KFuzzTest: RFC after suggestions

[ethangraham2001]

Subject: [PATCH v1 RFC 0/6] kfuzztest: a new framework for fuzzing
kernel functions

This patch series introduces KFuzzTest, a lightweight framework for creating
in-kernel fuzz targets for internal kernel functions.

The primary motivation for KFuzzTest is to simplify the fuzzing of low-level,
relatively stateless functions (e.g., data parsers, format converters) that are
difficult to exercise effectively from the syscall boundary. It is intended for
in-situ fuzzing of kernel code without requiring that it be built as a separate
userspace library or that its dependencies be stubbed out. Using a simple
macro-based API, developers can add a new fuzz target with minimal boilerplate
code.

The core design consists of three main parts:
1. A `FUZZ_TEST(name, struct_type)` macro that allows developers to
   easily define a fuzz test.
2. A binary input format that allows a userspace fuzzer to serialize
   complex, pointer-rich C structures into a single buffer.
3. Metadata for test targets, constraints, and annotations, which is
   emitted into dedicated ELF sections to allow for discovery and
   inspection by userspace tools.

To demonstrate this framework's viability, support for KFuzzTest has been
prototyped in a development fork of Syzkaller, enabling coverage-guided fuzzing.
Based on this work, we have also created a standalone tool that provides a
simple path for developers to write and run fuzz targets within a VM.

This series is an RFC to gather early feedback on the overall design and
approach. We are particularly interested in feedback on:
- The general utility of such a framework.
- The design of the binary serialization format.
- The use of ELF sections for metadata and discovery.

The patch series is structured as follows:
- Patch 1 refactors KASAN to expose functions needed by KFuzzTest.
- Patch 2 introduces the core KFuzzTest API and data structures.
- Patch 3 adds the runtime implementation for the framework.
- Patch 4 adds documentation.
- Patch 5 provides example fuzz targets.
- Patch 6 defines fuzz targets for real kernel functions.