from C++, and

  • [ ] It is defined as per the C17 specification by omitting the length[^1]

    • Note: Any array defined with length 1 is not a flexible array, and any access beyond the first element invokes undefined behaviour in both C and C++.

  • [ ] I've annotated the flexible array member with LIBPLDM_CC_COUNTED_BY()

[^1]: C17 draft specification, 6.7.2.1 Structure and union specifiers, paragraph 18.

• [ ] If my work interacts with the PLDM wire format, then I have done so using the msgbuf APIs found in src/msgbuf.h (and under src/msgbuf/) to minimise concerns around spatial memory safety and endian-correctness.

• [ ] All my error conditions are handled by returning an error code to the caller.

• [ ] All my invariants are tested using assert().

• [ ] I have not used assert() to evaluate any error conditions without also handling the error condition by returning an error code the the caller.

  • Release builds of the library are configured with assert() disabled (-Db_ndebug=if-release, which provides -DNDEBUG in CFLAGS).

• [ ] My new APIs return negative errno values on error and not PLDM completion codes.

  • [ ] The specific error values my function returns and their meaning in the context of the function call are listed in the API documentation.

• [ ] If I've added support for a new PLDM message type, then I've implemented both the encoder and decoder for that message. Note this applies for both request and response message types.

• [ ] My new function symbols are marked with LIBPLDM_ABI_TESTING in the implementation

• [ ] I've implemented test cases with reasonable branch coverage of each new function I've added

• [ ] I've guarded the test cases of functions marked LIBPLDM_ABI_TESTING so that they are not compiled when the corresponding function symbols aren't visible

• [ ] If I've added support for a new message type, then my commit message specifies all of:

  • [ ] The relevant DMTF specification by its DSP number and title
  • [ ] The relevant version of the specification
  • [ ] The section of the specification that defines the message type

• [ ] If my work impacts the public API of the library, then I've added an entry to CHANGELOG.md describing my work

Stabilising an existing API

• [ ] The API of interest is currently marked LIBPLDM_ABI_TESTING

• [ ] My commit message links to a publicly visible patch that makes use of the API

• [ ] My commit updates the annotation from LIBPLDM_ABI_TESTING to LIBPLDM_ABI_STABLE only for the function symbols demonstrated by the patch linked in the commit message.

• [ ] I've removed guards from the function's tests so they are always compiled

• [ ] If I've updated the ABI dump, then I've used the OpenBMC CI container to do so.

Updating an ABI dump

Each of the following must succeed:

• [ ] Enter the OpenBMC CI Docker container
  • Approximately: docker run --cap-add=sys_admin --rm=true --privileged=true -u $USER -w $(pwd) -v $(pwd):$(pwd) -e MAKEFLAGS= -it openbmc/ubuntu-unit-test:2024-W21-ce361f95ff4fa669
• [ ] CC=gcc CXX=g++; [ $(uname -m) = 'x86_64' ] && meson setup -Dabi=deprecated,stable build
• [ ] meson compile -C build
• [ ] ./scripts/abi-dump-formatter < build/src/current.dump > abi/x86_64/gcc.dump

Removing an API

• [ ] If the function is marked LIBPLDM_ABI_TESTING, then I have removed it

• [ ] If the function is marked LIBPLDM_ABI_STABLE, then I have changed the annotation to LIBPLDM_ABI_DEPRECATED and left it in-place.

  • [ ] I have updated the ABI dump, or will mark the change as WIP until it has been.

• [ ] If the function is marked LIBPLDM_ABI_DEPRECATED, then I have removed it only after satisfying myself that each of the following is true:

  • [ ] There are no known users of the function left in the community
  • [ ] There has been at least one tagged release of libpldm subsequent to the API being marked deprecated

Renaming an API

A change to an API is a pure rename only if there are no additional behavioural changes. Renaming an API with no other behavioural changes is really two actions:

1. Introducing the new API name
2. Deprecating the old API name

• [ ] Only the name of the function has changed. None of its behaviour has changed.

• [ ] Both the new and the old functions are declared in the public headers

• [ ] I've aliased the old function name to the new function name via the libpldm_deprecated_aliases list in meson.build

• [ ] I've added a semantic patch to migrate users from the old name to the new name

• [ ] I've updated the ABI dump to capture the rename, or will mark the change as WIP until it has been.

Testing my changes

Each of the following must succeed when executed in order. Note that to avoid googletest bug #4232 you must avoid using GCC 12 (shipped in Debian Bookworm).

• [ ] meson setup -Dabi-compliance-check=disabled build

• [ ] meson compile -C build && meson test -C build

• [ ] meson configure --buildtype=release build

• [ ] meson compile -C build && meson test -C build

• [ ] meson configure --buildtype=debug build

• [ ] meson configure -Dabi=deprecated,stable build

• [ ] meson compile -C build && meson test -C build

This process is captured in scripts/pre-submit for automation.