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, 203 insertions, 0 deletions

diff --git a/tools/objtool/trace.c b/tools/objtool/trace.c
new file mode 100644
index 000000000..61c6aa302
--- /dev/null
+++ b/tools/objtool/trace.c
@@ -0,0 +1,203 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * Copyright (c) 2025, Oracle and/or its affiliates.
+ */
+
+#include <objtool/trace.h>
+
+bool trace;
+int trace_depth;
+
+/*
+ * Macros to trace CFI state attributes changes.
+ */
+
+#define TRACE_CFI_ATTR(attr, prev, next, fmt, ...)		\
+({								\
+	if ((prev)->attr != (next)->attr)			\
+		TRACE("%s=" fmt " ", #attr, __VA_ARGS__);	\
+})
+
+#define TRACE_CFI_ATTR_BOOL(attr, prev, next)			\
+	TRACE_CFI_ATTR(attr, prev, next,			\
+		       "%s", (next)->attr ? "true" : "false")
+
+#define TRACE_CFI_ATTR_NUM(attr, prev, next, fmt)		\
+	TRACE_CFI_ATTR(attr, prev, next, fmt, (next)->attr)
+
+#define CFI_REG_NAME_MAXLEN   16
+
+/*
+ * Return the name of a register. Note that the same static buffer
+ * is returned if the name is dynamically generated.
+ */
+static const char *cfi_reg_name(unsigned int reg)
+{
+	static char rname_buffer[CFI_REG_NAME_MAXLEN];
+	const char *rname;
+
+	switch (reg) {
+	case CFI_UNDEFINED:
+		return "<undefined>";
+	case CFI_CFA:
+		return "cfa";
+	case CFI_SP_INDIRECT:
+		return "(sp)";
+	case CFI_BP_INDIRECT:
+		return "(bp)";
+	}
+
+	if (reg < CFI_NUM_REGS) {
+		rname = arch_reg_name[reg];
+		if (rname)
+			return rname;
+	}
+
+	if (snprintf(rname_buffer, CFI_REG_NAME_MAXLEN, "r%d", reg) == -1)
+		return "<error>";
+
+	return (const char *)rname_buffer;
+}
+
+/*
+ * Functions and macros to trace CFI registers changes.
+ */
+
+static void trace_cfi_reg(const char *prefix, int reg, const char *fmt,
+			  int base_prev, int offset_prev,
+			  int base_next, int offset_next)
+{
+	char *rname;
+
+	if (base_prev == base_next && offset_prev == offset_next)
+		return;
+
+	if (prefix)
+		TRACE("%s:", prefix);
+
+	if (base_next == CFI_UNDEFINED) {
+		TRACE("%1$s=<undef> ", cfi_reg_name(reg));
+	} else {
+		rname = strdup(cfi_reg_name(reg));
+		TRACE(fmt, rname, cfi_reg_name(base_next), offset_next);
+		free(rname);
+	}
+}
+
+static void trace_cfi_reg_val(const char *prefix, int reg,
+			      int base_prev, int offset_prev,
+			      int base_next, int offset_next)
+{
+	trace_cfi_reg(prefix, reg, "%1$s=%2$s%3$+d ",
+		      base_prev, offset_prev, base_next, offset_next);
+}
+
+static void trace_cfi_reg_ref(const char *prefix, int reg,
+			      int base_prev, int offset_prev,
+			      int base_next, int offset_next)
+{
+	trace_cfi_reg(prefix, reg, "%1$s=(%2$s%3$+d) ",
+		      base_prev, offset_prev, base_next, offset_next);
+}
+
+#define TRACE_CFI_REG_VAL(reg, prev, next)				\
+	trace_cfi_reg_val(NULL, reg, prev.base, prev.offset,		\
+			  next.base, next.offset)
+
+#define TRACE_CFI_REG_REF(reg, prev, next)				\
+	trace_cfi_reg_ref(NULL, reg, prev.base, prev.offset,		\
+			  next.base, next.offset)
+
+void trace_insn_state(struct instruction *insn, struct insn_state *sprev,
+		      struct insn_state *snext)
+{
+	struct cfi_state *cprev, *cnext;
+	int i;
+
+	if (!memcmp(sprev, snext, sizeof(struct insn_state)))
+		return;
+
+	cprev = &sprev->cfi;
+	cnext = &snext->cfi;
+
+	disas_print_insn(stderr, objtool_disas_ctx, insn,
+			 trace_depth - 1, "state: ");
+
+	/* print registers changes */
+	TRACE_CFI_REG_VAL(CFI_CFA, cprev->cfa, cnext->cfa);
+	for (i = 0; i < CFI_NUM_REGS; i++) {
+		TRACE_CFI_REG_VAL(i, cprev->vals[i], cnext->vals[i]);
+		TRACE_CFI_REG_REF(i, cprev->regs[i], cnext->regs[i]);
+	}
+
+	/* print attributes changes */
+	TRACE_CFI_ATTR_NUM(stack_size, cprev, cnext, "%d");
+	TRACE_CFI_ATTR_BOOL(drap, cprev, cnext);
+	if (cnext->drap) {
+		trace_cfi_reg_val("drap", cnext->drap_reg,
+				  cprev->drap_reg, cprev->drap_offset,
+				  cnext->drap_reg, cnext->drap_offset);
+	}
+	TRACE_CFI_ATTR_BOOL(bp_scratch, cprev, cnext);
+	TRACE_CFI_ATTR_NUM(instr, sprev, snext, "%d");
+	TRACE_CFI_ATTR_NUM(uaccess_stack, sprev, snext, "%u");
+
+	TRACE("\n");
+
+	insn->trace = 1;
+}
+
+void trace_alt_begin(struct instruction *orig_insn, struct alternative *alt,
+		     char *alt_name)
+{
+	struct instruction *alt_insn;
+	char suffix[2];
+
+	alt_insn = alt->insn;
+
+	if (alt->type == ALT_TYPE_EX_TABLE) {
+		/*
+		 * When there is an exception table then the instruction
+		 * at the original location is executed but it can cause
+		 * an exception. In that case, the execution will be
+		 * redirected to the alternative instruction.
+		 *
+		 * The instruction at the original location can have
+		 * instruction alternatives, so we just print the location
+		 * of the instruction that can cause the exception and
+		 * not the instruction itself.
+		 */
+		TRACE_ALT_INFO_NOADDR(orig_insn, "/ ", "%s for instruction at 0x%lx <%s+0x%lx>",
+				      alt_name,
+				      orig_insn->offset, insn_sym(orig_insn)->name,
+				      orig_insn->offset - insn_sym(orig_insn)->offset);
+	} else {
+		TRACE_ALT_INFO_NOADDR(orig_insn, "/ ", "%s", alt_name);
+	}
+
+	if (alt->type == ALT_TYPE_JUMP_TABLE) {
+		/*
+		 * For a jump alternative, if the default instruction is
+		 * a NOP then it is replaced with the jmp instruction,
+		 * otherwise it is replaced with a NOP instruction.
+		 */
+		trace_depth++;
+		if (orig_insn->type == INSN_NOP) {
+			suffix[0] = (orig_insn->len == 5) ? 'q' : '\0';
+			TRACE_ADDR(orig_insn, "jmp%-3s %lx <%s+0x%lx>", suffix,
+				   alt_insn->offset, insn_sym(alt_insn)->name,
+				   alt_insn->offset - insn_sym(alt_insn)->offset);
+		} else {
+			TRACE_ADDR(orig_insn, "nop%d", orig_insn->len);
+			trace_depth--;
+		}
+	}
+}
+
+void trace_alt_end(struct instruction *orig_insn, struct alternative *alt,
+		   char *alt_name)
+{
+	if (alt->type == ALT_TYPE_JUMP_TABLE && orig_insn->type == INSN_NOP)
+		trace_depth--;
+	TRACE_ALT_INFO_NOADDR(orig_insn, "\\ ", "%s", alt_name);
+}