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, 305 insertions, 0 deletions

diff --git a/scripts/elf-parse.h b/scripts/elf-parse.h
new file mode 100644
index 000000000..f4411e030
--- /dev/null
+++ b/scripts/elf-parse.h
@@ -0,0 +1,305 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+#ifndef _SCRIPTS_ELF_PARSE_H
+#define _SCRIPTS_ELF_PARSE_H
+
+#include <elf.h>
+
+#include <tools/be_byteshift.h>
+#include <tools/le_byteshift.h>
+
+typedef union {
+	Elf32_Ehdr	e32;
+	Elf64_Ehdr	e64;
+} Elf_Ehdr;
+
+typedef union {
+	Elf32_Shdr	e32;
+	Elf64_Shdr	e64;
+} Elf_Shdr;
+
+typedef union {
+	Elf32_Sym	e32;
+	Elf64_Sym	e64;
+} Elf_Sym;
+
+typedef union {
+	Elf32_Rela	e32;
+	Elf64_Rela	e64;
+} Elf_Rela;
+
+struct elf_funcs {
+	int (*compare_extable)(const void *a, const void *b);
+	uint64_t (*ehdr_shoff)(Elf_Ehdr *ehdr);
+	uint16_t (*ehdr_shstrndx)(Elf_Ehdr *ehdr);
+	uint16_t (*ehdr_shentsize)(Elf_Ehdr *ehdr);
+	uint16_t (*ehdr_shnum)(Elf_Ehdr *ehdr);
+	uint64_t (*shdr_addr)(Elf_Shdr *shdr);
+	uint64_t (*shdr_offset)(Elf_Shdr *shdr);
+	uint64_t (*shdr_size)(Elf_Shdr *shdr);
+	uint64_t (*shdr_entsize)(Elf_Shdr *shdr);
+	uint32_t (*shdr_link)(Elf_Shdr *shdr);
+	uint32_t (*shdr_name)(Elf_Shdr *shdr);
+	uint32_t (*shdr_type)(Elf_Shdr *shdr);
+	uint8_t (*sym_type)(Elf_Sym *sym);
+	uint32_t (*sym_name)(Elf_Sym *sym);
+	uint64_t (*sym_value)(Elf_Sym *sym);
+	uint16_t (*sym_shndx)(Elf_Sym *sym);
+	uint64_t (*rela_offset)(Elf_Rela *rela);
+	uint64_t (*rela_info)(Elf_Rela *rela);
+	uint64_t (*rela_addend)(Elf_Rela *rela);
+	void (*rela_write_addend)(Elf_Rela *rela, uint64_t val);
+	uint32_t (*r)(const uint32_t *);
+	uint16_t (*r2)(const uint16_t *);
+	uint64_t (*r8)(const uint64_t *);
+	void (*w)(uint32_t, uint32_t *);
+	void (*w8)(uint64_t, uint64_t *);
+};
+
+extern struct elf_funcs elf_parser;
+
+static inline uint64_t ehdr64_shoff(Elf_Ehdr *ehdr)
+{
+	return elf_parser.r8(&ehdr->e64.e_shoff);
+}
+
+static inline uint64_t ehdr32_shoff(Elf_Ehdr *ehdr)
+{
+	return elf_parser.r(&ehdr->e32.e_shoff);
+}
+
+static inline uint64_t ehdr_shoff(Elf_Ehdr *ehdr)
+{
+	return elf_parser.ehdr_shoff(ehdr);
+}
+
+#define EHDR_HALF(fn_name)				\
+static inline uint16_t ehdr64_##fn_name(Elf_Ehdr *ehdr)	\
+{							\
+	return elf_parser.r2(&ehdr->e64.e_##fn_name);	\
+}							\
+							\
+static inline uint16_t ehdr32_##fn_name(Elf_Ehdr *ehdr)	\
+{							\
+	return elf_parser.r2(&ehdr->e32.e_##fn_name);	\
+}							\
+							\
+static inline uint16_t ehdr_##fn_name(Elf_Ehdr *ehdr)	\
+{							\
+	return elf_parser.ehdr_##fn_name(ehdr);		\
+}
+
+EHDR_HALF(shentsize)
+EHDR_HALF(shstrndx)
+EHDR_HALF(shnum)
+
+#define SHDR_WORD(fn_name)				\
+static inline uint32_t shdr64_##fn_name(Elf_Shdr *shdr)	\
+{							\
+	return elf_parser.r(&shdr->e64.sh_##fn_name);	\
+}							\
+							\
+static inline uint32_t shdr32_##fn_name(Elf_Shdr *shdr)	\
+{							\
+	return elf_parser.r(&shdr->e32.sh_##fn_name);	\
+}							\
+							\
+static inline uint32_t shdr_##fn_name(Elf_Shdr *shdr)	\
+{							\
+	return elf_parser.shdr_##fn_name(shdr);	\
+}
+
+#define SHDR_ADDR(fn_name)				\
+static inline uint64_t shdr64_##fn_name(Elf_Shdr *shdr)	\
+{							\
+	return elf_parser.r8(&shdr->e64.sh_##fn_name);	\
+}							\
+							\
+static inline uint64_t shdr32_##fn_name(Elf_Shdr *shdr)	\
+{							\
+	return elf_parser.r(&shdr->e32.sh_##fn_name);	\
+}							\
+							\
+static inline uint64_t shdr_##fn_name(Elf_Shdr *shdr)	\
+{							\
+	return elf_parser.shdr_##fn_name(shdr);		\
+}
+
+#define SHDR_WORD(fn_name)				\
+static inline uint32_t shdr64_##fn_name(Elf_Shdr *shdr)	\
+{							\
+	return elf_parser.r(&shdr->e64.sh_##fn_name);	\
+}							\
+							\
+static inline uint32_t shdr32_##fn_name(Elf_Shdr *shdr)	\
+{							\
+	return elf_parser.r(&shdr->e32.sh_##fn_name);	\
+}							\
+static inline uint32_t shdr_##fn_name(Elf_Shdr *shdr)	\
+{							\
+	return elf_parser.shdr_##fn_name(shdr);		\
+}
+
+SHDR_ADDR(addr)
+SHDR_ADDR(offset)
+SHDR_ADDR(size)
+SHDR_ADDR(entsize)
+
+SHDR_WORD(link)
+SHDR_WORD(name)
+SHDR_WORD(type)
+
+#define SYM_ADDR(fn_name)				\
+static inline uint64_t sym64_##fn_name(Elf_Sym *sym)	\
+{							\
+	return elf_parser.r8(&sym->e64.st_##fn_name);	\
+}							\
+							\
+static inline uint64_t sym32_##fn_name(Elf_Sym *sym)	\
+{							\
+	return elf_parser.r(&sym->e32.st_##fn_name);	\
+}							\
+							\
+static inline uint64_t sym_##fn_name(Elf_Sym *sym)	\
+{							\
+	return elf_parser.sym_##fn_name(sym);		\
+}
+
+#define SYM_WORD(fn_name)				\
+static inline uint32_t sym64_##fn_name(Elf_Sym *sym)	\
+{							\
+	return elf_parser.r(&sym->e64.st_##fn_name);	\
+}							\
+							\
+static inline uint32_t sym32_##fn_name(Elf_Sym *sym)	\
+{							\
+	return elf_parser.r(&sym->e32.st_##fn_name);	\
+}							\
+							\
+static inline uint32_t sym_##fn_name(Elf_Sym *sym)	\
+{							\
+	return elf_parser.sym_##fn_name(sym);		\
+}
+
+#define SYM_HALF(fn_name)				\
+static inline uint16_t sym64_##fn_name(Elf_Sym *sym)	\
+{							\
+	return elf_parser.r2(&sym->e64.st_##fn_name);	\
+}							\
+							\
+static inline uint16_t sym32_##fn_name(Elf_Sym *sym)	\
+{							\
+	return elf_parser.r2(&sym->e32.st_##fn_name);	\
+}							\
+							\
+static inline uint16_t sym_##fn_name(Elf_Sym *sym)	\
+{							\
+	return elf_parser.sym_##fn_name(sym);		\
+}
+
+static inline uint8_t sym64_type(Elf_Sym *sym)
+{
+	return ELF64_ST_TYPE(sym->e64.st_info);
+}
+
+static inline uint8_t sym32_type(Elf_Sym *sym)
+{
+	return ELF32_ST_TYPE(sym->e32.st_info);
+}
+
+static inline uint8_t sym_type(Elf_Sym *sym)
+{
+	return elf_parser.sym_type(sym);
+}
+
+SYM_ADDR(value)
+SYM_WORD(name)
+SYM_HALF(shndx)
+
+#define __maybe_unused			__attribute__((__unused__))
+
+#define RELA_ADDR(fn_name)						\
+static inline uint64_t rela64_##fn_name(Elf_Rela *rela)			\
+{									\
+	return elf_parser.r8((uint64_t *)&rela->e64.r_##fn_name);	\
+}									\
+									\
+static inline uint64_t rela32_##fn_name(Elf_Rela *rela)			\
+{									\
+	return elf_parser.r((uint32_t *)&rela->e32.r_##fn_name);	\
+}									\
+									\
+static inline uint64_t __maybe_unused rela_##fn_name(Elf_Rela *rela)	\
+{									\
+	return elf_parser.rela_##fn_name(rela);				\
+}
+
+RELA_ADDR(offset)
+RELA_ADDR(info)
+RELA_ADDR(addend)
+
+static inline void rela64_write_addend(Elf_Rela *rela, uint64_t val)
+{
+	elf_parser.w8(val, (uint64_t *)&rela->e64.r_addend);
+}
+
+static inline void rela32_write_addend(Elf_Rela *rela, uint64_t val)
+{
+	elf_parser.w(val, (uint32_t *)&rela->e32.r_addend);
+}
+
+static inline uint32_t rbe(const uint32_t *x)
+{
+	return get_unaligned_be32(x);
+}
+
+static inline uint16_t r2be(const uint16_t *x)
+{
+	return get_unaligned_be16(x);
+}
+
+static inline uint64_t r8be(const uint64_t *x)
+{
+	return get_unaligned_be64(x);
+}
+
+static inline uint32_t rle(const uint32_t *x)
+{
+	return get_unaligned_le32(x);
+}
+
+static inline uint16_t r2le(const uint16_t *x)
+{
+	return get_unaligned_le16(x);
+}
+
+static inline uint64_t r8le(const uint64_t *x)
+{
+	return get_unaligned_le64(x);
+}
+
+static inline void wbe(uint32_t val, uint32_t *x)
+{
+	put_unaligned_be32(val, x);
+}
+
+static inline void wle(uint32_t val, uint32_t *x)
+{
+	put_unaligned_le32(val, x);
+}
+
+static inline void w8be(uint64_t val, uint64_t *x)
+{
+	put_unaligned_be64(val, x);
+}
+
+static inline void w8le(uint64_t val, uint64_t *x)
+{
+	put_unaligned_le64(val, x);
+}
+
+void *elf_map(char const *fname, size_t *size, uint32_t types);
+void elf_unmap(void *addr, size_t size);
+int elf_map_machine(void *addr);
+int elf_map_long_size(void *addr);
+
+#endif /* _SCRIPTS_ELF_PARSE_H */