0c5aa72fd3
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.
79 lines
2.4 KiB
Markdown
79 lines
2.4 KiB
Markdown
# Virtual Filesystem (VFS)
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## Overview
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The VFS provides a unified interface for file and directory operations across
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different filesystem implementations. Filesystem drivers register ops structs
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and are mounted at specific paths. Path resolution finds the longest-matching
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mount point and delegates to that filesystem.
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## Architecture
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```
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User/Kernel Code
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│
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▼
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vfs_open("/initrd/hello-world", 0)
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│
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▼
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VFS: find_mount("/initrd/hello-world")
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│ → mount "/initrd", rel_path = "hello-world"
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▼
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resolve_path → initrd_finddir("hello-world")
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│
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▼
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vfs_read(fd, buf, size)
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│ → initrd_read(node, offset, size, buf)
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▼
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Returns file data from CPIO archive
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```
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## Mount Points
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Filesystems are mounted at absolute paths. The VFS supports up to 16
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simultaneous mounts. Path resolution uses longest-prefix matching:
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```
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Mount: "/initrd" → handles /initrd/*
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Mount: "/sys" → handles /sys/*
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Mount: "/dev" → handles /dev/*
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```
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## File Operations
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| Function | Description |
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|----------|-------------|
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| `vfs_open(path, flags)` | Open a file, returns fd |
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| `vfs_close(fd)` | Close a file descriptor |
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| `vfs_read(fd, buf, size)` | Read bytes, advances offset |
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| `vfs_write(fd, buf, size)` | Write bytes, advances offset |
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| `vfs_seek(fd, offset, whence)` | Seek within file |
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| `vfs_readdir(path, idx, out)` | Read directory entry |
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| `vfs_stat(path, out)` | Get file info |
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## Filesystem Driver Interface
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Each filesystem provides a `vfs_fs_ops_t` struct:
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```c
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typedef struct vfs_fs_ops {
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int (*open)(vfs_node_t *node, uint32_t flags);
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void (*close)(vfs_node_t *node);
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int32_t (*read)(vfs_node_t *node, uint32_t offset, uint32_t size, void *buf);
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int32_t (*write)(vfs_node_t *node, uint32_t offset, uint32_t size, const void *buf);
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int (*readdir)(vfs_node_t *dir, uint32_t idx, vfs_dirent_t *out);
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int (*finddir)(vfs_node_t *dir, const char *name, vfs_node_t *out);
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} vfs_fs_ops_t;
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```
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## Initrd Filesystem Driver
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The initrd filesystem (`initrd_fs.c`) provides read-only access to the CPIO
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ramdisk. It is automatically mounted at `/initrd` during boot. Files are
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accessed via zero-copy reads directly from the CPIO archive in memory.
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## Files
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- `src/vfs.h` / `src/vfs.c` — VFS core: mount table, fd table, path resolution
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- `src/initrd_fs.h` / `src/initrd_fs.c` — CPIO ramdisk VFS driver
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