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>

3 files changed, 271 insertions, 0 deletions

diff --git a/samples/auxdisplay/.gitignore b/samples/auxdisplay/.gitignore
new file mode 100644
index 000000000..d02381684
--- /dev/null
+++ b/samples/auxdisplay/.gitignore
@@ -0,0 +1,2 @@
+# SPDX-License-Identifier: GPL-2.0-only
+/cfag12864b-example
diff --git a/samples/auxdisplay/Makefile b/samples/auxdisplay/Makefile
new file mode 100644
index 000000000..19d556893
--- /dev/null
+++ b/samples/auxdisplay/Makefile
@@ -0,0 +1,2 @@
+# SPDX-License-Identifier: GPL-2.0
+userprogs-always-y += cfag12864b-example
diff --git a/samples/auxdisplay/cfag12864b-example.c b/samples/auxdisplay/cfag12864b-example.c
new file mode 100644
index 000000000..2e3bb7375
--- /dev/null
+++ b/samples/auxdisplay/cfag12864b-example.c
@@ -0,0 +1,267 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ *    Filename: cfag12864b-example.c
+ *     Version: 0.1.0
+ * Description: cfag12864b LCD userspace example program
+ *
+ *      Author: Copyright (C) Miguel Ojeda <ojeda@kernel.org>
+ *        Date: 2006-10-31
+ */
+
+/*
+ * ------------------------
+ * start of cfag12864b code
+ * ------------------------
+ */
+
+#include <string.h>
+#include <fcntl.h>
+#include <unistd.h>
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <sys/mman.h>
+
+#define CFAG12864B_WIDTH		(128)
+#define CFAG12864B_HEIGHT		(64)
+#define CFAG12864B_SIZE			(128 * 64 / 8)
+#define CFAG12864B_BPB			(8)
+#define CFAG12864B_ADDRESS(x, y)	((y) * CFAG12864B_WIDTH / \
+					CFAG12864B_BPB + (x) / CFAG12864B_BPB)
+#define CFAG12864B_BIT(n)		(((unsigned char) 1) << (n))
+
+#undef CFAG12864B_DOCHECK
+#ifdef CFAG12864B_DOCHECK
+	#define CFAG12864B_CHECK(x, y)		((x) < CFAG12864B_WIDTH && \
+						(y) < CFAG12864B_HEIGHT)
+#else
+	#define CFAG12864B_CHECK(x, y)		(1)
+#endif
+
+int cfag12864b_fd;
+unsigned char * cfag12864b_mem;
+unsigned char cfag12864b_buffer[CFAG12864B_SIZE];
+
+/*
+ * init a cfag12864b framebuffer device
+ *
+ * No error:       return = 0
+ * Unable to open: return = -1
+ * Unable to mmap: return = -2
+ */
+static int cfag12864b_init(char *path)
+{
+	cfag12864b_fd = open(path, O_RDWR);
+	if (cfag12864b_fd == -1)
+		return -1;
+
+	cfag12864b_mem = mmap(0, CFAG12864B_SIZE, PROT_READ | PROT_WRITE,
+		MAP_SHARED, cfag12864b_fd, 0);
+	if (cfag12864b_mem == MAP_FAILED) {
+		close(cfag12864b_fd);
+		return -2;
+	}
+
+	return 0;
+}
+
+/*
+ * exit a cfag12864b framebuffer device
+ */
+static void cfag12864b_exit(void)
+{
+	munmap(cfag12864b_mem, CFAG12864B_SIZE);
+	close(cfag12864b_fd);
+}
+
+/*
+ * set (x, y) pixel
+ */
+static void cfag12864b_set(unsigned char x, unsigned char y)
+{
+	if (CFAG12864B_CHECK(x, y))
+		cfag12864b_buffer[CFAG12864B_ADDRESS(x, y)] |=
+			CFAG12864B_BIT(x % CFAG12864B_BPB);
+}
+
+/*
+ * unset (x, y) pixel
+ */
+static void cfag12864b_unset(unsigned char x, unsigned char y)
+{
+	if (CFAG12864B_CHECK(x, y))
+		cfag12864b_buffer[CFAG12864B_ADDRESS(x, y)] &=
+			~CFAG12864B_BIT(x % CFAG12864B_BPB);
+}
+
+/*
+ * is set (x, y) pixel?
+ *
+ * Pixel off: return = 0
+ * Pixel on:  return = 1
+ */
+static unsigned char cfag12864b_isset(unsigned char x, unsigned char y)
+{
+	if (CFAG12864B_CHECK(x, y))
+		if (cfag12864b_buffer[CFAG12864B_ADDRESS(x, y)] &
+			CFAG12864B_BIT(x % CFAG12864B_BPB))
+			return 1;
+
+	return 0;
+}
+
+/*
+ * not (x, y) pixel
+ */
+static void cfag12864b_not(unsigned char x, unsigned char y)
+{
+	if (cfag12864b_isset(x, y))
+		cfag12864b_unset(x, y);
+	else
+		cfag12864b_set(x, y);
+}
+
+/*
+ * fill (set all pixels)
+ */
+static void cfag12864b_fill(void)
+{
+	unsigned short i;
+
+	for (i = 0; i < CFAG12864B_SIZE; i++)
+		cfag12864b_buffer[i] = 0xFF;
+}
+
+/*
+ * clear (unset all pixels)
+ */
+static void cfag12864b_clear(void)
+{
+	unsigned short i;
+
+	for (i = 0; i < CFAG12864B_SIZE; i++)
+		cfag12864b_buffer[i] = 0;
+}
+
+/*
+ * format a [128*64] matrix
+ *
+ * Pixel off: src[i] = 0
+ * Pixel on:  src[i] > 0
+ */
+static void cfag12864b_format(unsigned char * matrix)
+{
+	unsigned char i, j, n;
+
+	for (i = 0; i < CFAG12864B_HEIGHT; i++)
+	for (j = 0; j < CFAG12864B_WIDTH / CFAG12864B_BPB; j++) {
+		cfag12864b_buffer[i * CFAG12864B_WIDTH / CFAG12864B_BPB +
+			j] = 0;
+		for (n = 0; n < CFAG12864B_BPB; n++)
+			if (matrix[i * CFAG12864B_WIDTH +
+				j * CFAG12864B_BPB + n])
+				cfag12864b_buffer[i * CFAG12864B_WIDTH /
+					CFAG12864B_BPB + j] |=
+					CFAG12864B_BIT(n);
+	}
+}
+
+/*
+ * blit buffer to lcd
+ */
+static void cfag12864b_blit(void)
+{
+	memcpy(cfag12864b_mem, cfag12864b_buffer, CFAG12864B_SIZE);
+}
+
+/*
+ * ----------------------
+ * end of cfag12864b code
+ * ----------------------
+ */
+
+#include <stdio.h>
+
+#define EXAMPLES	6
+
+static void example(unsigned char n)
+{
+	unsigned short i, j;
+	unsigned char matrix[CFAG12864B_WIDTH * CFAG12864B_HEIGHT];
+
+	if (n > EXAMPLES)
+		return;
+
+	printf("Example %i/%i - ", n, EXAMPLES);
+
+	switch (n) {
+	case 1:
+		printf("Draw points setting bits");
+		cfag12864b_clear();
+		for (i = 0; i < CFAG12864B_WIDTH; i += 2)
+			for (j = 0; j < CFAG12864B_HEIGHT; j += 2)
+				cfag12864b_set(i, j);
+		break;
+
+	case 2:
+		printf("Clear the LCD");
+		cfag12864b_clear();
+		break;
+
+	case 3:
+		printf("Draw rows formatting a [128*64] matrix");
+		memset(matrix, 0, CFAG12864B_WIDTH * CFAG12864B_HEIGHT);
+		for (i = 0; i < CFAG12864B_WIDTH; i++)
+			for (j = 0; j < CFAG12864B_HEIGHT; j += 2)
+				matrix[j * CFAG12864B_WIDTH + i] = 1;
+		cfag12864b_format(matrix);
+		break;
+
+	case 4:
+		printf("Fill the lcd");
+		cfag12864b_fill();
+		break;
+
+	case 5:
+		printf("Draw columns unsetting bits");
+		for (i = 0; i < CFAG12864B_WIDTH; i += 2)
+			for (j = 0; j < CFAG12864B_HEIGHT; j++)
+				cfag12864b_unset(i, j);
+		break;
+
+	case 6:
+		printf("Do negative not-ing all bits");
+		for (i = 0; i < CFAG12864B_WIDTH; i++)
+			for (j = 0; j < CFAG12864B_HEIGHT; j ++)
+				cfag12864b_not(i, j);
+		break;
+	}
+
+	puts(" - [Press Enter]");
+}
+
+int main(int argc, char *argv[])
+{
+	unsigned char n;
+
+	if (argc != 2) {
+		printf(
+			"Syntax:  %s fbdev\n"
+			"Usually: /dev/fb0, /dev/fb1...\n", argv[0]);
+		return -1;
+	}
+
+	if (cfag12864b_init(argv[1])) {
+		printf("Can't init %s fbdev\n", argv[1]);
+		return -2;
+	}
+
+	for (n = 1; n <= EXAMPLES; n++) {
+		example(n);
+		cfag12864b_blit();
+		while (getchar() != '\n');
+	}
+
+	cfag12864b_exit();
+
+	return 0;
+}