Files

AI 0c5aa72fd3 Add VFS subsystem and initrd filesystem driver (AI)

VFS subsystem (vfs.c/h):
- Mount table with up to 16 mount points, longest-prefix path matching.
- File descriptor table (256 entries, fds 0-2 reserved for std streams).
- Path resolution walks mount table then delegates to filesystem's
  finddir() for each path component.
- Operations: open, close, read, write, seek, readdir, stat.
- Each filesystem driver provides a vfs_fs_ops_t with callbacks.

Initrd filesystem driver (initrd_fs.c/h):
- Read-only VFS driver backed by the CPIO ramdisk.
- Mounted at '/initrd' during boot.
- Zero-copy reads: file data points directly into the CPIO archive
  memory, no allocation or copying needed.
- Supports readdir (flat iteration) and finddir (name lookup).

Bug fix: resolve_path was overwriting file-specific fs_data (set by
finddir, e.g. pointer to CPIO file data) with the mount's fs_data
(NULL). Fixed to preserve fs_data from finddir.

Verified in QEMU: kernel reads /initrd/README via VFS and prints its
contents. Ring 3 user process continues to work.

2026-02-23 12:23:32 +00:00

2.4 KiB

Raw Blame History

Virtual Filesystem (VFS)

Overview

The VFS provides a unified interface for file and directory operations across different filesystem implementations. Filesystem drivers register ops structs and are mounted at specific paths. Path resolution finds the longest-matching mount point and delegates to that filesystem.

Architecture

 User/Kernel Code
      │
      ▼
  vfs_open("/initrd/hello-world", 0)
      │
      ▼
  VFS: find_mount("/initrd/hello-world")
      │  → mount "/initrd", rel_path = "hello-world"
      ▼
  resolve_path → initrd_finddir("hello-world")
      │
      ▼
  vfs_read(fd, buf, size)
      │  → initrd_read(node, offset, size, buf)
      ▼
  Returns file data from CPIO archive

Mount Points

Filesystems are mounted at absolute paths. The VFS supports up to 16 simultaneous mounts. Path resolution uses longest-prefix matching:

Mount: "/initrd"  → handles /initrd/*
Mount: "/sys"     → handles /sys/*
Mount: "/dev"     → handles /dev/*

File Operations

Function	Description
`vfs_open(path, flags)`	Open a file, returns fd
`vfs_close(fd)`	Close a file descriptor
`vfs_read(fd, buf, size)`	Read bytes, advances offset
`vfs_write(fd, buf, size)`	Write bytes, advances offset
`vfs_seek(fd, offset, whence)`	Seek within file
`vfs_readdir(path, idx, out)`	Read directory entry
`vfs_stat(path, out)`	Get file info

Filesystem Driver Interface

Each filesystem provides a vfs_fs_ops_t struct:

typedef struct vfs_fs_ops {
    int (*open)(vfs_node_t *node, uint32_t flags);
    void (*close)(vfs_node_t *node);
    int32_t (*read)(vfs_node_t *node, uint32_t offset, uint32_t size, void *buf);
    int32_t (*write)(vfs_node_t *node, uint32_t offset, uint32_t size, const void *buf);
    int (*readdir)(vfs_node_t *dir, uint32_t idx, vfs_dirent_t *out);
    int (*finddir)(vfs_node_t *dir, const char *name, vfs_node_t *out);
} vfs_fs_ops_t;

Initrd Filesystem Driver

The initrd filesystem (initrd_fs.c) provides read-only access to the CPIO ramdisk. It is automatically mounted at /initrd during boot. Files are accessed via zero-copy reads directly from the CPIO archive in memory.

Files

src/vfs.h / src/vfs.c — VFS core: mount table, fd table, path resolution
src/initrd_fs.h / src/initrd_fs.c — CPIO ramdisk VFS driver

2.4 KiB Raw Blame History