platform/posix: Unbreak fuzzer support

Upstream Zephyr moved the LLVM fuzzer entry point out of the arch
layer and made it an app responsibility, so we broke.  Add back the
support here that got removed.

Signed-off-by: Andy Ross <[email protected]>

src/platform/Kconfig

@@ -55,6 +55,28 @@ config ZEPHYR_POSIX
 	  and thus able to instrument and test the whole OS
 	  environment.
 
+if ZEPHYR_POSIX && ARCH_POSIX_LIBFUZZER
+
+config ZEPHYR_POSIX_FUZZ_IRQ
+       int "OS interrupt via which to deliver fuzz cases"
+       default 31
+       help
+         New fuzz cases are delivered to Zephyr via interrupts.  The
+         IRQ should be otherwise unused, but can be any value desired
+         by the app.
+
+config ZEPHYR_POSIX_FUZZ_TICKS
+       int "Ticks to allow for fuzz case processing"
+       default 2
+       help
+         Fuzz interrupts are delivered, from the perspective of the
+         OS, at a steady cadence in simulated time.  In general most
+         apps won't require much time to reach an idle state
+         following a unit-test style case, so the default is short to
+         prevent interaction with regular timer workloads.
+
+endif # ZEPHYR_POSIX && ARCH_POSIX_LIBFUZZER
+
 config IMX8
 	bool "Build for NXP i.MX8"
 	select XT_HAVE_RESET_VECTOR_ROM

src/platform/posix/fuzz.c

@@ -0,0 +1,47 @@
+// SPDX-License-Identifier: BSD-3-Clause
+// Copyright(c) 2024 Google LLC.  All rights reserved.
+// Author: Andy Ross <[email protected]>
+
+#include <stdint.h>
+#include <stddef.h>
+#include <stdbool.h>
+
+#include <irq_ctrl.h>
+#include <zephyr/sys/time_units.h>
+
+/* Zephyr arch APIs, not in a header (native_sim has them though) */
+void posix_init(int argc, char *argv[]);
+void posix_exec_for(uint64_t us);
+
+const uint8_t *posix_fuzz_buf;
+size_t posix_fuzz_sz;
+
+/**
+ * Entry point for fuzzing. Works by placing the data
+ * into two known symbols, triggering an app-visible interrupt, and
+ * then letting the simulator run for a fixed amount of time (intended to be
+ * "long enough" to handle the event and reach a quiescent state
+ * again)
+ */
+int LLVMFuzzerTestOneInput(const uint8_t *data, size_t sz)
+{
+	static bool runner_initialized;
+
+	if (!runner_initialized) {
+		posix_init(0, NULL);
+		runner_initialized = true;
+	}
+
+	/* Provide the fuzz data to the embedded OS as an interrupt, with
+	 * "DMA-like" data placed into native_fuzz_buf/sz
+	 */
+	posix_fuzz_buf = (void *)data;
+	posix_fuzz_sz = sz;
+	hw_irq_ctrl_set_irq(CONFIG_ZEPHYR_POSIX_FUZZ_IRQ);
+
+	/* Give the OS time to process whatever happened in that
+	 * interrupt and reach an idle state.
+	 */
+	posix_exec_for(k_ticks_to_us_ceil64(CONFIG_ZEPHYR_POSIX_FUZZ_TICKS));
+	return 0;
+}

src/platform/posix/ipc.c

@@ -43,7 +43,7 @@ static uint8_t fuzz_in_sz;
 // synchronously as another message after receipt of "complete_cmd()"
 // from the SOF engine, etc...  Eventually we'll receive another fuzz
 // input after some amount of simulated time has passed (c.f.
-// CONFIG_ARCH_POSIX_FUZZ_TICKS)
+// CONFIG_ZEPHYR_POSIX_FUZZ_TICKS)
 static void fuzz_isr(const void *arg)
 {
 	size_t rem, i, n = MIN(posix_fuzz_sz, sizeof(fuzz_in) - fuzz_in_sz);
@@ -179,7 +179,7 @@ void ipc_platform_complete_cmd(struct ipc *ipc)
 
 	if (fuzz_in_sz > 0) {
 		posix_fuzz_sz = 0;
-		posix_sw_set_pending_IRQ(CONFIG_ARCH_POSIX_FUZZ_IRQ);
+		posix_sw_set_pending_IRQ(CONFIG_ZEPHYR_POSIX_FUZZ_IRQ);
 	}
 }
 
@@ -203,8 +203,8 @@ void ipc_platform_send_msg_direct(const struct ipc_msg *msg)
 
 int platform_ipc_init(struct ipc *ipc)
 {
-	IRQ_CONNECT(CONFIG_ARCH_POSIX_FUZZ_IRQ, 0, fuzz_isr, NULL, 0);
-	irq_enable(CONFIG_ARCH_POSIX_FUZZ_IRQ);
+	IRQ_CONNECT(CONFIG_ZEPHYR_POSIX_FUZZ_IRQ, 0, fuzz_isr, NULL, 0);
+	irq_enable(CONFIG_ZEPHYR_POSIX_FUZZ_IRQ);
 
 	global_ipc = ipc;
 	schedule_task_init_edf(&ipc->ipc_task, SOF_UUID(ipc_task_uuid),

zephyr/CMakeLists.txt

@@ -335,6 +335,7 @@ zephyr_library_sources_ifdef(CONFIG_ZEPHYR_POSIX
 	${SOF_PLATFORM_PATH}/posix/dai.c
 	${SOF_PLATFORM_PATH}/posix/ipc.c
 	${SOF_PLATFORM_PATH}/posix/posix.c
+	${SOF_PLATFORM_PATH}/posix/fuzz.c
 	${SOF_PLATFORM_PATH}/posix/base_fw_platform.c
 )