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, 136 insertions, 0 deletions

diff --git a/lib/iomem_copy.c b/lib/iomem_copy.c
new file mode 100644
index 000000000..dec7eaea6
--- /dev/null
+++ b/lib/iomem_copy.c
@@ -0,0 +1,136 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * Copyright 2024 Kalray, Inc.  All Rights Reserved.
+ */
+
+#include <linux/align.h>
+#include <linux/export.h>
+#include <linux/io.h>
+#include <linux/types.h>
+#include <linux/unaligned.h>
+
+#ifndef memset_io
+/**
+ * memset_io() - Set a range of I/O memory to a constant value
+ * @addr: The beginning of the I/O-memory range to set
+ * @val: The value to set the memory to
+ * @count: The number of bytes to set
+ *
+ * Set a range of I/O memory to a given value.
+ */
+void memset_io(volatile void __iomem *addr, int val, size_t count)
+{
+	long qc = (u8)val;
+
+	qc *= ~0UL / 0xff;
+
+	while (count && !IS_ALIGNED((long)addr, sizeof(long))) {
+		__raw_writeb(val, addr);
+		addr++;
+		count--;
+	}
+
+	while (count >= sizeof(long)) {
+#ifdef CONFIG_64BIT
+		__raw_writeq(qc, addr);
+#else
+		__raw_writel(qc, addr);
+#endif
+
+		addr += sizeof(long);
+		count -= sizeof(long);
+	}
+
+	while (count) {
+		__raw_writeb(val, addr);
+		addr++;
+		count--;
+	}
+}
+EXPORT_SYMBOL(memset_io);
+#endif
+
+#ifndef memcpy_fromio
+/**
+ * memcpy_fromio() - Copy a block of data from I/O memory
+ * @dst: The (RAM) destination for the copy
+ * @src: The (I/O memory) source for the data
+ * @count: The number of bytes to copy
+ *
+ * Copy a block of data from I/O memory.
+ */
+void memcpy_fromio(void *dst, const volatile void __iomem *src, size_t count)
+{
+	while (count && !IS_ALIGNED((long)src, sizeof(long))) {
+		*(u8 *)dst = __raw_readb(src);
+		src++;
+		dst++;
+		count--;
+	}
+
+	while (count >= sizeof(long)) {
+#ifdef CONFIG_64BIT
+		long val = __raw_readq(src);
+#else
+		long val = __raw_readl(src);
+#endif
+		put_unaligned(val, (long *)dst);
+
+
+		src += sizeof(long);
+		dst += sizeof(long);
+		count -= sizeof(long);
+	}
+
+	while (count) {
+		*(u8 *)dst = __raw_readb(src);
+		src++;
+		dst++;
+		count--;
+	}
+}
+EXPORT_SYMBOL(memcpy_fromio);
+#endif
+
+#ifndef memcpy_toio
+/**
+ * memcpy_toio() -Copy a block of data into I/O memory
+ * @dst: The (I/O memory) destination for the copy
+ * @src: The (RAM) source for the data
+ * @count: The number of bytes to copy
+ *
+ * Copy a block of data to I/O memory.
+ */
+void memcpy_toio(volatile void __iomem *dst, const void *src, size_t count)
+{
+	while (count && !IS_ALIGNED((long)dst, sizeof(long))) {
+		__raw_writeb(*(u8 *)src, dst);
+		src++;
+		dst++;
+		count--;
+	}
+
+	while (count >= sizeof(long)) {
+		long val = get_unaligned((long *)src);
+#ifdef CONFIG_64BIT
+		__raw_writeq(val, dst);
+#else
+		__raw_writel(val, dst);
+#endif
+
+		src += sizeof(long);
+		dst += sizeof(long);
+		count -= sizeof(long);
+	}
+
+	while (count) {
+		__raw_writeb(*(u8 *)src, dst);
+		src++;
+		dst++;
+		count--;
+	}
+}
+EXPORT_SYMBOL(memcpy_toio);
+#endif
+
+