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, 219 insertions, 0 deletions

diff --git a/arch/powerpc/boot/oflib.c b/arch/powerpc/boot/oflib.c
new file mode 100644
index 000000000..8759c985e
--- /dev/null
+++ b/arch/powerpc/boot/oflib.c
@@ -0,0 +1,219 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * Copyright (C) Paul Mackerras 1997.
+ */
+#include <stddef.h>
+#include "types.h"
+#include "elf.h"
+#include "string.h"
+#include "stdio.h"
+#include "page.h"
+#include "ops.h"
+
+#include "of.h"
+
+typedef u32 prom_arg_t;
+
+/* The following structure is used to communicate with open firmware.
+ * All arguments in and out are in big endian format. */
+struct prom_args {
+	__be32 service;	/* Address of service name string. */
+	__be32 nargs;	/* Number of input arguments. */
+	__be32 nret;	/* Number of output arguments. */
+	__be32 args[10];	/* Input/output arguments. */
+};
+
+#ifdef __powerpc64__
+extern int prom(void *);
+#else
+static int (*prom) (void *);
+#endif
+
+void of_init(void *promptr)
+{
+#ifndef __powerpc64__
+	prom = (int (*)(void *))promptr;
+#endif
+}
+
+#define ADDR(x)		(u32)(unsigned long)(x)
+
+int of_call_prom(const char *service, int nargs, int nret, ...)
+{
+	int i;
+	struct prom_args args;
+	va_list list;
+
+	args.service = cpu_to_be32(ADDR(service));
+	args.nargs = cpu_to_be32(nargs);
+	args.nret = cpu_to_be32(nret);
+
+	va_start(list, nret);
+	for (i = 0; i < nargs; i++)
+		args.args[i] = cpu_to_be32(va_arg(list, prom_arg_t));
+	va_end(list);
+
+	for (i = 0; i < nret; i++)
+		args.args[nargs+i] = 0;
+
+	if (prom(&args) < 0)
+		return PROM_ERROR;
+
+	return (nret > 0) ? be32_to_cpu(args.args[nargs]) : 0;
+}
+
+static int of_call_prom_ret(const char *service, int nargs, int nret,
+			    prom_arg_t *rets, ...)
+{
+	int i;
+	struct prom_args args;
+	va_list list;
+
+	args.service = cpu_to_be32(ADDR(service));
+	args.nargs = cpu_to_be32(nargs);
+	args.nret = cpu_to_be32(nret);
+
+	va_start(list, rets);
+	for (i = 0; i < nargs; i++)
+		args.args[i] = cpu_to_be32(va_arg(list, prom_arg_t));
+	va_end(list);
+
+	for (i = 0; i < nret; i++)
+		args.args[nargs+i] = 0;
+
+	if (prom(&args) < 0)
+		return PROM_ERROR;
+
+	if (rets != NULL)
+		for (i = 1; i < nret; ++i)
+			rets[i-1] = be32_to_cpu(args.args[nargs+i]);
+
+	return (nret > 0) ? be32_to_cpu(args.args[nargs]) : 0;
+}
+
+/* returns true if s2 is a prefix of s1 */
+static int string_match(const char *s1, const char *s2)
+{
+	for (; *s2; ++s2)
+		if (*s1++ != *s2)
+			return 0;
+	return 1;
+}
+
+/*
+ * Older OF's require that when claiming a specific range of addresses,
+ * we claim the physical space in the /memory node and the virtual
+ * space in the chosen mmu node, and then do a map operation to
+ * map virtual to physical.
+ */
+static int need_map = -1;
+static ihandle chosen_mmu;
+static ihandle memory;
+
+static int check_of_version(void)
+{
+	phandle oprom, chosen;
+	char version[64];
+
+	oprom = of_finddevice("/openprom");
+	if (oprom == (phandle) -1)
+		return 0;
+	if (of_getprop(oprom, "model", version, sizeof(version)) <= 0)
+		return 0;
+	version[sizeof(version)-1] = 0;
+	printf("OF version = '%s'\r\n", version);
+	if (!string_match(version, "Open Firmware, 1.")
+	    && !string_match(version, "FirmWorks,3."))
+		return 0;
+	chosen = of_finddevice("/chosen");
+	if (chosen == (phandle) -1) {
+		chosen = of_finddevice("/chosen@0");
+		if (chosen == (phandle) -1) {
+			printf("no chosen\n");
+			return 0;
+		}
+	}
+	if (of_getprop(chosen, "mmu", &chosen_mmu, sizeof(chosen_mmu)) <= 0) {
+		printf("no mmu\n");
+		return 0;
+	}
+	memory = of_call_prom("open", 1, 1, "/memory");
+	if (memory == PROM_ERROR) {
+		memory = of_call_prom("open", 1, 1, "/memory@0");
+		if (memory == PROM_ERROR) {
+			printf("no memory node\n");
+			return 0;
+		}
+	}
+	printf("old OF detected\r\n");
+	return 1;
+}
+
+unsigned int of_claim(unsigned long virt, unsigned long size,
+		      unsigned long align)
+{
+	int ret;
+	prom_arg_t result;
+
+	if (need_map < 0)
+		need_map = check_of_version();
+	if (align || !need_map)
+		return of_call_prom("claim", 3, 1, virt, size, align);
+
+	ret = of_call_prom_ret("call-method", 5, 2, &result, "claim", memory,
+			       align, size, virt);
+	if (ret != 0 || result == -1)
+		return  -1;
+	ret = of_call_prom_ret("call-method", 5, 2, &result, "claim", chosen_mmu,
+			       align, size, virt);
+	/* 0x12 == coherent + read/write */
+	ret = of_call_prom("call-method", 6, 1, "map", chosen_mmu,
+			   0x12, size, virt, virt);
+	return virt;
+}
+
+void *of_vmlinux_alloc(unsigned long size)
+{
+	unsigned long start = (unsigned long)_start, end = (unsigned long)_end;
+	unsigned long addr;
+	void *p;
+
+	/* With some older POWER4 firmware we need to claim the area the kernel
+	 * will reside in.  Newer firmwares don't need this so we just ignore
+	 * the return value.
+	 */
+	addr = (unsigned long) of_claim(start, end - start, 0);
+	printf("Trying to claim from 0x%lx to 0x%lx (0x%lx) got %lx\r\n",
+	       start, end, end - start, addr);
+
+	p = malloc(size);
+	if (!p)
+		fatal("Can't allocate memory for kernel image!\n\r");
+
+	return p;
+}
+
+void of_exit(void)
+{
+	of_call_prom("exit", 0, 0);
+}
+
+/*
+ * OF device tree routines
+ */
+void *of_finddevice(const char *name)
+{
+	return (void *) (unsigned long) of_call_prom("finddevice", 1, 1, name);
+}
+
+int of_getprop(const void *phandle, const char *name, void *buf,
+	       const int buflen)
+{
+	return of_call_prom("getprop", 4, 1, phandle, name, buf, buflen);
+}
+
+int of_setprop(const void *phandle, const char *name, const void *buf,
+	       const int buflen)
+{
+	return of_call_prom("setprop", 4, 1, phandle, name, buf, buflen);
+}