How do I use file in user's space?

[gamingjam60]

I want to use FatFs Filesystem library in my x86 architecture based 64 bit system. Here my plan is to use fatfs f_open function by following way

f_open => kernel space system call defination => userspace system call is showing invalid file pointer. Here my code was

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case INT_SYSCALL_FATFS_OPEN: {          // 0x33 : Open a file

            const char *path = (const char *)regs->rbx;
            FIL *file = (FIL *)regs->rcx;    // File pointer (output)
            // FIL *file = (FIL *)kheap_alloc(sizeof(FIL), ALLOCATE_DATA);
            BYTE mode = (BYTE)regs->rdx;        // File open mode

            printf("int_syscall_fatfs_open: path: %x, file: %x, mode: %x\n", (uint64_t)path, (uint64_t)file, (uint64_t)mode);

            if (!path || !file || !mode) {
                regs->rax = -1;                 // Invalid path, file, mode
                break;
            }

            FRESULT res = f_open(file, path, mode);
            if (res != FR_OK) {
                kheap_free((void *)file, sizeof(FIL));
                regs->rax = -1;                 // Open file failed
            }
            regs->rax = (uint64_t)file;         // Return file pointer
            
            break;
        }

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uint64_t syscall_fatfs_open(const char *path, uint64_t mode) {

    uint64_t file = syscall_uheap_alloc(128, ALLOCATE_DATA);

    if (!path || !mode) {
        return -1; // Invalid path
    }

    printf("syscall_fatfs_open: path: %x, mode: %x\n", (uint64_t)path, mode);

    uint64_t file_handle;

    asm volatile (
        "mov %[path], %%rbx\n"      // path pointer store into rbx
        "mov %[file], %%rcx\n"
        "mov %[mode], %%rdx\n"      // Store mode into rcx
        "int $0x33\n"
        "mov %%rax, %[handle]\n"
        : [handle] "=r" (file_handle)
        : [path] "r" ((uint64_t)path), [file] "r" ((uint64_t)file), [mode] "r" (mode)
        : "rbx", "rcx", "rax"
    );

    return file_handle;
}

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    uint64_t opened_file = syscall_fatfs_open(file_name, FA_WRITE | FA_READ | FA_CREATE_ALWAYS);

Here I have allocated FIL structure pointer in Lower half which is showing invalid pointer in f_open's output . I also tried to allocate file pointer in Upper half in kernelspace is also causing same proble.

How do I need to allocate file pointer which will be valid in userspace and writable, readable etc.

[Octocontrabass]

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        "mov %[mode], %%rdx\n"      // Store mode into rcx

Your inline asm clobbers RDX without telling the compiler.

How do I need to allocate file pointer which will be valid in userspace and writable, readable etc.

Valid for what? What are you trying to do with that pointer? Userspace must use system calls to read and write files.