// https://syzkaller.appspot.com/bug?id=1d99662faa9759f2b707e39fe1332876c9aab411
#define _GNU_SOURCE
#include <errno.h>
#include <fcntl.h>
#include <pthread.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/mount.h>
#include <sys/sendfile.h>
#include <sys/stat.h>
#include <unistd.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
//        }


volatile int start_flag = 0;
volatile int stop_flag = 0;

int in_fd;
int out_fd;

void *thread_func(void *arg)
{
    long id = (long)arg;
    PIN_TO_CPU(id % 2);

    off_t base_offset = (id == 0) ? 0 : 524288;

    WAIT_ON(&start_flag, 1);

    while (!__atomic_load_n(&stop_flag, __ATOMIC_RELAXED)) {
        off_t offset = base_offset;
        ssize_t res = sendfile(out_fd, in_fd, &offset, 65536);
        if (res < 0) {
            if (errno == EAGAIN || errno == EINTR)
                continue;
            printf("[-] Failed to sendfile in thread %ld: %s\n", id, strerror(errno));
            exit(1);
        }
    }
    return NULL;
}

int main()
{
    SETUP_UNBUFFERED_IO();
    printf("[*] Starting reproducer...\n");

    int res = mkdir("/tmp/ramfs", 0777);
    if (res < 0 && errno != EEXIST) {
        printf("[-] Failed to mkdir /tmp/ramfs: %s\n", strerror(errno));
        exit(1);
    }
    printf("[+] mkdir /tmp/ramfs successful.\n");

    res = mount("ramfs", "/tmp/ramfs", "ramfs", 0, "");
    if (res < 0) {
        printf("[-] Failed to mount ramfs: %s\n", strerror(errno));
        exit(1);
    }
    printf("[+] mount ramfs successful.\n");

    in_fd = open("/tmp/ramfs/test_file", O_RDWR | O_CREAT | O_TRUNC, 0666);
    if (in_fd < 0) {
        printf("[-] Failed to open /tmp/ramfs/test_file: %s\n", strerror(errno));
        exit(1);
    }
    printf("[+] open /tmp/ramfs/test_file successful.\n");

    char *buf = malloc(65536);
    if (!buf) {
        printf("[-] Failed to malloc: %s\n", strerror(errno));
        exit(1);
    }
    memset(buf, 'A', 65536);
    for (int i = 0; i < 16; i++) {
        ssize_t wres = write(in_fd, buf, 65536);
        if (wres < 0) {
            printf("[-] Failed to write: %s\n", strerror(errno));
            exit(1);
        }
    }
    free(buf);
    printf("[+] write 1MB successful.\n");

    out_fd = open("/dev/null", O_WRONLY);
    if (out_fd < 0) {
        printf("[-] Failed to open /dev/null: %s\n", strerror(errno));
        exit(1);
    }
    printf("[+] open /dev/null successful.\n");

    pthread_t threads[2];
    for (long i = 0; i < 2; i++) {
        res = pthread_create(&threads[i], NULL, thread_func, (void *)i);
        if (res != 0) {
            printf("[-] Failed to pthread_create: %s\n", strerror(res));
            exit(1);
        }
    }
    printf("[+] pthread_create successful.\n");

    printf("[*] Starting race...\n");
    SIGNAL(&start_flag, 1);

    TIMER_START(start);
    while (TIMER_NOT_EXPIRED(start, 5.0)) {
        usleep(100000);
    }

    __atomic_store_n(&stop_flag, 1, __ATOMIC_RELAXED);

    for (int i = 0; i < 2; i++) {
        res = pthread_join(threads[i], NULL);
        if (res != 0) {
            printf("[-] Failed to pthread_join: %s\n", strerror(res));
            exit(1);
        }
    }
    printf("[+] pthread_join successful.\n");

    res = close(in_fd);
    if (res < 0) {
        printf("[-] Failed to close in_fd: %s\n", strerror(errno));
        exit(1);
    }
    printf("[+] close in_fd successful.\n");

    res = close(out_fd);
    if (res < 0) {
        printf("[-] Failed to close out_fd: %s\n", strerror(errno));
        exit(1);
    }
    printf("[+] close out_fd successful.\n");

    res = unlink("/tmp/ramfs/test_file");
    if (res < 0) {
        printf("[-] Failed to unlink: %s\n", strerror(errno));
        exit(1);
    }
    printf("[+] unlink successful.\n");

    res = umount2("/tmp/ramfs", MNT_DETACH);
    if (res < 0) {
        printf("[-] Failed to umount2: %s\n", strerror(errno));
        exit(1);
    }
    printf("[+] umount2 successful.\n");

    printf("[+] Reproducer completed successfully.\n");
    return 0;
}