How do I use file in user's space?
Posted: Fri Jul 04, 2025 9:43 am
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
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.
f_open => kernel space system call defination => userspace system call is showing invalid file pointer. Here my code was
Code: Select all
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);
How do I need to allocate file pointer which will be valid in userspace and writable, readable etc.