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, 75 insertions, 0 deletions

diff --git a/drivers/clocksource/mmio.c b/drivers/clocksource/mmio.c
new file mode 100644
index 000000000..0fee8edb8
--- /dev/null
+++ b/drivers/clocksource/mmio.c
@@ -0,0 +1,75 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * Generic MMIO clocksource support
+ */
+#include <linux/clocksource.h>
+#include <linux/errno.h>
+#include <linux/init.h>
+#include <linux/slab.h>
+
+struct clocksource_mmio {
+	void __iomem *reg;
+	struct clocksource clksrc;
+};
+
+static inline struct clocksource_mmio *to_mmio_clksrc(struct clocksource *c)
+{
+	return container_of(c, struct clocksource_mmio, clksrc);
+}
+
+u64 clocksource_mmio_readl_up(struct clocksource *c)
+{
+	return (u64)readl_relaxed(to_mmio_clksrc(c)->reg);
+}
+EXPORT_SYMBOL_GPL(clocksource_mmio_readl_up);
+
+u64 clocksource_mmio_readl_down(struct clocksource *c)
+{
+	return ~(u64)readl_relaxed(to_mmio_clksrc(c)->reg) & c->mask;
+}
+EXPORT_SYMBOL_GPL(clocksource_mmio_readl_down);
+
+u64 clocksource_mmio_readw_up(struct clocksource *c)
+{
+	return (u64)readw_relaxed(to_mmio_clksrc(c)->reg);
+}
+EXPORT_SYMBOL_GPL(clocksource_mmio_readw_up);
+
+u64 clocksource_mmio_readw_down(struct clocksource *c)
+{
+	return ~(u64)readw_relaxed(to_mmio_clksrc(c)->reg) & c->mask;
+}
+EXPORT_SYMBOL_GPL(clocksource_mmio_readw_down);
+
+/**
+ * clocksource_mmio_init - Initialize a simple mmio based clocksource
+ * @base:	Virtual address of the clock readout register
+ * @name:	Name of the clocksource
+ * @hz:		Frequency of the clocksource in Hz
+ * @rating:	Rating of the clocksource
+ * @bits:	Number of valid bits
+ * @read:	One of clocksource_mmio_read*() above
+ */
+int clocksource_mmio_init(void __iomem *base, const char *name,
+			  unsigned long hz, int rating, unsigned bits,
+			  u64 (*read)(struct clocksource *))
+{
+	struct clocksource_mmio *cs;
+
+	if (bits > 64 || bits < 16)
+		return -EINVAL;
+
+	cs = kzalloc_obj(struct clocksource_mmio);
+	if (!cs)
+		return -ENOMEM;
+
+	cs->reg = base;
+	cs->clksrc.name = name;
+	cs->clksrc.rating = rating;
+	cs->clksrc.read = read;
+	cs->clksrc.mask = CLOCKSOURCE_MASK(bits);
+	cs->clksrc.flags = CLOCK_SOURCE_IS_CONTINUOUS;
+
+	return clocksource_register_hz(&cs->clksrc, hz);
+}
+EXPORT_SYMBOL_GPL(clocksource_mmio_init);