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, 76 insertions, 0 deletions

diff --git a/scripts/misc-check b/scripts/misc-check
new file mode 100755
index 000000000..40e5a4b01
--- /dev/null
+++ b/scripts/misc-check
@@ -0,0 +1,76 @@
+#!/bin/sh
+# SPDX-License-Identifier: GPL-2.0-only
+
+set -e
+
+# Detect files that are tracked but ignored by git.
+check_tracked_ignored_files () {
+	git -C "${srctree:-.}" ls-files -i -c --exclude-per-directory=.gitignore 2>/dev/null |
+		sed 's/$/: warning: ignored by one of the .gitignore files/' >&2
+}
+
+# Check for missing #include <linux/export.h>
+#
+# The rule for including <linux/export.h> is very simple:
+# Include <linux/export.h> only when you use EXPORT_SYMBOL(). That's it.
+#
+# However, some headers include <linux/export.h> even though they are completely
+# unrelated to EXPORT_SYMBOL().
+#
+# One example is include/linux/module.h. Please note <linux/module.h> and
+# <linux/export.h> are orthogonal. <linux/module.h> should be included by files
+# that can be compiled as modules. In other words, <linux/module.h> should be
+# included by EXPORT_SYMBOL consumers. In contrast, <linux/export.h> should be
+# included from EXPORT_SYMBOL providers, which may or may not be modular.
+# Hence, include/linux/module.h should *not* include <linux/export.h>.
+#
+# Another example is include/linux/linkage.h, which is completely unrelated to
+# EXPORT_SYMBOL(). Worse, it is included by most C files, which means, most C
+# files end up including <linux/export.h>, even though only some of them
+# actually export symbols. Hence, include/linux/linkage.h should *not* include
+# <linux/export.h>.
+#
+# Before fixing such headers, we must ensure that C files using EXPORT_SYMBOL()
+# include <linux/export.h> directly, since many C files currently rely on
+# <linux/export.h> being included indirectly (likely, via <linux/linkage> etc.).
+#
+# Therefore, this check.
+#
+# The problem is simple - the warned files use EXPORT_SYMBOL(), but do not
+# include <linux/export.h>. Please add #include <linux/export.h> to them.
+#
+# If the included headers are sorted alphabetically, please insert
+# <linux/export.h> in the appropriate position to maintain the sort order.
+# For this reason, this script only checks missing <linux/export.h>, but
+# does not automatically fix it.
+check_missing_include_linux_export_h () {
+
+	git -C "${srctree:-.}" grep --files-with-matches -E 'EXPORT_SYMBOL((_NS)?(_GPL)?|_FOR_MODULES)\(.*\)' \
+	    -- '*.[ch]' :^tools/ :^include/linux/export.h |
+	xargs -r git -C "${srctree:-.}" grep --files-without-match '#include[[:space:]]*<linux/export\.h>' |
+	xargs -r printf "%s: warning: EXPORT_SYMBOL() is used, but #include <linux/export.h> is missing\n" >&2
+}
+
+# If you do not use EXPORT_SYMBOL(), please do not include <linux/export.h>.
+# Currently, this is checked for *.c files, but not for *.h files, because some
+# *.c files rely on <linux/export.h> being included indirectly.
+check_unnecessary_include_linux_export_h () {
+
+	git -C "${srctree:-.}" grep --files-with-matches '#include[[:space:]]*<linux/export\.h>' \
+	    -- '*.[c]' :^tools/ |
+	xargs -r git -C "${srctree:-.}" grep --files-without-match -E 'EXPORT_SYMBOL((_NS)?(_GPL)?|_FOR_MODULES)\(.*\)' |
+	xargs -r printf "%s: warning: EXPORT_SYMBOL() is not used, but #include <linux/export.h> is present\n" >&2
+}
+
+case "${KBUILD_EXTRA_WARN}" in
+*1*)
+	check_tracked_ignored_files
+	;;
+esac
+
+case "${KBUILD_EXTRA_WARN}" in
+*2*)
+	check_missing_include_linux_export_h
+	check_unnecessary_include_linux_export_h
+	;;
+esac