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, 94 insertions, 0 deletions

diff --git a/arch/microblaze/lib/memset.c b/arch/microblaze/lib/memset.c
new file mode 100644
index 000000000..7c2352d56
--- /dev/null
+++ b/arch/microblaze/lib/memset.c
@@ -0,0 +1,94 @@
+/*
+ * Copyright (C) 2008-2009 Michal Simek <monstr@monstr.eu>
+ * Copyright (C) 2008-2009 PetaLogix
+ * Copyright (C) 2007 John Williams
+ *
+ * Reasonably optimised generic C-code for memset on Microblaze
+ * This is generic C code to do efficient, alignment-aware memcpy.
+ *
+ * It is based on demo code originally Copyright 2001 by Intel Corp, taken from
+ * http://www.embedded.com/showArticle.jhtml?articleID=19205567
+ *
+ * Attempts were made, unsuccessfully, to contact the original
+ * author of this code (Michael Morrow, Intel).  Below is the original
+ * copyright notice.
+ *
+ * This software has been developed by Intel Corporation.
+ * Intel specifically disclaims all warranties, express or
+ * implied, and all liability, including consequential and
+ * other indirect damages, for the use of this program, including
+ * liability for infringement of any proprietary rights,
+ * and including the warranties of merchantability and fitness
+ * for a particular purpose. Intel does not assume any
+ * responsibility for and errors which may appear in this program
+ * not any responsibility to update it.
+ */
+
+#include <linux/export.h>
+#include <linux/types.h>
+#include <linux/stddef.h>
+#include <linux/compiler.h>
+#include <linux/string.h>
+
+#ifdef CONFIG_OPT_LIB_FUNCTION
+void *memset(void *v_src, int c, __kernel_size_t n)
+{
+	char *src = v_src;
+	uint32_t *i_src;
+	uint32_t w32 = 0;
+
+	/* Truncate c to 8 bits */
+	c = (c & 0xFF);
+
+	if (unlikely(c)) {
+		/* Make a repeating word out of it */
+		w32 = c;
+		w32 |= w32 << 8;
+		w32 |= w32 << 16;
+	}
+
+	if (likely(n >= 4)) {
+		/* Align the destination to a word boundary */
+		/* This is done in an endian independent manner */
+		switch ((unsigned) src & 3) {
+		case 1:
+			*src++ = c;
+			--n;
+			fallthrough;
+		case 2:
+			*src++ = c;
+			--n;
+			fallthrough;
+		case 3:
+			*src++ = c;
+			--n;
+		}
+
+		i_src  = (void *)src;
+
+		/* Do as many full-word copies as we can */
+		for (; n >= 4; n -= 4)
+			*i_src++ = w32;
+
+		src  = (void *)i_src;
+	}
+
+	/* Simple, byte oriented memset or the rest of count. */
+	switch (n) {
+	case 3:
+		*src++ = c;
+		fallthrough;
+	case 2:
+		*src++ = c;
+		fallthrough;
+	case 1:
+		*src++ = c;
+		break;
+	default:
+		break;
+	}
+
+	return v_src;
+}
+EXPORT_SYMBOL(memset);
+#endif /* CONFIG_OPT_LIB_FUNCTION */