3d5fb4c267
Build system changes: - scripts/gen_initrd.sh packs all files from apps/ into a newc-format CPIO archive at build/isodir/boot/initrd.cpio. - CMakeLists.txt adds 'initrd' target as ISO dependency. GRUB loads the archive as a Multiboot2 module via 'module2 /boot/initrd.cpio'. - apps/README added as placeholder file for initial ramdisk content. Kernel changes: - kernel.c scans Multiboot2 tags for MULTIBOOT_TAG_TYPE_MODULE to find the initrd's physical address range, then passes it to cpio_init(). - cpio.c/h implements a parser for the SVR4/newc CPIO format: - cpio_init(): lists archive contents on startup - cpio_find(): look up a file by name (handles ./ prefix) - cpio_next(): iterate through all entries - cpio_count(): count files in archive - The initrd lives in identity-mapped physical memory, so no additional mapping is needed to access it. Verified in QEMU: GRUB loads the module at 0x0014A000, CPIO parser finds the README file (38 bytes). All existing functionality (Ring 3 processes, syscalls) continues to work.
82 lines
2.0 KiB
Markdown
82 lines
2.0 KiB
Markdown
# CPIO Initial Ramdisk
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## Overview
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The initial ramdisk (initrd) provides files to the kernel at boot time before
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any filesystem drivers are available. It is a CPIO archive in SVR4/newc format,
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loaded by GRUB as a Multiboot2 module.
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## Build Process
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During the build, the script `scripts/gen_initrd.sh` packs all files from the
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`apps/` directory into a CPIO archive:
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```
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apps/
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├── README (placeholder)
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├── hello-world (future: flat binary)
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└── sh (future: shell binary)
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```
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The archive is placed at `build/isodir/boot/initrd.cpio` and included in the
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ISO image. GRUB loads it as a module via:
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```
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module2 /boot/initrd.cpio
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```
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## CPIO Format
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The newc (SVR4) CPIO format uses 110-byte headers with hex ASCII fields:
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```
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Offset Size Field
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0 6 Magic ("070701")
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6 8 Inode
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14 8 Mode
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22 8 UID
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30 8 GID
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38 8 Nlink
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46 8 Mtime
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54 8 Filesize
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62 8 Devmajor
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70 8 Devminor
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78 8 Rdevmajor
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86 8 Rdevminor
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94 8 Namesize
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102 8 Check
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```
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After the header: filename (namesize bytes, padded to 4-byte boundary),
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then file data (filesize bytes, padded to 4-byte boundary). The archive
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ends with a `TRAILER!!!` entry.
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## Kernel Interface
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The kernel finds the initrd by scanning Multiboot2 boot information for
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`MULTIBOOT_TAG_TYPE_MODULE` (type 3). The module's physical memory range
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is identity-mapped, so it can be read directly.
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```c
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#include "cpio.h"
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// Find a file
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cpio_entry_t entry;
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if (cpio_find("hello-world", &entry) == 0) {
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// entry.data = pointer to file contents
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// entry.datasize = file size
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}
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// Iterate all files
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uint32_t offset = 0;
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while (cpio_next(&offset, &entry) == 0) {
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// entry.name, entry.data, entry.datasize
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}
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```
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## Files
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- `scripts/gen_initrd.sh` — Build script to generate CPIO archive
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- `src/cpio.h` / `src/cpio.c` — CPIO parser (find, iterate, count)
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- `CMakeLists.txt` — `initrd` target and `module2` in grub.cfg
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