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, 233 insertions, 0 deletions

diff --git a/arch/alpha/lib/fpreg.c b/arch/alpha/lib/fpreg.c
new file mode 100644
index 000000000..3d3216504
--- /dev/null
+++ b/arch/alpha/lib/fpreg.c
@@ -0,0 +1,233 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * arch/alpha/lib/fpreg.c
+ *
+ * (C) Copyright 1998 Linus Torvalds
+ */
+
+#include <linux/compiler.h>
+#include <linux/export.h>
+#include <linux/preempt.h>
+#include <asm/fpu.h>
+#include <asm/thread_info.h>
+
+#if defined(CONFIG_ALPHA_EV6) || defined(CONFIG_ALPHA_EV67)
+#define STT(reg,val)  asm volatile ("ftoit $f"#reg",%0" : "=r"(val));
+#else
+#define STT(reg,val)  asm volatile ("stt $f"#reg",%0" : "=m"(val));
+#endif
+
+unsigned long
+alpha_read_fp_reg (unsigned long reg)
+{
+	unsigned long val;
+
+	if (unlikely(reg >= 32))
+		return 0;
+	preempt_disable();
+	if (current_thread_info()->status & TS_SAVED_FP)
+		val = current_thread_info()->fp[reg];
+	else switch (reg) {
+	      case  0: STT( 0, val); break;
+	      case  1: STT( 1, val); break;
+	      case  2: STT( 2, val); break;
+	      case  3: STT( 3, val); break;
+	      case  4: STT( 4, val); break;
+	      case  5: STT( 5, val); break;
+	      case  6: STT( 6, val); break;
+	      case  7: STT( 7, val); break;
+	      case  8: STT( 8, val); break;
+	      case  9: STT( 9, val); break;
+	      case 10: STT(10, val); break;
+	      case 11: STT(11, val); break;
+	      case 12: STT(12, val); break;
+	      case 13: STT(13, val); break;
+	      case 14: STT(14, val); break;
+	      case 15: STT(15, val); break;
+	      case 16: STT(16, val); break;
+	      case 17: STT(17, val); break;
+	      case 18: STT(18, val); break;
+	      case 19: STT(19, val); break;
+	      case 20: STT(20, val); break;
+	      case 21: STT(21, val); break;
+	      case 22: STT(22, val); break;
+	      case 23: STT(23, val); break;
+	      case 24: STT(24, val); break;
+	      case 25: STT(25, val); break;
+	      case 26: STT(26, val); break;
+	      case 27: STT(27, val); break;
+	      case 28: STT(28, val); break;
+	      case 29: STT(29, val); break;
+	      case 30: STT(30, val); break;
+	      case 31: STT(31, val); break;
+	}
+	preempt_enable();
+	return val;
+}
+EXPORT_SYMBOL(alpha_read_fp_reg);
+
+#if defined(CONFIG_ALPHA_EV6) || defined(CONFIG_ALPHA_EV67)
+#define LDT(reg,val)  asm volatile ("itoft %0,$f"#reg : : "r"(val));
+#else
+#define LDT(reg,val)  asm volatile ("ldt $f"#reg",%0" : : "m"(val));
+#endif
+
+void
+alpha_write_fp_reg (unsigned long reg, unsigned long val)
+{
+	if (unlikely(reg >= 32))
+		return;
+
+	preempt_disable();
+	if (current_thread_info()->status & TS_SAVED_FP) {
+		current_thread_info()->status |= TS_RESTORE_FP;
+		current_thread_info()->fp[reg] = val;
+	} else switch (reg) {
+	      case  0: LDT( 0, val); break;
+	      case  1: LDT( 1, val); break;
+	      case  2: LDT( 2, val); break;
+	      case  3: LDT( 3, val); break;
+	      case  4: LDT( 4, val); break;
+	      case  5: LDT( 5, val); break;
+	      case  6: LDT( 6, val); break;
+	      case  7: LDT( 7, val); break;
+	      case  8: LDT( 8, val); break;
+	      case  9: LDT( 9, val); break;
+	      case 10: LDT(10, val); break;
+	      case 11: LDT(11, val); break;
+	      case 12: LDT(12, val); break;
+	      case 13: LDT(13, val); break;
+	      case 14: LDT(14, val); break;
+	      case 15: LDT(15, val); break;
+	      case 16: LDT(16, val); break;
+	      case 17: LDT(17, val); break;
+	      case 18: LDT(18, val); break;
+	      case 19: LDT(19, val); break;
+	      case 20: LDT(20, val); break;
+	      case 21: LDT(21, val); break;
+	      case 22: LDT(22, val); break;
+	      case 23: LDT(23, val); break;
+	      case 24: LDT(24, val); break;
+	      case 25: LDT(25, val); break;
+	      case 26: LDT(26, val); break;
+	      case 27: LDT(27, val); break;
+	      case 28: LDT(28, val); break;
+	      case 29: LDT(29, val); break;
+	      case 30: LDT(30, val); break;
+	      case 31: LDT(31, val); break;
+	}
+	preempt_enable();
+}
+EXPORT_SYMBOL(alpha_write_fp_reg);
+
+#if defined(CONFIG_ALPHA_EV6) || defined(CONFIG_ALPHA_EV67)
+#define STS(reg,val)  asm volatile ("ftois $f"#reg",%0" : "=r"(val));
+#else
+#define STS(reg,val)  asm volatile ("sts $f"#reg",%0" : "=m"(val));
+#endif
+
+unsigned long
+alpha_read_fp_reg_s (unsigned long reg)
+{
+	unsigned long val;
+
+	if (unlikely(reg >= 32))
+		return 0;
+
+	preempt_disable();
+	if (current_thread_info()->status & TS_SAVED_FP) {
+		LDT(0, current_thread_info()->fp[reg]);
+		STS(0, val);
+	} else switch (reg) {
+	      case  0: STS( 0, val); break;
+	      case  1: STS( 1, val); break;
+	      case  2: STS( 2, val); break;
+	      case  3: STS( 3, val); break;
+	      case  4: STS( 4, val); break;
+	      case  5: STS( 5, val); break;
+	      case  6: STS( 6, val); break;
+	      case  7: STS( 7, val); break;
+	      case  8: STS( 8, val); break;
+	      case  9: STS( 9, val); break;
+	      case 10: STS(10, val); break;
+	      case 11: STS(11, val); break;
+	      case 12: STS(12, val); break;
+	      case 13: STS(13, val); break;
+	      case 14: STS(14, val); break;
+	      case 15: STS(15, val); break;
+	      case 16: STS(16, val); break;
+	      case 17: STS(17, val); break;
+	      case 18: STS(18, val); break;
+	      case 19: STS(19, val); break;
+	      case 20: STS(20, val); break;
+	      case 21: STS(21, val); break;
+	      case 22: STS(22, val); break;
+	      case 23: STS(23, val); break;
+	      case 24: STS(24, val); break;
+	      case 25: STS(25, val); break;
+	      case 26: STS(26, val); break;
+	      case 27: STS(27, val); break;
+	      case 28: STS(28, val); break;
+	      case 29: STS(29, val); break;
+	      case 30: STS(30, val); break;
+	      case 31: STS(31, val); break;
+	}
+	preempt_enable();
+	return val;
+}
+EXPORT_SYMBOL(alpha_read_fp_reg_s);
+
+#if defined(CONFIG_ALPHA_EV6) || defined(CONFIG_ALPHA_EV67)
+#define LDS(reg,val)  asm volatile ("itofs %0,$f"#reg : : "r"(val));
+#else
+#define LDS(reg,val)  asm volatile ("lds $f"#reg",%0" : : "m"(val));
+#endif
+
+void
+alpha_write_fp_reg_s (unsigned long reg, unsigned long val)
+{
+	if (unlikely(reg >= 32))
+		return;
+
+	preempt_disable();
+	if (current_thread_info()->status & TS_SAVED_FP) {
+		current_thread_info()->status |= TS_RESTORE_FP;
+		LDS(0, val);
+		STT(0, current_thread_info()->fp[reg]);
+	} else switch (reg) {
+	      case  0: LDS( 0, val); break;
+	      case  1: LDS( 1, val); break;
+	      case  2: LDS( 2, val); break;
+	      case  3: LDS( 3, val); break;
+	      case  4: LDS( 4, val); break;
+	      case  5: LDS( 5, val); break;
+	      case  6: LDS( 6, val); break;
+	      case  7: LDS( 7, val); break;
+	      case  8: LDS( 8, val); break;
+	      case  9: LDS( 9, val); break;
+	      case 10: LDS(10, val); break;
+	      case 11: LDS(11, val); break;
+	      case 12: LDS(12, val); break;
+	      case 13: LDS(13, val); break;
+	      case 14: LDS(14, val); break;
+	      case 15: LDS(15, val); break;
+	      case 16: LDS(16, val); break;
+	      case 17: LDS(17, val); break;
+	      case 18: LDS(18, val); break;
+	      case 19: LDS(19, val); break;
+	      case 20: LDS(20, val); break;
+	      case 21: LDS(21, val); break;
+	      case 22: LDS(22, val); break;
+	      case 23: LDS(23, val); break;
+	      case 24: LDS(24, val); break;
+	      case 25: LDS(25, val); break;
+	      case 26: LDS(26, val); break;
+	      case 27: LDS(27, val); break;
+	      case 28: LDS(28, val); break;
+	      case 29: LDS(29, val); break;
+	      case 30: LDS(30, val); break;
+	      case 31: LDS(31, val); break;
+	}
+	preempt_enable();
+}
+EXPORT_SYMBOL(alpha_write_fp_reg_s);