Implement filesystem syscalls (open/read/write/lseek/close)

[pbalduino]

Goal

Implement filesystem-related system calls, wiring them through the VFS layer to provide POSIX-like file operations.

Context

This layer connects userspace file operations to the kernel VFS system, enabling user programs to work with files and directories.

Definition of Done

• File operations: open, read, write, lseek, close syscalls
• Directory operations: opendir, readdir, closedir functionality
• Metadata operations: stat, fstat, access syscalls
• File creation: creat, mkdir, unlink, rmdir syscalls
• File positioning: lseek with SEEK_SET, SEEK_CUR, SEEK_END
• Error handling: Proper errno setting and error propagation
• Security: Basic permission checking and validation

Syscall Implementation

File Operations

Directory Operations

Metadata Operations

Implementation Details

• Add syscall numbers to syscall table
• Implement syscall handlers that validate parameters
• Wire syscall handlers to VFS operations
• Add proper error handling and errno setting
• Implement file descriptor validation
• Add userspace pointer validation for safety

File Descriptor Management

• Extend existing fd table for file operations
• Handle fd allocation, deallocation, and reuse
• Support standard descriptors (stdin=0, stdout=1, stderr=2)
• Implement fd limits and resource management

Permission and Security

• Basic permission checking (read/write/execute)
• Path validation and canonicalization
• Protection against directory traversal attacks
• Proper handling of special files and devices

Error Handling

• Standard errno values (ENOENT, EACCES, EINVAL, etc.)
• Consistent error propagation from VFS to userspace
• Proper cleanup on error conditions
• Resource leak prevention

Files to Create/Modify

• syscall/fs_syscalls.c - Filesystem syscall implementations
• syscall/syscall_table.c - Add new syscall numbers
• include/syscall_numbers.h - Define syscall constants
• vfs integration points

Userspace Interface

• Update syscall number definitions for userspace
• Provide basic libc-style wrappers
• Add error handling examples
• Create simple file operation examples

Testing Strategy

• Test basic file operations (create, read, write, delete)
• Test directory operations (create, list, delete)
• Test error conditions (invalid paths, permissions)
• Test file positioning and seeking
• Test concurrent file access

POSIX Compatibility

• Follow POSIX semantics where possible
• Standard return values and error codes
• Compatible file descriptor behavior
• Standard file permission model

Dependencies

• VFS layer implementation
• Extended syscall dispatcher
• Userspace memory validation
• Process file descriptor tables

Related Issues

Part of filesystem support stack - enables userspace file operations.