kbuild: rust: keep Rust objects out of Clang LTO with inline helpersgrafted

Under `CONFIG_LTO_CLANG` + `CONFIG_RUST_INLINE_HELPERS`, one may hit
`objtool` errors such as:

    vmlinux.o: error: objtool: _R..._3Gsp4boot+0xd6a:
    can't find jump dest instruction at .text._R..._3Gsp4boot+0x1dfd

The reason is that in such builds, the Clang invocation that compiles
the combined Rust plus helpers bitcode emits LLVM bitcode (again) --
the final code generation happens in the linker's LTO step, which the
`-mllvm` trap options passed to Clang do not reach.

This, in turn, means that unreachable traps are missing, and the
impossible paths do not merely fallthrough to the next symbol, but past
the end of their own section, since LTO builds place each function in
its own section.

Thus filter `CC_FLAGS_LTO` out of the Clang invocation, so that it always
emits machine code directly, with the traps in place.

Assisted-by: LLM
Cc: Gary Guo <gary@garyguo.net>
Cc: Boqun Feng <boqun@kernel.org>
Cc: Alice Ryhl <aliceryhl@google.com>
Cc: Matthew Maurer <mmaurer@google.com>
Cc: Josh Poimboeuf <jpoimboe@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: stable@vger.kernel.org
Fixes: 3a2486cc1da5 ("kbuild: rust: provide an option to inline C helpers into Rust")
Acked-by: Gary Guo <gary@garyguo.net>
Link: https://patch.msgid.link/20260816133233.197500-2-ojeda@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

1 files changed, 169 insertions, 0 deletions

diff --git a/Documentation/dev-tools/context-analysis.rst b/Documentation/dev-tools/context-analysis.rst
new file mode 100644
index 000000000..8e71e1e75
--- /dev/null
+++ b/Documentation/dev-tools/context-analysis.rst
@@ -0,0 +1,169 @@
+.. SPDX-License-Identifier: GPL-2.0
+.. Copyright (C) 2025, Google LLC.
+
+.. _context-analysis:
+
+Compiler-Based Context Analysis
+===============================
+
+Context Analysis is a language extension, which enables statically checking
+that required contexts are active (or inactive) by acquiring and releasing
+user-definable "context locks". An obvious application is lock-safety checking
+for the kernel's various synchronization primitives (each of which represents a
+"context lock"), and checking that locking rules are not violated.
+
+The Clang compiler currently supports the full set of context analysis
+features. To enable for Clang, configure the kernel with::
+
+    CONFIG_WARN_CONTEXT_ANALYSIS=y
+
+The feature requires Clang 23 or later.
+
+The analysis is *opt-in by default*, and requires declaring which modules and
+subsystems should be analyzed in the respective `Makefile`::
+
+    CONTEXT_ANALYSIS_mymodule.o := y
+
+Or for all translation units in the directory::
+
+    CONTEXT_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_CONTEXT_ANALYSIS_ALL=y
+
+Programming Model
+-----------------
+
+The below describes the programming model around using context lock types.
+
+.. note::
+   Enabling context analysis can be seen as enabling a dialect of Linux C with
+   a Context System. Some valid patterns involving complex control-flow are
+   constrained (such as conditional acquisition and later conditional release
+   in the same function).
+
+Context analysis is a way to specify permissibility of operations to depend on
+context locks being held (or not held). Typically we are interested in
+protecting data and code in a critical section by requiring a specific context
+to be active, for example by holding a specific lock. The analysis ensures that
+callers cannot perform an operation without the required context being active.
+
+Context locks are associated with named structs, along with functions that
+operate on struct instances to acquire and release the associated context lock.
+
+Context locks can be held either exclusively or shared. This mechanism allows
+assigning more precise privileges when a context is active, typically to
+distinguish where a thread may only read (shared) or also write (exclusive) to
+data guarded within a context.
+
+The set of contexts that are actually active in 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 context environment. The
+context environment is calculated for every program point, and describes the
+set of contexts that are statically known to be active, or inactive, at that
+particular point. This environment is a conservative approximation of the full
+set of contexts that will actually be active in 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 context locks acquired or
+   released by inline functions. It requires explicit annotations to (a) assert
+   that it's not a bug if a context lock 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:
+`raw_spinlock_t`, `spinlock_t`, `rwlock_t`, `mutex`, `seqlock_t`,
+`bit_spinlock`, RCU, SRCU (`srcu_struct`), `rw_semaphore`, `local_lock_t`,
+`ww_mutex`.
+
+To initialize variables guarded by a context lock with an initialization
+function (``type_init(&lock)``), prefer using ``guard(type_init)(&lock)`` or
+``scoped_guard(type_init, &lock) { ... }`` to initialize such guarded members
+or globals in the enclosing scope. This initializes the context lock and treats
+the context as active within the initialization scope (initialization implies
+exclusive access to the underlying object).
+
+For example::
+
+    struct my_data {
+            spinlock_t lock;
+            int counter __guarded_by(&lock);
+    };
+
+    void init_my_data(struct my_data *d)
+    {
+            ...
+            guard(spinlock_init)(&d->lock);
+            d->counter = 0;
+            ...
+    }
+
+Alternatively, initializing guarded variables can be done with context analysis
+disabled, preferably in the smallest possible scope (due to lack of any other
+checking): either with a ``context_unsafe(var = init)`` expression, or by
+marking small initialization functions with the ``__context_unsafe(init)``
+attribute.
+
+Lockdep assertions, such as `lockdep_assert_held()`, inform the compiler's
+context 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-context-analysis.h
+   :identifiers: context_lock_struct
+                 token_context_lock token_context_lock_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
+                 __acquire_shared
+                 __release_shared
+                 __acquire_ret
+                 __acquire_shared_ret
+                 context_unsafe
+                 __context_unsafe
+                 disable_context_analysis enable_context_analysis
+
+.. note::
+   The function attribute `__no_context_analysis` is reserved for internal
+   implementation of context lock types, 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 keywords
+and documentation still using the thread-safety-analysis-only terminology. This
+was later changed and the feature became more flexible, gaining the ability to
+define custom "capabilities". 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, and "capability" is already overloaded in the
+kernel, the naming chosen for the kernel departs from Clang's initial "Thread
+Safety" and "capability" nomenclature; we refer to the feature as "Context
+Analysis" to avoid confusion. The internal implementation still makes
+references to Clang's terminology in a few places, such as `-Wthread-safety`
+being the warning option that also still appears in diagnostic messages.