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, 295 insertions, 0 deletions

diff --git a/crypto/jitterentropy-testing.c b/crypto/jitterentropy-testing.c
new file mode 100644
index 000000000..21c9d7c32
--- /dev/null
+++ b/crypto/jitterentropy-testing.c
@@ -0,0 +1,295 @@
+/* SPDX-License-Identifier: GPL-2.0 OR BSD-2-Clause */
+/*
+ * Test interface for Jitter RNG.
+ *
+ * Copyright (C) 2023, Stephan Mueller <smueller@chronox.de>
+ */
+
+#include <linux/debugfs.h>
+#include <linux/module.h>
+#include <linux/uaccess.h>
+
+#include "jitterentropy.h"
+
+#define JENT_TEST_RINGBUFFER_SIZE	(1<<10)
+#define JENT_TEST_RINGBUFFER_MASK	(JENT_TEST_RINGBUFFER_SIZE - 1)
+
+struct jent_testing {
+	u64 jent_testing_rb[JENT_TEST_RINGBUFFER_SIZE];
+	u32 rb_reader;
+	atomic_t rb_writer;
+	atomic_t jent_testing_enabled;
+	spinlock_t lock;
+	wait_queue_head_t read_wait;
+};
+
+static struct dentry *jent_raw_debugfs_root = NULL;
+
+/*************************** Generic Data Handling ****************************/
+
+/*
+ * boot variable:
+ * 0 ==> No boot test, gathering of runtime data allowed
+ * 1 ==> Boot test enabled and ready for collecting data, gathering runtime
+ *	 data is disabled
+ * 2 ==> Boot test completed and disabled, gathering of runtime data is
+ *	 disabled
+ */
+
+static void jent_testing_reset(struct jent_testing *data)
+{
+	unsigned long flags;
+
+	spin_lock_irqsave(&data->lock, flags);
+	data->rb_reader = 0;
+	atomic_set(&data->rb_writer, 0);
+	spin_unlock_irqrestore(&data->lock, flags);
+}
+
+static void jent_testing_data_init(struct jent_testing *data, u32 boot)
+{
+	/*
+	 * The boot time testing implies we have a running test. If the
+	 * caller wants to clear it, he has to unset the boot_test flag
+	 * at runtime via sysfs to enable regular runtime testing
+	 */
+	if (boot)
+		return;
+
+	jent_testing_reset(data);
+	atomic_set(&data->jent_testing_enabled, 1);
+	pr_warn("Enabling data collection\n");
+}
+
+static void jent_testing_fini(struct jent_testing *data, u32 boot)
+{
+	/* If we have boot data, we do not reset yet to allow data to be read */
+	if (boot)
+		return;
+
+	atomic_set(&data->jent_testing_enabled, 0);
+	jent_testing_reset(data);
+	pr_warn("Disabling data collection\n");
+}
+
+static bool jent_testing_store(struct jent_testing *data, u64 value,
+			       u32 *boot)
+{
+	unsigned long flags;
+
+	if (!atomic_read(&data->jent_testing_enabled) && (*boot != 1))
+		return false;
+
+	spin_lock_irqsave(&data->lock, flags);
+
+	/*
+	 * Disable entropy testing for boot time testing after ring buffer
+	 * is filled.
+	 */
+	if (*boot) {
+		if (((u32)atomic_read(&data->rb_writer)) >
+		     JENT_TEST_RINGBUFFER_SIZE) {
+			*boot = 2;
+			pr_warn_once("One time data collection test disabled\n");
+			spin_unlock_irqrestore(&data->lock, flags);
+			return false;
+		}
+
+		if (atomic_read(&data->rb_writer) == 1)
+			pr_warn("One time data collection test enabled\n");
+	}
+
+	data->jent_testing_rb[((u32)atomic_read(&data->rb_writer)) &
+			      JENT_TEST_RINGBUFFER_MASK] = value;
+	atomic_inc(&data->rb_writer);
+
+	spin_unlock_irqrestore(&data->lock, flags);
+
+	if (wq_has_sleeper(&data->read_wait))
+		wake_up_interruptible(&data->read_wait);
+
+	return true;
+}
+
+static bool jent_testing_have_data(struct jent_testing *data)
+{
+	return ((((u32)atomic_read(&data->rb_writer)) &
+		 JENT_TEST_RINGBUFFER_MASK) !=
+		 (data->rb_reader & JENT_TEST_RINGBUFFER_MASK));
+}
+
+static int jent_testing_reader(struct jent_testing *data, u32 *boot,
+			       u8 *outbuf, u32 outbuflen)
+{
+	unsigned long flags;
+	int collected_data = 0;
+
+	jent_testing_data_init(data, *boot);
+
+	while (outbuflen) {
+		u32 writer = (u32)atomic_read(&data->rb_writer);
+
+		spin_lock_irqsave(&data->lock, flags);
+
+		/* We have no data or reached the writer. */
+		if (!writer || (writer == data->rb_reader)) {
+
+			spin_unlock_irqrestore(&data->lock, flags);
+
+			/*
+			 * Now we gathered all boot data, enable regular data
+			 * collection.
+			 */
+			if (*boot) {
+				*boot = 0;
+				goto out;
+			}
+
+			wait_event_interruptible(data->read_wait,
+						 jent_testing_have_data(data));
+			if (signal_pending(current)) {
+				collected_data = -ERESTARTSYS;
+				goto out;
+			}
+
+			continue;
+		}
+
+		/* We copy out word-wise */
+		if (outbuflen < sizeof(u64)) {
+			spin_unlock_irqrestore(&data->lock, flags);
+			goto out;
+		}
+
+		memcpy(outbuf, &data->jent_testing_rb[data->rb_reader],
+		       sizeof(u64));
+		data->rb_reader++;
+
+		spin_unlock_irqrestore(&data->lock, flags);
+
+		outbuf += sizeof(u64);
+		outbuflen -= sizeof(u64);
+		collected_data += sizeof(u64);
+	}
+
+out:
+	jent_testing_fini(data, *boot);
+	return collected_data;
+}
+
+static int jent_testing_extract_user(struct file *file, char __user *buf,
+				     size_t nbytes, loff_t *ppos,
+				     int (*reader)(u8 *outbuf, u32 outbuflen))
+{
+	u8 *tmp, *tmp_aligned;
+	int ret = 0, large_request = (nbytes > 256);
+
+	if (!nbytes)
+		return 0;
+
+	/*
+	 * The intention of this interface is for collecting at least
+	 * 1000 samples due to the SP800-90B requirements. However, due to
+	 * memory and performance constraints, it is not desirable to allocate
+	 * 8000 bytes of memory. Instead, we allocate space for only 125
+	 * samples, which will allow the user to collect all 1000 samples using
+	 * 8 calls to this interface.
+	 */
+	tmp = kmalloc(125 * sizeof(u64) + sizeof(u64), GFP_KERNEL);
+	if (!tmp)
+		return -ENOMEM;
+
+	tmp_aligned = PTR_ALIGN(tmp, sizeof(u64));
+
+	while (nbytes) {
+		int i;
+
+		if (large_request && need_resched()) {
+			if (signal_pending(current)) {
+				if (ret == 0)
+					ret = -ERESTARTSYS;
+				break;
+			}
+			schedule();
+		}
+
+		i = min_t(int, nbytes, 125 * sizeof(u64));
+		i = reader(tmp_aligned, i);
+		if (i <= 0) {
+			if (i < 0)
+				ret = i;
+			break;
+		}
+		if (copy_to_user(buf, tmp_aligned, i)) {
+			ret = -EFAULT;
+			break;
+		}
+
+		nbytes -= i;
+		buf += i;
+		ret += i;
+	}
+
+	kfree_sensitive(tmp);
+
+	if (ret > 0)
+		*ppos += ret;
+
+	return ret;
+}
+
+/************** Raw High-Resolution Timer Entropy Data Handling **************/
+
+static u32 boot_raw_hires_test = 0;
+module_param(boot_raw_hires_test, uint, 0644);
+MODULE_PARM_DESC(boot_raw_hires_test,
+		 "Enable gathering boot time high resolution timer entropy of the first Jitter RNG entropy events");
+
+static struct jent_testing jent_raw_hires = {
+	.rb_reader = 0,
+	.rb_writer = ATOMIC_INIT(0),
+	.lock      = __SPIN_LOCK_UNLOCKED(jent_raw_hires.lock),
+	.read_wait = __WAIT_QUEUE_HEAD_INITIALIZER(jent_raw_hires.read_wait)
+};
+
+int jent_raw_hires_entropy_store(__u64 value)
+{
+	return jent_testing_store(&jent_raw_hires, value, &boot_raw_hires_test);
+}
+EXPORT_SYMBOL(jent_raw_hires_entropy_store);
+
+static int jent_raw_hires_entropy_reader(u8 *outbuf, u32 outbuflen)
+{
+	return jent_testing_reader(&jent_raw_hires, &boot_raw_hires_test,
+				   outbuf, outbuflen);
+}
+
+static ssize_t jent_raw_hires_read(struct file *file, char __user *to,
+				   size_t count, loff_t *ppos)
+{
+	return jent_testing_extract_user(file, to, count, ppos,
+					 jent_raw_hires_entropy_reader);
+}
+
+static const struct file_operations jent_raw_hires_fops = {
+	.owner = THIS_MODULE,
+	.read = jent_raw_hires_read,
+};
+
+/******************************* Initialization *******************************/
+
+void jent_testing_init(void)
+{
+	jent_raw_debugfs_root = debugfs_create_dir(KBUILD_MODNAME, NULL);
+
+	debugfs_create_file_unsafe("jent_raw_hires", 0400,
+				   jent_raw_debugfs_root, NULL,
+				   &jent_raw_hires_fops);
+}
+EXPORT_SYMBOL(jent_testing_init);
+
+void jent_testing_exit(void)
+{
+	debugfs_remove_recursive(jent_raw_debugfs_root);
+}
+EXPORT_SYMBOL(jent_testing_exit);