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, 69 insertions, 0 deletions

diff --git a/include/drm/drm_util.h b/include/drm/drm_util.h
new file mode 100644
index 000000000..440199618
--- /dev/null
+++ b/include/drm/drm_util.h
@@ -0,0 +1,69 @@
+/*
+ * Internal Header for the Direct Rendering Manager
+ *
+ * Copyright 2018 Intel Corporation
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a
+ * copy of this software and associated documentation files (the "Software"),
+ * to deal in the Software without restriction, including without limitation
+ * the rights to use, copy, modify, merge, publish, distribute, sublicense,
+ * and/or sell copies of the Software, and to permit persons to whom the
+ * Software is furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice (including the next
+ * paragraph) shall be included in all copies or substantial portions of the
+ * Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.  IN NO EVENT SHALL
+ * VA LINUX SYSTEMS AND/OR ITS SUPPLIERS BE LIABLE FOR ANY CLAIM, DAMAGES OR
+ * OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE,
+ * ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
+ * OTHER DEALINGS IN THE SOFTWARE.
+ */
+
+#ifndef _DRM_UTIL_H_
+#define _DRM_UTIL_H_
+
+/**
+ * DOC: drm utils
+ *
+ * Macros and inline functions that does not naturally belong in other places
+ */
+
+#include <linux/interrupt.h>
+#include <linux/kgdb.h>
+#include <linux/preempt.h>
+#include <linux/smp.h>
+#include <linux/util_macros.h>
+
+/*
+ * Use EXPORT_SYMBOL_FOR_TESTS_ONLY() for functions that shall
+ * only be visible for drmselftests.
+ */
+#if defined(CONFIG_DRM_EXPORT_FOR_TESTS)
+#define EXPORT_SYMBOL_FOR_TESTS_ONLY(x) EXPORT_SYMBOL(x)
+#else
+#define EXPORT_SYMBOL_FOR_TESTS_ONLY(x)
+#endif
+
+/**
+ * drm_can_sleep - returns true if currently okay to sleep
+ *
+ * This function shall not be used in new code.
+ * The check for running in atomic context may not work - see linux/preempt.h.
+ *
+ * FIXME: All users of drm_can_sleep should be removed (see todo.rst)
+ *
+ * Returns:
+ * False if kgdb is active, we are in atomic context or irqs are disabled.
+ */
+static inline bool drm_can_sleep(void)
+{
+	if (in_atomic() || in_dbg_master() || irqs_disabled())
+		return false;
+	return true;
+}
+
+#endif