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, 63 insertions, 0 deletions

diff --git a/drivers/video/fbdev/sbuslib.h b/drivers/video/fbdev/sbuslib.h
new file mode 100644
index 000000000..e9af2dc93
--- /dev/null
+++ b/drivers/video/fbdev/sbuslib.h
@@ -0,0 +1,63 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/* sbuslib.h: SBUS fb helper library interfaces */
+#ifndef _SBUSLIB_H
+#define _SBUSLIB_H
+
+struct device_node;
+struct fb_info;
+struct fb_var_screeninfo;
+struct vm_area_struct;
+
+struct sbus_mmap_map {
+	unsigned long voff;
+	unsigned long poff;
+	unsigned long size;
+};
+
+#define SBUS_MMAP_FBSIZE(n) (-n)
+#define SBUS_MMAP_EMPTY	0x80000000
+
+extern void sbusfb_fill_var(struct fb_var_screeninfo *var,
+			    struct device_node *dp, int bpp);
+extern int sbusfb_mmap_helper(const struct sbus_mmap_map *map,
+			      unsigned long physbase, unsigned long fbsize,
+			      unsigned long iospace,
+			      struct vm_area_struct *vma);
+int sbusfb_ioctl_helper(unsigned long cmd, unsigned long arg,
+			struct fb_info *info,
+			int type, int fb_depth, unsigned long fb_size);
+int sbusfb_compat_ioctl(struct fb_info *info, unsigned int cmd,
+			unsigned long arg);
+
+/*
+ * Initialize struct fb_ops for SBUS I/O.
+ */
+
+#define __FB_DEFAULT_SBUS_OPS_RDWR(__prefix) \
+	.fb_read	= fb_io_read, \
+	.fb_write	= fb_io_write
+
+#define __FB_DEFAULT_SBUS_OPS_DRAW(__prefix) \
+	.fb_fillrect	= cfb_fillrect, \
+	.fb_copyarea	= cfb_copyarea, \
+	.fb_imageblit	= cfb_imageblit
+
+#ifdef CONFIG_COMPAT
+#define __FB_DEFAULT_SBUS_OPS_IOCTL(__prefix) \
+	.fb_ioctl		= __prefix ## _sbusfb_ioctl, \
+	.fb_compat_ioctl	= sbusfb_compat_ioctl
+#else
+#define __FB_DEFAULT_SBUS_OPS_IOCTL(__prefix) \
+	.fb_ioctl	= __prefix ## _sbusfb_ioctl
+#endif
+
+#define __FB_DEFAULT_SBUS_OPS_MMAP(__prefix) \
+	.fb_mmap	= __prefix ## _sbusfb_mmap
+
+#define FB_DEFAULT_SBUS_OPS(__prefix) \
+	__FB_DEFAULT_SBUS_OPS_RDWR(__prefix), \
+	__FB_DEFAULT_SBUS_OPS_DRAW(__prefix), \
+	__FB_DEFAULT_SBUS_OPS_IOCTL(__prefix), \
+	__FB_DEFAULT_SBUS_OPS_MMAP(__prefix)
+
+#endif /* _SBUSLIB_H */