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, 224 insertions, 0 deletions

diff --git a/lib/tests/kfifo_kunit.c b/lib/tests/kfifo_kunit.c
new file mode 100644
index 000000000..a85eedc31
--- /dev/null
+++ b/lib/tests/kfifo_kunit.c
@@ -0,0 +1,224 @@
+// SPDX-License-Identifier: GPL-2.0
+/*
+ * KUnit test for the generic kernel FIFO implementation.
+ *
+ * Copyright (C) 2024 Diego Vieira <diego.daniel.professional@gmail.com>
+ */
+#include <kunit/test.h>
+
+#include <linux/kfifo.h>
+
+#define KFIFO_SIZE 32
+#define N_ELEMENTS 5
+
+static void kfifo_test_reset_should_clear_the_fifo(struct kunit *test)
+{
+	DEFINE_KFIFO(my_fifo, u8, KFIFO_SIZE);
+
+	kfifo_put(&my_fifo, 1);
+	kfifo_put(&my_fifo, 2);
+	kfifo_put(&my_fifo, 3);
+	KUNIT_EXPECT_EQ(test, kfifo_len(&my_fifo), 3);
+
+	kfifo_reset(&my_fifo);
+
+	KUNIT_EXPECT_EQ(test, kfifo_len(&my_fifo), 0);
+	KUNIT_EXPECT_TRUE(test, kfifo_is_empty(&my_fifo));
+}
+
+static void kfifo_test_define_should_define_an_empty_fifo(struct kunit *test)
+{
+	DEFINE_KFIFO(my_fifo, u8, KFIFO_SIZE);
+
+	KUNIT_EXPECT_TRUE(test, kfifo_initialized(&my_fifo));
+	KUNIT_EXPECT_TRUE(test, kfifo_is_empty(&my_fifo));
+	KUNIT_EXPECT_EQ(test, kfifo_len(&my_fifo), 0);
+}
+
+static void kfifo_test_len_should_ret_n_of_stored_elements(struct kunit *test)
+{
+	u8 buffer1[N_ELEMENTS];
+
+	for (int i = 0; i < N_ELEMENTS; i++)
+		buffer1[i] = i + 1;
+
+	DEFINE_KFIFO(my_fifo, u8, KFIFO_SIZE);
+
+	KUNIT_EXPECT_EQ(test, kfifo_len(&my_fifo), 0);
+
+	kfifo_in(&my_fifo, buffer1, N_ELEMENTS);
+	KUNIT_EXPECT_EQ(test, kfifo_len(&my_fifo), N_ELEMENTS);
+
+	kfifo_in(&my_fifo, buffer1, N_ELEMENTS);
+	KUNIT_EXPECT_EQ(test, kfifo_len(&my_fifo), N_ELEMENTS * 2);
+
+	kfifo_reset(&my_fifo);
+	KUNIT_EXPECT_EQ(test, kfifo_len(&my_fifo), 0);
+}
+
+static void kfifo_test_put_should_insert_and_get_should_pop(struct kunit *test)
+{
+	u8 out_data = 0;
+	int processed_elements;
+	u8 elements[] = { 3, 5, 11 };
+
+	DEFINE_KFIFO(my_fifo, u8, KFIFO_SIZE);
+
+	// If the fifo is empty, get returns 0
+	processed_elements = kfifo_get(&my_fifo, &out_data);
+	KUNIT_EXPECT_EQ(test, processed_elements, 0);
+	KUNIT_EXPECT_EQ(test, out_data, 0);
+
+	for (int i = 0; i < 3; i++)
+		kfifo_put(&my_fifo, elements[i]);
+
+	for (int i = 0; i < 3; i++) {
+		processed_elements = kfifo_get(&my_fifo, &out_data);
+		KUNIT_EXPECT_EQ(test, processed_elements, 1);
+		KUNIT_EXPECT_EQ(test, out_data, elements[i]);
+	}
+}
+
+static void kfifo_test_in_should_insert_multiple_elements(struct kunit *test)
+{
+	u8 in_buffer[] = { 11, 25, 65 };
+	u8 out_data;
+	int processed_elements;
+
+	DEFINE_KFIFO(my_fifo, u8, KFIFO_SIZE);
+
+	kfifo_in(&my_fifo, in_buffer, 3);
+
+	for (int i = 0; i < 3; i++) {
+		processed_elements = kfifo_get(&my_fifo, &out_data);
+		KUNIT_EXPECT_EQ(test, processed_elements, 1);
+		KUNIT_EXPECT_EQ(test, out_data, in_buffer[i]);
+	}
+}
+
+static void kfifo_test_out_should_pop_multiple_elements(struct kunit *test)
+{
+	u8 in_buffer[] = { 11, 25, 65 };
+	u8 out_buffer[3];
+	int copied_elements;
+
+	DEFINE_KFIFO(my_fifo, u8, KFIFO_SIZE);
+
+	for (int i = 0; i < 3; i++)
+		kfifo_put(&my_fifo, in_buffer[i]);
+
+	copied_elements = kfifo_out(&my_fifo, out_buffer, 3);
+	KUNIT_EXPECT_EQ(test, copied_elements, 3);
+
+	for (int i = 0; i < 3; i++)
+		KUNIT_EXPECT_EQ(test, out_buffer[i], in_buffer[i]);
+	KUNIT_EXPECT_TRUE(test, kfifo_is_empty(&my_fifo));
+}
+
+static void kfifo_test_dec_init_should_define_an_empty_fifo(struct kunit *test)
+{
+	DECLARE_KFIFO(my_fifo, u8, KFIFO_SIZE);
+
+	INIT_KFIFO(my_fifo);
+
+	// my_fifo is a struct with an inplace buffer
+	KUNIT_EXPECT_FALSE(test, __is_kfifo_ptr(&my_fifo));
+
+	KUNIT_EXPECT_TRUE(test, kfifo_initialized(&my_fifo));
+}
+
+static void kfifo_test_define_should_equal_declare_init(struct kunit *test)
+{
+	// declare a variable my_fifo of type struct kfifo of u8
+	DECLARE_KFIFO(my_fifo1, u8, KFIFO_SIZE);
+	// initialize the my_fifo variable
+	INIT_KFIFO(my_fifo1);
+
+	// DEFINE_KFIFO declares the variable with the initial value
+	// essentially the same as calling DECLARE_KFIFO and INIT_KFIFO
+	DEFINE_KFIFO(my_fifo2, u8, KFIFO_SIZE);
+
+	// my_fifo1 and my_fifo2 have the same size
+	KUNIT_EXPECT_EQ(test, sizeof(my_fifo1), sizeof(my_fifo2));
+	KUNIT_EXPECT_EQ(test, kfifo_initialized(&my_fifo1),
+			kfifo_initialized(&my_fifo2));
+	KUNIT_EXPECT_EQ(test, kfifo_is_empty(&my_fifo1),
+			kfifo_is_empty(&my_fifo2));
+}
+
+static void kfifo_test_alloc_should_initiliaze_a_ptr_fifo(struct kunit *test)
+{
+	int ret;
+	DECLARE_KFIFO_PTR(my_fifo, u8);
+
+	INIT_KFIFO(my_fifo);
+
+	// kfifo_initialized returns false signaling the buffer pointer is NULL
+	KUNIT_EXPECT_FALSE(test, kfifo_initialized(&my_fifo));
+
+	// kfifo_alloc allocates the buffer
+	ret = kfifo_alloc(&my_fifo, KFIFO_SIZE, GFP_KERNEL);
+	KUNIT_EXPECT_EQ_MSG(test, ret, 0, "Memory allocation should succeed");
+	KUNIT_EXPECT_TRUE(test, kfifo_initialized(&my_fifo));
+
+	// kfifo_free frees the buffer
+	kfifo_free(&my_fifo);
+}
+
+static void kfifo_test_peek_should_not_remove_elements(struct kunit *test)
+{
+	u8 out_data;
+	int processed_elements;
+
+	DEFINE_KFIFO(my_fifo, u8, KFIFO_SIZE);
+
+	// If the fifo is empty, peek returns 0
+	processed_elements = kfifo_peek(&my_fifo, &out_data);
+	KUNIT_EXPECT_EQ(test, processed_elements, 0);
+
+	kfifo_put(&my_fifo, 3);
+	kfifo_put(&my_fifo, 5);
+	kfifo_put(&my_fifo, 11);
+
+	KUNIT_EXPECT_EQ(test, kfifo_len(&my_fifo), 3);
+
+	processed_elements = kfifo_peek(&my_fifo, &out_data);
+	KUNIT_EXPECT_EQ(test, processed_elements, 1);
+	KUNIT_EXPECT_EQ(test, out_data, 3);
+
+	KUNIT_EXPECT_EQ(test, kfifo_len(&my_fifo), 3);
+
+	// Using peek doesn't remove the element
+	// so the read element and the fifo length
+	// remains the same
+	processed_elements = kfifo_peek(&my_fifo, &out_data);
+	KUNIT_EXPECT_EQ(test, processed_elements, 1);
+	KUNIT_EXPECT_EQ(test, out_data, 3);
+
+	KUNIT_EXPECT_EQ(test, kfifo_len(&my_fifo), 3);
+}
+
+static struct kunit_case kfifo_test_cases[] = {
+	KUNIT_CASE(kfifo_test_reset_should_clear_the_fifo),
+	KUNIT_CASE(kfifo_test_define_should_define_an_empty_fifo),
+	KUNIT_CASE(kfifo_test_len_should_ret_n_of_stored_elements),
+	KUNIT_CASE(kfifo_test_put_should_insert_and_get_should_pop),
+	KUNIT_CASE(kfifo_test_in_should_insert_multiple_elements),
+	KUNIT_CASE(kfifo_test_out_should_pop_multiple_elements),
+	KUNIT_CASE(kfifo_test_dec_init_should_define_an_empty_fifo),
+	KUNIT_CASE(kfifo_test_define_should_equal_declare_init),
+	KUNIT_CASE(kfifo_test_alloc_should_initiliaze_a_ptr_fifo),
+	KUNIT_CASE(kfifo_test_peek_should_not_remove_elements),
+	{},
+};
+
+static struct kunit_suite kfifo_test_module = {
+	.name = "kfifo",
+	.test_cases = kfifo_test_cases,
+};
+
+kunit_test_suites(&kfifo_test_module);
+
+MODULE_LICENSE("GPL");
+MODULE_AUTHOR("Diego Vieira <diego.daniel.professional@gmail.com>");
+MODULE_DESCRIPTION("KUnit test for the kernel FIFO");