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, 140 insertions, 0 deletions

diff --git a/drivers/firmware/sysfb_simplefb.c b/drivers/firmware/sysfb_simplefb.c
new file mode 100644
index 000000000..71f542e37
--- /dev/null
+++ b/drivers/firmware/sysfb_simplefb.c
@@ -0,0 +1,140 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * Generic System Framebuffers
+ * Copyright (c) 2012-2013 David Herrmann <dh.herrmann@gmail.com>
+ */
+
+/*
+ * simple-framebuffer probing
+ * Try to convert "screen_info" into a "simple-framebuffer" compatible mode.
+ * If the mode is incompatible, we return "false" and let the caller create
+ * legacy nodes instead.
+ */
+
+#include <linux/err.h>
+#include <linux/init.h>
+#include <linux/kernel.h>
+#include <linux/mm.h>
+#include <linux/platform_data/simplefb.h>
+#include <linux/platform_device.h>
+#include <linux/screen_info.h>
+#include <linux/sysfb.h>
+
+static const char simplefb_resname[] = "BOOTFB";
+static const struct simplefb_format formats[] = SIMPLEFB_FORMATS;
+
+/* try parsing screen_info into a simple-framebuffer mode struct */
+__init bool sysfb_parse_mode(const struct screen_info *si,
+			     struct simplefb_platform_data *mode)
+{
+	__u8 type;
+	u32 bits_per_pixel;
+	unsigned int i;
+
+	type = si->orig_video_isVGA;
+	if (type != VIDEO_TYPE_VLFB && type != VIDEO_TYPE_EFI)
+		return false;
+
+	bits_per_pixel = __screen_info_lfb_bits_per_pixel(si);
+
+	for (i = 0; i < ARRAY_SIZE(formats); ++i) {
+		const struct simplefb_format *f = &formats[i];
+
+		if (f->transp.length)
+			continue; /* transparent formats are unsupported by VESA/EFI */
+
+		if (bits_per_pixel == f->bits_per_pixel &&
+		    si->red_size == f->red.length &&
+		    si->red_pos == f->red.offset &&
+		    si->green_size == f->green.length &&
+		    si->green_pos == f->green.offset &&
+		    si->blue_size == f->blue.length &&
+		    si->blue_pos == f->blue.offset) {
+			mode->format = f->name;
+			mode->width = si->lfb_width;
+			mode->height = si->lfb_height;
+			mode->stride = si->lfb_linelength;
+			return true;
+		}
+	}
+
+	return false;
+}
+
+__init struct platform_device *sysfb_create_simplefb(const struct screen_info *si,
+						     const struct simplefb_platform_data *mode,
+						     struct device *parent)
+{
+	struct platform_device *pd;
+	struct resource res;
+	u64 base, size;
+	u32 length;
+	int ret;
+
+	/*
+	 * If the 64BIT_BASE capability is set, ext_lfb_base will contain the
+	 * upper half of the base address. Assemble the address, then make sure
+	 * it is valid and we can actually access it.
+	 */
+	base = si->lfb_base;
+	if (si->capabilities & VIDEO_CAPABILITY_64BIT_BASE)
+		base |= (u64)si->ext_lfb_base << 32;
+	if (!base || (u64)(resource_size_t)base != base) {
+		printk(KERN_DEBUG "sysfb: inaccessible VRAM base\n");
+		return ERR_PTR(-EINVAL);
+	}
+
+	/*
+	 * Don't use lfb_size as IORESOURCE size, since it may contain the
+	 * entire VMEM, and thus require huge mappings. Use just the part we
+	 * need, that is, the part where the framebuffer is located. But verify
+	 * that it does not exceed the advertised VMEM.
+	 * Note that in case of VBE, the lfb_size is shifted by 16 bits for
+	 * historical reasons.
+	 */
+	size = si->lfb_size;
+	if (si->orig_video_isVGA == VIDEO_TYPE_VLFB)
+		size <<= 16;
+	length = mode->height * mode->stride;
+	if (length > size) {
+		printk(KERN_WARNING "sysfb: VRAM smaller than advertised\n");
+		return ERR_PTR(-EINVAL);
+	}
+	length = PAGE_ALIGN(length);
+
+	/* setup IORESOURCE_MEM as framebuffer memory */
+	memset(&res, 0, sizeof(res));
+	res.flags = IORESOURCE_MEM;
+	res.name = simplefb_resname;
+	res.start = base;
+	res.end = res.start + length - 1;
+	if (res.end <= res.start)
+		return ERR_PTR(-EINVAL);
+
+	pd = platform_device_alloc("simple-framebuffer", 0);
+	if (!pd)
+		return ERR_PTR(-ENOMEM);
+
+	pd->dev.parent = parent;
+
+	sysfb_set_efifb_fwnode(si, pd);
+
+	ret = platform_device_add_resources(pd, &res, 1);
+	if (ret)
+		goto err_put_device;
+
+	ret = platform_device_add_data(pd, mode, sizeof(*mode));
+	if (ret)
+		goto err_put_device;
+
+	ret = platform_device_add(pd);
+	if (ret)
+		goto err_put_device;
+
+	return pd;
+
+err_put_device:
+	platform_device_put(pd);
+
+	return ERR_PTR(ret);
+}