BoundsSafetyAdoptionGuide.rst

Adoption Guide for -fbounds-safety

• Where to get -fbounds-safety
• Feature flag
• Include ptrcheck.h
• Add bounds annotations on pointers as necessary
• Address compiler diagnostics
• Run test suites to fix new run-time traps
• Repeat the process for each remaining file

Where to get -fbounds-safety

The open sourcing to llvm.org's llvm-project is still on going and the feature is not available yet. In the mean time, the preview implementation is available here in a fork of llvm-project. Please follow Building LLVM with CMake to build the compiler.

Feature flag

Pass -fbounds-safety as a Clang compilation flag for the C file that you want to adopt. We recommend adopting the model file by file, because adoption requires some effort to add bounds annotations and fix compiler diagnostics.

Include ptrcheck.h

ptrcheck.h is a Clang toolchain header to provide definition of the bounds annotations such as __counted_by, __counted_by_or_null, __sized_by, etc. In the LLVM source tree, the header is located in llvm-project/clang/lib/Headers/ptrcheck.h.

Add bounds annotations on pointers as necessary

Annotate pointers on struct fields and function parameters if they are pointing to an array of object, with appropriate bounds annotations. Please see :doc:`BoundsSafety` to learn what kind of bounds annotations are available and their semantics. Note that local pointer variables typically don't need bounds annotations because they are implicitely a wide pointer (__bidi_indexable) that automatically carries the bounds information.

Address compiler diagnostics

Once you pass -fbounds-safety to compiler a C file, you will see some new compiler warnings and errors, which guide adoption of -fbounds-safety. Consider the following example:

#include <ptrcheck.h>

void init_buf(int *p, int n) {
   for (int i = 0; i < n; ++i)
      p[i] = 0; // error: array subscript on single pointer 'p' must use a constant index of 0 to be in bounds
}

The parameter int *p doesn't have a bounds annotation, so the compiler will complain about the code indexing into it (p[i]) as it assumes that p is pointing to a single int object or null. To address the diagnostics, you should add a bounds annotation on int *p so that the compiler can reason about the safety of the array subscript. In the following example, p is now int *__counted_by(n), so the compiler will allow the array subscript with additional run-time checks as necessary.

#include <ptrcheck.h>

void init_buf(int *__counted_by(n) p, int n) {
   for (int i = 0; i < n; ++i)
      p[i] = 0; // ok; `p` is now has a type with bounds annotation.
}

Run test suites to fix new run-time traps

Adopting -fbounds-safety may cause your program to trap if it violates bounds safety or it has incorrect adoption. Thus, it is necessary to perform run-time testing of your program to gain confidence that it won't trap at run time.

Repeat the process for each remaining file

Once you've done with adopting a single C file, please repeat the same process for each remaining C file that you want to adopt.