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, 95 insertions, 0 deletions

diff --git a/include/asm-generic/iomap.h b/include/asm-generic/iomap.h
new file mode 100644
index 000000000..9f3f25d7f
--- /dev/null
+++ b/include/asm-generic/iomap.h
@@ -0,0 +1,95 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef __GENERIC_IO_H
+#define __GENERIC_IO_H
+
+#include <linux/linkage.h>
+#include <asm/byteorder.h>
+
+/*
+ * These are the "generic" interfaces for doing new-style
+ * memory-mapped or PIO accesses. Architectures may do
+ * their own arch-optimized versions, these just act as
+ * wrappers around the old-style IO register access functions:
+ * read[bwl]/write[bwl]/in[bwl]/out[bwl]
+ *
+ * Don't include this directly, include it from <asm/io.h>.
+ */
+
+/*
+ * Read/write from/to an (offsettable) iomem cookie. It might be a PIO
+ * access or a MMIO access, these functions don't care. The info is
+ * encoded in the hardware mapping set up by the mapping functions
+ * (or the cookie itself, depending on implementation and hw).
+ *
+ * The generic routines just encode the PIO/MMIO as part of the
+ * cookie, and coldly assume that the MMIO IO mappings are not
+ * in the low address range. Architectures for which this is not
+ * true can't use this generic implementation.
+ */
+extern unsigned int ioread8(const void __iomem *);
+extern unsigned int ioread16(const void __iomem *);
+extern unsigned int ioread16be(const void __iomem *);
+extern unsigned int ioread32(const void __iomem *);
+extern unsigned int ioread32be(const void __iomem *);
+
+extern u64 __ioread64_lo_hi(const void __iomem *addr);
+extern u64 __ioread64_hi_lo(const void __iomem *addr);
+extern u64 __ioread64be_lo_hi(const void __iomem *addr);
+extern u64 __ioread64be_hi_lo(const void __iomem *addr);
+
+extern void iowrite8(u8, void __iomem *);
+extern void iowrite16(u16, void __iomem *);
+extern void iowrite16be(u16, void __iomem *);
+extern void iowrite32(u32, void __iomem *);
+extern void iowrite32be(u32, void __iomem *);
+
+extern void __iowrite64_lo_hi(u64 val, void __iomem *addr);
+extern void __iowrite64_hi_lo(u64 val, void __iomem *addr);
+extern void __iowrite64be_lo_hi(u64 val, void __iomem *addr);
+extern void __iowrite64be_hi_lo(u64 val, void __iomem *addr);
+
+/*
+ * "string" versions of the above. Note that they
+ * use native byte ordering for the accesses (on
+ * the assumption that IO and memory agree on a
+ * byte order, and CPU byteorder is irrelevant).
+ *
+ * They do _not_ update the port address. If you
+ * want MMIO that copies stuff laid out in MMIO
+ * memory across multiple ports, use "memcpy_toio()"
+ * and friends.
+ */
+extern void ioread8_rep(const void __iomem *port, void *buf, unsigned long count);
+extern void ioread16_rep(const void __iomem *port, void *buf, unsigned long count);
+extern void ioread32_rep(const void __iomem *port, void *buf, unsigned long count);
+
+extern void iowrite8_rep(void __iomem *port, const void *buf, unsigned long count);
+extern void iowrite16_rep(void __iomem *port, const void *buf, unsigned long count);
+extern void iowrite32_rep(void __iomem *port, const void *buf, unsigned long count);
+
+#ifdef CONFIG_HAS_IOPORT_MAP
+/* Create a virtual mapping cookie for an IO port range */
+extern void __iomem *ioport_map(unsigned long port, unsigned int nr);
+extern void ioport_unmap(void __iomem *);
+#endif
+
+#ifndef ioremap_wc
+#define ioremap_wc ioremap
+#endif
+
+#ifndef ioremap_wt
+#define ioremap_wt ioremap
+#endif
+
+#ifndef ioremap_np
+/* See the comment in asm-generic/io.h about ioremap_np(). */
+#define ioremap_np ioremap_np
+static inline void __iomem *ioremap_np(phys_addr_t offset, size_t size)
+{
+	return NULL;
+}
+#endif
+
+#include <asm-generic/pci_iomap.h>
+
+#endif