[PATCH v2 04/34] Documentation: Add documentation for Compiler-Based Capability Analysis

[Marco Elver <elver@google.com>]

Adds documentation in Documentation/dev-tools/capability-analysis.rst,
and adds it to the index and cross-references from Sparse's document.

Signed-off-by: Marco Elver <elver@google.com>
---
v2:
* Remove cross-reference to Sparse, since we plan to remove Sparse
  support anyway.
* Mention __no_capability_analysis should be avoided.
---
 .../dev-tools/capability-analysis.rst         | 145 ++++++++++++++++++
 Documentation/dev-tools/index.rst             |   1 +
 2 files changed, 146 insertions(+)
 create mode 100644 Documentation/dev-tools/capability-analysis.rst

diff --git a/Documentation/dev-tools/capability-analysis.rst b/Documentation/dev-tools/capability-analysis.rst
new file mode 100644
index 000000000000..4b9c93cc8fcd
--- /dev/null
+++ b/Documentation/dev-tools/capability-analysis.rst
@@ -0,0 +1,145 @@
+.. SPDX-License-Identifier: GPL-2.0
+.. Copyright (C) 2025, Google LLC.
+
+.. _capability-analysis:
+
+Compiler-Based Capability Analysis
+==================================
+
+Capability analysis is a C language extension, which enables statically
+checking that user-definable "capabilities" are acquired and released where
+required. An obvious application is lock-safety checking for the kernel's
+various synchronization primitives (each of which represents a "capability"),
+and checking that locking rules are not violated.
+
+The Clang compiler currently supports the full set of capability analysis
+features. To enable for Clang, configure the kernel with::
+
+    CONFIG_WARN_CAPABILITY_ANALYSIS=y
+
+The analysis is *opt-in by default*, and requires declaring which modules and
+subsystems should be analyzed in the respective `Makefile`::
+
+    CAPABILITY_ANALYSIS_mymodule.o := y
+
+Or for all translation units in the directory::
+
+    CAPABILITY_ANALYSIS := y
+
+It is possible to enable the analysis tree-wide, however, which will result in
+numerous false positive warnings currently and is *not* generally recommended::
+
+    CONFIG_WARN_CAPABILITY_ANALYSIS_ALL=y
+
+Programming Model
+-----------------
+
+The below describes the programming model around using capability-enabled
+types.
+
+.. note::
+   Enabling capability analysis can be seen as enabling a dialect of Linux C with
+   a Capability System. Some valid patterns involving complex control-flow are
+   constrained (such as conditional acquisition and later conditional release
+   in the same function, or returning pointers to capabilities from functions.
+
+Capability analysis is a way to specify permissibility of operations to depend
+on capabilities being held (or not held). Typically we are interested in
+protecting data and code by requiring some capability to be held, for example a
+specific lock. The analysis ensures that the caller cannot perform the
+operation without holding the appropriate capability.
+
+Capabilities are associated with named structs, along with functions that
+operate on capability-enabled struct instances to acquire and release the
+associated capability.
+
+Capabilities can be held either exclusively or shared. This mechanism allows
+assign more precise privileges when holding a capability, typically to
+distinguish where a thread may only read (shared) or also write (exclusive) to
+guarded data.
+
+The set of capabilities that are actually held by a given thread at a given
+point in program execution is a run-time concept. The static analysis works by
+calculating an approximation of that set, called the capability environment.
+The capability environment is calculated for every program point, and describes
+the set of capabilities that are statically known to be held, or not held, at
+that particular point. This environment is a conservative approximation of the
+full set of capabilities that will actually held by a thread at run-time.
+
+More details are also documented `here
+<https://clang.llvm.org/docs/ThreadSafetyAnalysis.html>`_.
+
+.. note::
+   Clang's analysis explicitly does not infer capabilities acquired or released
+   by inline functions. It requires explicit annotations to (a) assert that
+   it's not a bug if a capability is released or acquired, and (b) to retain
+   consistency between inline and non-inline function declarations.
+
+Supported Kernel Primitives
+~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+.. Currently the following synchronization primitives are supported:
+
+For capabilities with an initialization function (e.g., `spin_lock_init()`),
+calling this function on the capability instance before initializing any
+guarded members or globals prevents the compiler from issuing warnings about
+unguarded initialization.
+
+Lockdep assertions, such as `lockdep_assert_held()`, inform the compiler's
+capability analysis that the associated synchronization primitive is held after
+the assertion. This avoids false positives in complex control-flow scenarios
+and encourages the use of Lockdep where static analysis is limited. For
+example, this is useful when a function doesn't *always* require a lock, making
+`__must_hold()` inappropriate.
+
+Keywords
+~~~~~~~~
+
+.. kernel-doc:: include/linux/compiler-capability-analysis.h
+   :identifiers: struct_with_capability
+                 token_capability token_capability_instance
+                 __guarded_by __pt_guarded_by
+                 __must_hold
+                 __must_not_hold
+                 __acquires
+                 __cond_acquires
+                 __releases
+                 __must_hold_shared
+                 __acquires_shared
+                 __cond_acquires_shared
+                 __releases_shared
+                 __acquire
+                 __release
+                 __cond_lock
+                 __acquire_shared
+                 __release_shared
+                 __cond_lock_shared
+                 capability_unsafe
+                 __capability_unsafe
+                 disable_capability_analysis enable_capability_analysis
+
+.. note::
+   The function attribute `__no_capability_analysis` is reserved for internal
+   implementation of capability-enabled primitives, and should be avoided in
+   normal code.
+
+Background
+----------
+
+Clang originally called the feature `Thread Safety Analysis
+<https://clang.llvm.org/docs/ThreadSafetyAnalysis.html>`_, with some
+terminology still using the thread-safety-analysis-only names. This was later
+changed and the feature became more flexible, gaining the ability to define
+custom "capabilities".
+
+Indeed, its foundations can be found in `capability systems
+<https://www.cs.cornell.edu/talc/papers/capabilities.pdf>`_, used to specify
+the permissibility of operations to depend on some capability being held (or
+not held).
+
+Because the feature is not just able to express capabilities related to
+synchronization primitives, the naming chosen for the kernel departs from
+Clang's initial "Thread Safety" nomenclature and refers to the feature as
+"Capability Analysis" to avoid confusion. The implementation still makes
+references to the older terminology in some places, such as `-Wthread-safety`
+being the warning option that also still appears in diagnostic messages.
diff --git a/Documentation/dev-tools/index.rst b/Documentation/dev-tools/index.rst
index 65c54b27a60b..62ac23f797cd 100644
--- a/Documentation/dev-tools/index.rst
+++ b/Documentation/dev-tools/index.rst
@@ -18,6 +18,7 @@ Documentation/process/debugging/index.rst
    :maxdepth: 2
 
    testing-overview
+   capability-analysis
    checkpatch
    clang-format
    coccinelle
-- 
2.48.1.711.g2feabab25a-goog