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, 237 insertions, 0 deletions

diff --git a/tools/perf/util/dwarf-regs.c b/tools/perf/util/dwarf-regs.c
new file mode 100644
index 000000000..797f455eb
--- /dev/null
+++ b/tools/perf/util/dwarf-regs.c
@@ -0,0 +1,237 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * dwarf-regs.c : Mapping of DWARF debug register numbers into register names.
+ *
+ * Written by: Masami Hiramatsu <mhiramat@kernel.org>
+ */
+
+#include <stdlib.h>
+#include <string.h>
+#include <debug.h>
+#include <dwarf-regs.h>
+#include <elf.h>
+#include <errno.h>
+#include <linux/kernel.h>
+
+/* Define const char * {arch}_register_tbl[] */
+#define DEFINE_DWARF_REGSTR_TABLE
+#include "../arch/x86/include/dwarf-regs-table.h"
+#include "../arch/arm/include/dwarf-regs-table.h"
+#include "../arch/arm64/include/dwarf-regs-table.h"
+#include "../arch/sh/include/dwarf-regs-table.h"
+#include "../arch/powerpc/include/dwarf-regs-table.h"
+#include "../arch/riscv/include/dwarf-regs-table.h"
+#include "../arch/s390/include/dwarf-regs-table.h"
+#include "../arch/sparc/include/dwarf-regs-table.h"
+#include "../arch/xtensa/include/dwarf-regs-table.h"
+#include "../arch/mips/include/dwarf-regs-table.h"
+#include "../arch/loongarch/include/dwarf-regs-table.h"
+
+/* Return architecture dependent register string (for kprobe-tracer) */
+const char *get_dwarf_regstr(unsigned int n, unsigned int machine, unsigned int flags)
+{
+	#define __get_dwarf_regstr(tbl, n) (((n) < ARRAY_SIZE(tbl)) ? (tbl)[(n)] : NULL)
+
+	if (machine == EM_NONE) {
+		/* Generic arch - use host arch */
+		machine = EM_HOST;
+	}
+	switch (machine) {
+	case EM_386:
+		return __get_dwarf_regstr(x86_32_regstr_tbl, n);
+	case EM_X86_64:
+		return __get_dwarf_regstr(x86_64_regstr_tbl, n);
+	case EM_ARM:
+		return __get_dwarf_regstr(arm_regstr_tbl, n);
+	case EM_AARCH64:
+		return __get_dwarf_regstr(aarch64_regstr_tbl, n);
+	case EM_CSKY:
+		return __get_csky_regstr(n, flags);
+	case EM_SH:
+		return __get_dwarf_regstr(sh_regstr_tbl, n);
+	case EM_S390:
+		return __get_dwarf_regstr(s390_regstr_tbl, n);
+	case EM_PPC:
+	case EM_PPC64:
+		return __get_dwarf_regstr(powerpc_regstr_tbl, n);
+	case EM_RISCV:
+		return __get_dwarf_regstr(riscv_regstr_tbl, n);
+	case EM_SPARC:
+	case EM_SPARCV9:
+		return __get_dwarf_regstr(sparc_regstr_tbl, n);
+	case EM_XTENSA:
+		return __get_dwarf_regstr(xtensa_regstr_tbl, n);
+	case EM_MIPS:
+		return __get_dwarf_regstr(mips_regstr_tbl, n);
+	case EM_LOONGARCH:
+		return __get_dwarf_regstr(loongarch_regstr_tbl, n);
+	default:
+		pr_err("ELF MACHINE %x is not supported.\n", machine);
+	}
+	return NULL;
+
+	#undef __get_dwarf_regstr
+}
+
+static int __get_dwarf_regnum(const char *const *regstr, size_t num_regstr, const char *name)
+{
+	for (size_t i = 0; i < num_regstr; i++) {
+		if (regstr[i] && !strcmp(regstr[i], name))
+			return i;
+	}
+	return -ENOENT;
+}
+
+/* Return DWARF register number from architecture register name */
+int get_dwarf_regnum(const char *name, unsigned int machine, unsigned int flags)
+{
+	char *regname = strdup(name);
+	int reg = -1;
+	char *p;
+
+	#define _get_dwarf_regnum(tbl, name) __get_dwarf_regnum(tbl, ARRAY_SIZE(tbl), name)
+
+	if (regname == NULL)
+		return -EINVAL;
+
+	/* For convenience, remove trailing characters */
+	p = strpbrk(regname, " ,)");
+	if (p)
+		*p = '\0';
+
+	if (machine == EM_NONE) {
+		/* Generic arch - use host arch */
+		machine = EM_HOST;
+	}
+	switch (machine) {
+	case EM_X86_64:
+		reg = __get_dwarf_regnum_x86_64(name);
+		break;
+	case EM_386:
+		reg = __get_dwarf_regnum_i386(name);
+		break;
+	case EM_ARM:
+		reg = _get_dwarf_regnum(arm_regstr_tbl, name);
+		break;
+	case EM_AARCH64:
+		reg = _get_dwarf_regnum(aarch64_regstr_tbl, name);
+		break;
+	case EM_CSKY:
+		reg = __get_csky_regnum(name, flags);
+		break;
+	case EM_SH:
+		reg = _get_dwarf_regnum(sh_regstr_tbl, name);
+		break;
+	case EM_S390:
+		reg = _get_dwarf_regnum(s390_regstr_tbl, name);
+		break;
+	case EM_PPC:
+	case EM_PPC64:
+		reg = _get_dwarf_regnum(powerpc_regstr_tbl, name);
+		break;
+	case EM_RISCV:
+		reg = _get_dwarf_regnum(riscv_regstr_tbl, name);
+		break;
+	case EM_SPARC:
+	case EM_SPARCV9:
+		reg = _get_dwarf_regnum(sparc_regstr_tbl, name);
+		break;
+	case EM_XTENSA:
+		reg = _get_dwarf_regnum(xtensa_regstr_tbl, name);
+		break;
+	case EM_MIPS:
+		reg = _get_dwarf_regnum(mips_regstr_tbl, name);
+		break;
+	case EM_LOONGARCH:
+		reg = _get_dwarf_regnum(loongarch_regstr_tbl, name);
+		break;
+	default:
+		pr_err("ELF MACHINE %x is not supported.\n", machine);
+	}
+	free(regname);
+	return reg;
+
+	#undef _get_dwarf_regnum
+}
+
+static int get_libdw_frame_nregs(unsigned int machine, unsigned int flags __maybe_unused)
+{
+	switch (machine) {
+	case EM_X86_64:
+		return 17;
+	case EM_386:
+		return 9;
+	case EM_ARM:
+		return 16;
+	case EM_AARCH64:
+		return 97;
+	case EM_CSKY:
+		return 38;
+	case EM_S390:
+		return 32;
+	case EM_PPC:
+	case EM_PPC64:
+		return 145;
+	case EM_RISCV:
+		return 66;
+	case EM_SPARC:
+	case EM_SPARCV9:
+		return 103;
+	case EM_LOONGARCH:
+		return 74;
+	case EM_MIPS:
+		return 71;
+	default:
+		return 0;
+	}
+}
+
+int get_dwarf_regnum_for_perf_regnum(int perf_regnum, unsigned int machine,
+				     unsigned int flags, bool only_libdw_supported)
+{
+	int reg;
+
+	switch (machine) {
+	case EM_X86_64:
+		reg = __get_dwarf_regnum_for_perf_regnum_x86_64(perf_regnum);
+		break;
+	case EM_386:
+		reg = __get_dwarf_regnum_for_perf_regnum_i386(perf_regnum);
+		break;
+	case EM_ARM:
+		reg = __get_dwarf_regnum_for_perf_regnum_arm(perf_regnum);
+		break;
+	case EM_AARCH64:
+		reg = __get_dwarf_regnum_for_perf_regnum_arm64(perf_regnum);
+		break;
+	case EM_CSKY:
+		reg = __get_dwarf_regnum_for_perf_regnum_csky(perf_regnum, flags);
+		break;
+	case EM_PPC:
+	case EM_PPC64:
+		reg = __get_dwarf_regnum_for_perf_regnum_powerpc(perf_regnum);
+		break;
+	case EM_RISCV:
+		reg = __get_dwarf_regnum_for_perf_regnum_riscv(perf_regnum);
+		break;
+	case EM_S390:
+		reg = __get_dwarf_regnum_for_perf_regnum_s390(perf_regnum);
+		break;
+	case EM_LOONGARCH:
+		reg = __get_dwarf_regnum_for_perf_regnum_loongarch(perf_regnum);
+		break;
+	case EM_MIPS:
+		reg = __get_dwarf_regnum_for_perf_regnum_mips(perf_regnum);
+		break;
+	default:
+		pr_err("ELF MACHINE %x is not supported.\n", machine);
+		return -ENOENT;
+	}
+	if (reg >= 0 && only_libdw_supported) {
+		int nregs = get_libdw_frame_nregs(machine, flags);
+
+		if (reg >= nregs)
+			reg = -ENOENT;
+	}
+	return reg;
+}