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, 92 insertions, 0 deletions

diff --git a/drivers/firmware/efi/efibc.c b/drivers/firmware/efi/efibc.c
new file mode 100644
index 000000000..345a10aa7
--- /dev/null
+++ b/drivers/firmware/efi/efibc.c
@@ -0,0 +1,92 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * efibc: control EFI bootloaders which obey LoaderEntryOneShot var
+ * Copyright (c) 2013-2016, Intel Corporation.
+ */
+
+#define pr_fmt(fmt) "efibc: " fmt
+
+#include <linux/efi.h>
+#include <linux/module.h>
+#include <linux/reboot.h>
+#include <linux/slab.h>
+#include <linux/ucs2_string.h>
+
+#define MAX_DATA_LEN	512
+
+static int efibc_set_variable(efi_char16_t *name, efi_char16_t *value,
+			      unsigned long len)
+{
+	efi_status_t status;
+
+	status = efi.set_variable(name, &LINUX_EFI_LOADER_ENTRY_GUID,
+				  EFI_VARIABLE_NON_VOLATILE
+				  | EFI_VARIABLE_BOOTSERVICE_ACCESS
+				  | EFI_VARIABLE_RUNTIME_ACCESS,
+				  len * sizeof(efi_char16_t), value);
+
+	if (status != EFI_SUCCESS) {
+		pr_err("failed to set EFI variable: 0x%lx\n", status);
+		return -EIO;
+	}
+	return 0;
+}
+
+static int efibc_reboot_notifier_call(struct notifier_block *notifier,
+				      unsigned long event, void *data)
+{
+	efi_char16_t *reason = event == SYS_RESTART ? L"reboot"
+						    : L"shutdown";
+	const u8 *str = data;
+	efi_char16_t *wdata;
+	unsigned long l;
+	int ret;
+
+	ret = efibc_set_variable(L"LoaderEntryRebootReason", reason,
+				 ucs2_strlen(reason));
+	if (ret || !data)
+		return NOTIFY_DONE;
+
+	wdata = kmalloc_objs(efi_char16_t, MAX_DATA_LEN);
+	if (!wdata)
+		return NOTIFY_DONE;
+
+	for (l = 0; l < MAX_DATA_LEN - 1 && str[l] != '\0'; l++)
+		wdata[l] = str[l];
+	wdata[l] = L'\0';
+
+	efibc_set_variable(L"LoaderEntryOneShot", wdata, l);
+
+	kfree(wdata);
+	return NOTIFY_DONE;
+}
+
+static struct notifier_block efibc_reboot_notifier = {
+	.notifier_call = efibc_reboot_notifier_call,
+};
+
+static int __init efibc_init(void)
+{
+	int ret;
+
+	if (!efi_rt_services_supported(EFI_RT_SUPPORTED_SET_VARIABLE))
+		return -ENODEV;
+
+	ret = register_reboot_notifier(&efibc_reboot_notifier);
+	if (ret)
+		pr_err("unable to register reboot notifier\n");
+
+	return ret;
+}
+module_init(efibc_init);
+
+static void __exit efibc_exit(void)
+{
+	unregister_reboot_notifier(&efibc_reboot_notifier);
+}
+module_exit(efibc_exit);
+
+MODULE_AUTHOR("Jeremy Compostella <jeremy.compostella@intel.com>");
+MODULE_AUTHOR("Matt Gumbel <matthew.k.gumbel@intel.com");
+MODULE_DESCRIPTION("EFI Bootloader Control");
+MODULE_LICENSE("GPL v2");