// https://syzkaller.appspot.com/bug?id=86ced160b3c394018a4d5dd1db69824b7b1b6734
#define _GNU_SOURCE
#include <pthread.h>
#include <fcntl.h>
#include <unistd.h>
#include <stdio.h>
#include <string.h>
#include <sys/syscall.h>
#include <linux/futex.h>
#include <stdlib.h>
#include <limits.h>
#include <errno.h>
// Copyright 2026 syzkaller project authors. All rights reserved.
// Use of this source code is governed by Apache 2 LICENSE that can be found in the LICENSE file.

// IMPORTANT: Do not copy the macros or definitions below directly into your reproducer.
// Instead, add the following line to your reproducer:
// #include "race_toolkit.h"

// --- Race Condition Toolkit ---
// Macros and snippets for CPU pinning, memory barriers, and userfaultfd.

#define _GNU_SOURCE
#include <errno.h>
#include <fcntl.h>
#include <linux/futex.h>
#include <linux/userfaultfd.h>
#include <poll.h>
#include <pthread.h>
#include <sched.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/ioctl.h>
#include <sys/syscall.h>
#include <time.h>
#include <unistd.h>

// Unbuffered I/O: Ensure logs are written immediately.
#define SETUP_UNBUFFERED_IO() setvbuf(stdout, NULL, _IONBF, 0)

// CPU Pinning: Pin the current thread to a specific CPU core.
#define PIN_TO_CPU(cpu)                                                \
	do {                                                           \
		cpu_set_t mask;                                        \
		CPU_ZERO(&mask);                                       \
		CPU_SET(cpu, &mask);                                   \
		if (sched_setaffinity(0, sizeof(mask), &mask) == -1) { \
			perror("sched_setaffinity");                   \
		}                                                      \
	} while (0)

// Memory Barrier: Ensure memory ordering.
#define MB() __atomic_thread_fence(__ATOMIC_SEQ_CST)

// Spin-wait Barrier: Wait until a memory location has a specific value.
// Best for tight race windows (low latency, no context switches).
#define WAIT_ON(addr, val)                                               \
	do {                                                             \
		while (__atomic_load_n(addr, __ATOMIC_ACQUIRE) != (val)) \
			;                                                \
	} while (0)

// Signal: Set a memory location to a specific value to release a WAIT_ON.
#define SIGNAL(addr, val) __atomic_store_n(addr, val, __ATOMIC_RELEASE)

// --- Timing Primitives ---
// Robust timing loops in VM environments (using CLOCK_MONOTONIC to avoid time(NULL) jumps).

static inline double timer_elapsed_sec(struct timespec* start)
{
	struct timespec now;
	if (clock_gettime(CLOCK_MONOTONIC, &now) == -1) {
		perror("clock_gettime(CLOCK_MONOTONIC) elapsed");
		exit(1);
	}
	return (double)(now.tv_sec - start->tv_sec) + (double)(now.tv_nsec - start->tv_nsec) / 1e9;
}

// Initialize a monotonic timer variable.
#define TIMER_START(t)                                          \
	struct timespec t;                                      \
	if (clock_gettime(CLOCK_MONOTONIC, &t) == -1) {         \
		perror("clock_gettime(CLOCK_MONOTONIC) start"); \
		exit(1);                                        \
	}

// Check if the elapsed time since 't' is less than 'sec' seconds.
#define TIMER_NOT_EXPIRED(t, sec) (timer_elapsed_sec(&(t)) < (double)(sec))

// Futex-based Event: Shared with syzkaller executor.
// Best for general synchronization or longer waits to save CPU.
typedef struct {
	int state;
} event_t;

static void event_init(event_t* ev)
{
	ev->state = 0;
}
static void event_reset(event_t* ev)
{
	ev->state = 0;
}

static void event_set(event_t* ev)
{
	if (__atomic_load_n(&ev->state, __ATOMIC_ACQUIRE)) {
		fprintf(stderr, "event already set\n");
		exit(1);
	}
	__atomic_store_n(&ev->state, 1, __ATOMIC_RELEASE);
	syscall(SYS_futex, &ev->state, FUTEX_WAKE | FUTEX_PRIVATE_FLAG, 1000000);
}

static void event_wait(event_t* ev)
{
	while (!__atomic_load_n(&ev->state, __ATOMIC_ACQUIRE))
		syscall(SYS_futex, &ev->state, FUTEX_WAIT | FUTEX_PRIVATE_FLAG, 0, 0);
}

// userfaultfd setup: Register a memory range for page fault handling.
static int setup_uffd(void* addr, size_t len)
{
	int uffd = syscall(__NR_userfaultfd, O_CLOEXEC | O_NONBLOCK);
	if (uffd == -1)
		return -1;
	struct uffdio_api api = {.api = UFFD_API, .features = 0};
	if (ioctl(uffd, UFFDIO_API, &api) == -1) {
		close(uffd);
		return -1;
	}
	struct uffdio_register reg = {
	    .range = {.start = (uintptr_t)addr, .len = len},
	    .mode = UFFDIO_REGISTER_MODE_MISSING};
	if (ioctl(uffd, UFFDIO_REGISTER, &reg) == -1) {
		close(uffd);
		return -1;
	}
	return uffd;
}

// --- Guidance on Usage ---
// 1. Use WAIT_ON/SIGNAL for tight race conditions to avoid scheduling overhead.
// 2. Use event_t (futexes) for general coordination or when waiting for longer periods.
// 3. Always use PIN_TO_CPU to increase race probability on multi-core systems.
// 4. Use setup_uffd to register a memory range for page fault handling. This allows you to
//    pause a thread accessing that memory until you handle the fault, creating a reliable
//    and controllable race window.
// 5. Call SETUP_UNBUFFERED_IO() at the start of main() to ensure that logs are printed
//    immediately. This is essential for understanding the exact interleaving of events
//    when debugging race conditions.
// 6. For timing-based loops (e.g., running a race for 10 seconds), do NOT use time(NULL)
//    or loops relying on real-time clocks, as VM clocks are highly unreliable and can fail or drift.
//    Instead, use the robust monotonic timing primitives TIMER_START and TIMER_NOT_EXPIRED:
//        TIMER_START(start);
//        while (TIMER_NOT_EXPIRED(start, 10.0)) {
//            // Your race logic here
//        }


int futex_word = 0;

char trace_base[256];

/* Thread 1: Continuously toggles record-cmd */
void *toggler(void *arg) {
    PIN_TO_CPU(0);
    char path[512];
    snprintf(path, sizeof(path), "%s/options/record-cmd", trace_base);
    
    int fd = open(path, O_WRONLY);
    if (fd < 0) {
        printf("[-] Failed to open %s: %s\n", path, strerror(errno));
        return NULL;
    }

    while (1) {
        if (write(fd, "0\n", 2) < 0) {}
        if (write(fd, "1\n", 2) < 0) {}
    }
    return NULL;
}

/* Threads 2+: Continuously wake up waiting threads */
void *waker(void *arg) {
    PIN_TO_CPU(1);
    while (1) {
        syscall(SYS_futex, &futex_word, FUTEX_WAKE, 1, NULL, NULL, 0);
    }
    return NULL;
}

/* Threads 3+: Continuously go to sleep on a futex */
void *waiter(void *arg) {
    PIN_TO_CPU(1);
    while (1) {
        syscall(SYS_futex, &futex_word, FUTEX_WAIT, futex_word, NULL, NULL, 0);
    }
    return NULL;
}

int write_file(const char *path, const char *val) {
    int fd = open(path, O_WRONLY | O_TRUNC);
    if (fd < 0) return -1;
    int ret = write(fd, val, strlen(val));
    close(fd);
    return ret;
}

int main() {
    SETUP_UNBUFFERED_IO();

    const char *base_dirs[] = {
        "/sys/kernel/tracing",
        "/sys/kernel/debug/tracing",
        NULL
    };

    int found = 0;
    for (int i = 0; base_dirs[i]; i++) {
        if (access(base_dirs[i], F_OK) == 0) {
            strcpy(trace_base, base_dirs[i]);
            found = 1;
            break;
        }
    }

    if (!found) {
        printf("[-] Tracefs not found.\n");
        exit(1);
    }
    printf("[+] Tracefs found at %s\n", trace_base);

    char path[512];
    
    /* Clear all events */
    snprintf(path, sizeof(path), "%s/set_event", trace_base);
    if (write_file(path, "") < 0) {
        printf("[-] Failed to clear events: %s\n", strerror(errno));
        exit(1);
    }
    printf("[+] Cleared events.\n");

    /* Ensure record-cmd is enabled */
    snprintf(path, sizeof(path), "%s/options/record-cmd", trace_base);
    if (write_file(path, "1\n") < 0) {
        printf("[-] Failed to enable record-cmd: %s\n", strerror(errno));
    } else {
        printf("[+] Enabled record-cmd.\n");
    }

    /* Enable sched_wakeup */
    snprintf(path, sizeof(path), "%s/events/sched/sched_wakeup/enable", trace_base);
    if (write_file(path, "1\n") < 0) {
        printf("[-] Failed to enable sched_wakeup: %s\n", strerror(errno));
        exit(1);
    }
    printf("[+] Enabled sched_wakeup.\n");

    /* Enable record-tgid */
    /* This is critical: it keeps sched_tgid_ref > 0, which prevents 
       tracing_sched_unregister() from being called when record-cmd is toggled.
       This completely bypasses the RCU sync delay, allowing the race window
       to be hit thousands of times per second. */
    snprintf(path, sizeof(path), "%s/options/record-tgid", trace_base);
    if (write_file(path, "1\n") < 0) {
        printf("[-] Failed to enable record-tgid: %s\n", strerror(errno));
    } else {
        printf("[+] Enabled record-tgid.\n");
    }

    pthread_t t_toggler;
    if (pthread_create(&t_toggler, NULL, toggler, NULL) != 0) {
        printf("[-] Failed to create toggler thread.\n");
        exit(1);
    }

    #define NUM_THREADS 4
    pthread_t t_wakers[NUM_THREADS];
    pthread_t t_waiters[NUM_THREADS];

    for (int i = 0; i < NUM_THREADS; i++) {
        pthread_create(&t_wakers[i], NULL, waker, NULL);
        pthread_create(&t_waiters[i], NULL, waiter, NULL);
    }
    printf("[+] Started waker and waiter threads.\n");

    /* Infinite loop, let syzbot timeout and kill us.
       This avoids triggering unrelated data races during process exit. */
    while (1) {
        sleep(1);
    }

    return 0;
}