Patch to enable O_SYNC on RandomAccessFile

rafpatch.c

/*
 * Compile: gcc -I $JAVA_HOME/include -I $JAVA_HOME/include/linux -shared -fPIC -O3 -olibrafpatch.so -Wl,-soname,librafpatch.so rafpatch.c $JAVA_HOME/lib/amd64/libjava.so
 * Run:     java -agentpath:/path/to/librafpatch.so MainClass
 */

#include <jvmti.h>
#include <jni.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <unistd.h>
#include <fcntl.h>
#include <stdio.h>
#include <string.h>
#include <errno.h>


static jfieldID randomAccessFile_fd;
static jfieldID fileDescriptor_fd;

JNIEXPORT const char* JNICALL
JNU_GetStringPlatformChars(JNIEnv* env, jstring jstr, jboolean* isCopy);

JNIEXPORT void JNICALL
JNU_ReleaseStringPlatformChars(JNIEnv* env, jstring jstr, const char* str);

JNIEXPORT void JNICALL
JNU_ThrowNullPointerException(JNIEnv *env, const char* msg);

void throwFileNotFoundException(JNIEnv* env, jstring path);


#define RESTARTABLE(cmd, result) \
    do { \
        result = cmd; \
    } while(result == -1 && errno == EINTR);

int handleOpen(const char* path, int oflag, int mode) {
    int fd;
    RESTARTABLE(open64(path, oflag, mode), fd);
    if (fd != -1) {
        struct stat buf64;
        int result;
        RESTARTABLE(fstat64(fd, &buf64), result);
        if (result == -1) {
            close(fd);
            return -1;
        } else if (S_ISDIR(buf64.st_mode)) {
            close(fd);
            errno = EISDIR;
            return -1;
        }
    }
    return fd;
}


JNIEXPORT void JNICALL
Hook_java_io_RandomAccessFile_open0(JNIEnv* env, jobject this, jstring path, jint mode) {
    int flags = 0;
    if (mode & 1) {
        flags = O_RDONLY;
    } else if (mode & 2) {
        flags = O_RDWR | O_CREAT;
        if (mode & 4) {
            flags |= O_SYNC;
        } else if (mode & 8) {
            flags |= O_DSYNC;
        }
    }

    if (path == NULL) {
        JNU_ThrowNullPointerException(env, NULL);
        return;
    }

    const char* ps = JNU_GetStringPlatformChars(env, path, NULL);
    if (ps == NULL) return;

    // Strip trailing slashes
    char* p = (char*)ps + strlen(ps) - 1;
    while (p > ps && *p == '/') {
        *p-- = '\0';
    }

    int fd = handleOpen(ps, flags, 0666);
    if (fd == -1) {
        throwFileNotFoundException(env, path);
    } else {
        jobject fileDescriptor = (*env)->GetObjectField(env, this, randomAccessFile_fd);
        if (fileDescriptor != NULL) {
            (*env)->SetIntField(env, fileDescriptor, fileDescriptor_fd, fd);
        }
    }
    JNU_ReleaseStringPlatformChars(env, path, ps);                \
}


void JNICALL VMInit(jvmtiEnv* jvmti, JNIEnv* env, jthread thread) {
    jclass randomAccessFile = (*env)->FindClass(env, "java/io/RandomAccessFile");
    jclass fileDescriptor = (*env)->FindClass(env, "java/io/FileDescriptor");

    randomAccessFile_fd = (*env)->GetFieldID(env, randomAccessFile, "fd", "Ljava/io/FileDescriptor;");
    fileDescriptor_fd = (*env)->GetFieldID(env, fileDescriptor, "fd", "I");

    JNINativeMethod open0 = {"open0", "(Ljava/lang/String;I)V", Hook_java_io_RandomAccessFile_open0};
    (*env)->RegisterNatives(env, randomAccessFile, &open0, 1);

    printf("RandomAccessFile patch installed\n");
}

JNIEXPORT jint JNICALL
Agent_OnLoad(JavaVM* vm, char *options, void* reserved) {
    jvmtiEnv* jvmti;
    jvmtiEventCallbacks callbacks;

    (*vm)->GetEnv(vm, (void**)&jvmti, JVMTI_VERSION_1_0);

    memset(&callbacks, 0, sizeof(callbacks));
    callbacks.VMInit = VMInit;
    (*jvmti)->SetEventCallbacks(jvmti, &callbacks, sizeof(callbacks));
    (*jvmti)->SetEventNotificationMode(jvmti, JVMTI_ENABLE, JVMTI_EVENT_VM_INIT, NULL);

    return 0;
}