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, 103 insertions, 0 deletions

diff --git a/arch/alpha/lib/stacktrace.c b/arch/alpha/lib/stacktrace.c
new file mode 100644
index 000000000..2b1176dd5
--- /dev/null
+++ b/arch/alpha/lib/stacktrace.c
@@ -0,0 +1,103 @@
+// SPDX-License-Identifier: GPL-2.0
+#include <linux/kernel.h>
+
+typedef unsigned int instr;
+
+#define MAJOR_OP	0xfc000000
+#define LDA_OP		0x20000000
+#define STQ_OP		0xb4000000
+#define BR_OP		0xc0000000
+
+#define STK_ALLOC_1	0x23de8000 /* lda $30,-X($30) */
+#define STK_ALLOC_1M	0xffff8000
+#define STK_ALLOC_2	0x43c0153e /* subq $30,X,$30 */
+#define STK_ALLOC_2M	0xffe01fff
+
+#define MEM_REG		0x03e00000
+#define MEM_BASE	0x001f0000
+#define MEM_OFF		0x0000ffff
+#define MEM_OFF_SIGN	0x00008000
+#define	BASE_SP		0x001e0000
+
+#define STK_ALLOC_MATCH(INSTR)			\
+  (((INSTR) & STK_ALLOC_1M) == STK_ALLOC_1	\
+   || ((INSTR) & STK_ALLOC_2M) == STK_ALLOC_2)
+#define STK_PUSH_MATCH(INSTR) \
+  (((INSTR) & (MAJOR_OP | MEM_BASE | MEM_OFF_SIGN)) == (STQ_OP | BASE_SP))
+#define MEM_OP_OFFSET(INSTR) \
+  (((long)((INSTR) & MEM_OFF) << 48) >> 48)
+#define MEM_OP_REG(INSTR) \
+  (((INSTR) & MEM_REG) >> 22)
+
+/* Branches, jumps, PAL calls, and illegal opcodes end a basic block. */
+#define BB_END(INSTR)						\
+  (((instr)(INSTR) >= BR_OP) | ((instr)(INSTR) < LDA_OP) |	\
+   ((((instr)(INSTR) ^ 0x60000000) < 0x20000000) &		\
+    (((instr)(INSTR) & 0x0c000000) != 0)))
+
+#define IS_KERNEL_TEXT(PC) ((unsigned long)(PC) > START_ADDR)
+
+static char reg_name[][4] = {
+	"v0 ", "t0 ", "t1 ", "t2 ", "t3 ", "t4 ", "t5 ", "t6 ", "t7 ",
+	"s0 ", "s1 ", "s2 ", "s3 ", "s4 ", "s5 ", "s6 ", "a0 ", "a1 ",
+	"a2 ", "a3 ", "a4 ", "a5 ", "t8 ", "t9 ", "t10", "t11", "ra ",
+	"pv ", "at ", "gp ", "sp ", "0"
+};
+
+
+static instr *
+display_stored_regs(instr * pro_pc, unsigned char * sp)
+{
+	instr * ret_pc = 0;
+	int reg;
+	unsigned long value;
+
+	printk("Prologue [<%p>], Frame %p:\n", pro_pc, sp);
+	while (!BB_END(*pro_pc))
+		if (STK_PUSH_MATCH(*pro_pc)) {
+			reg = (*pro_pc & MEM_REG) >> 21;
+			value = *(unsigned long *)(sp + (*pro_pc & MEM_OFF));
+			if (reg == 26)
+				ret_pc = (instr *)value;
+			printk("\t\t%s / 0x%016lx\n", reg_name[reg], value);
+		}
+	return ret_pc;
+}
+
+static instr *
+seek_prologue(instr * pc)
+{
+	while (!STK_ALLOC_MATCH(*pc))
+		--pc;
+	while (!BB_END(*(pc - 1)))
+		--pc;
+	return pc;
+}
+
+static long
+stack_increment(instr * prologue_pc)
+{
+	while (!STK_ALLOC_MATCH(*prologue_pc))
+		++prologue_pc;
+
+	/* Count the bytes allocated. */
+	if ((*prologue_pc & STK_ALLOC_1M) == STK_ALLOC_1M)
+		return -(((long)(*prologue_pc) << 48) >> 48);
+	else
+		return (*prologue_pc >> 13) & 0xff;
+}
+
+void
+stacktrace(void)
+{
+	instr * ret_pc;
+	instr * prologue = (instr *)stacktrace;
+	unsigned char *sp = (unsigned char *)current_stack_pointer;
+
+	printk("\tstack trace:\n");
+	do {
+		ret_pc = display_stored_regs(prologue, sp);
+		sp += stack_increment(prologue);
+		prologue = seek_prologue(ret_pc);
+	} while (IS_KERNEL_TEXT(ret_pc));
+}