| Filename | Latest commit message | Latest commit date |
|---|---|---|
370605452c
`rename()` refused an existing destination, so write-to-a-temporary-then-rename
-- how every careful program saves a file -- failed on the second save of
anything. zsh's history was the visible case: `.zsh_history.new` accumulated
for ever and the history itself never updated.
An earlier attempt to fix this was reverted after it lost both files, and the
note it left behind blamed HTree slot reuse. That was the wrong suspect.
**sys_rename was removing the old name by hand.** memset a `struct dirent` to
zero and writei() it into the parent directory at the offset dirlookup()
returned. That is the v1 filesystem's on-disk layout, written directly,
bypassing dp->ops entirely. On SchFS a directory is a B-tree with an index over
it -- "the record at that offset" is not a thing, and writing zeroes there
corrupted the directory while leaving the name in the index. Adding a second
removal on top of a corrupting one is how both files disappeared.
Every other caller in the file already used dirunlink(). This one had been
open-coding a filesystem it no longer ran on, and the bug stayed invisible for
as long as nothing asked it to remove two names instead of one.
With that corrected, replacing the destination is the straightforward thing it
should always have been: look it up, refuse if it is a directory, unlink it,
drop its nlink. rename(a, a) returns 0 and touches nothing, per POSIX -- both
names already reach the same file, and removing one to add it back is only
equivalent when nothing else is watching.
The lock discipline is inherited rather than invented, and it is the delicate
part: dp_old and dp_new are frequently the *same* directory reached through two
nameiparent() calls, which is two references to one inode. So no two directory
locks are held at once -- each is locked, used, unlocked before the next -- and
the source inode stays unlocked throughout, held alive by an nlink bump.
Directories are still refused, in both roles: moving one needs the `..` fixup
and the ancestor-loop check that are not written, and replacing one needs them
plus an emptiness test. EISDIR either way.
mv a c -> 0, c has a's contents
mv c b -> 0, b replaced, b has a's contents (this is the fix)
mv b b -> 0, nothing happens
mv nope z -> ENOENT
mv b d/ -> 0, b lands inside d
60 create-and-rename cycles leave one file and 4 KB of disk, not 60. /root
holds .zsh_history and no .zsh_history.new, and a marker written in one boot is
still in the history file on the next.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
|
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| .. | ||
| boot.md | ||
| debugging.md | ||
| devices.md | ||
| filesystem.md | ||
| graphics.md | ||
| init.md | ||
| kernel.md | ||
| ldls.md | ||
| networking.md | ||
| README.md | ||
| schfs.md | ||
| syscalls.md | ||
| terminals.md | ||
| usb.md | ||
The system, layer by layer
What SchOS is today. Everything in this directory is built and running; what is merely intended lives in ../plan/.
The names, and what each one covers
uname -s reports SchIFT, the way every Unix reports its kernel rather than
its distribution.
| Layer | Name | What it is |
|---|---|---|
| System | SchOS | The distribution — kernel, userland, tooling |
| Kernel | SchIFT | Multi-core x86-64 kernel with a modular VFS |
| Filesystem | SchFS | Transactional, copy-on-write, the root filesystem |
| API | SchAPI | The native programming interface — designed, not built |
| Compositor | Schcom | Owns the display; v0 runs |
Six virtual terminals sit under all of it -- /dev/tty1..tty6, AltGr+F1..F6
or chvt N --
each with its own screen, line discipline and foreground process group. See
terminals.md.
The stack, bottom to top
┌─────────────────────────────────────────────────┐
│ toybox kilo mksh zsh musl first-party cmd/ │ userland
├─────────────────────────────────────────────────┤
│ vgpud — a driver that is an ordinary process │
├─────────────────────────────────────────────────┤
│ schinit — PID 1, job manifests, supervision │
├─────────────────────────────────────────────────┤
│ syscall table — Linux numbering, one .inc file │ ← the ABI seam
╞═════════════════════════════════════════════════╡
│ VFS │ sockets │ signals │ mmap │ poll │ procfs │
├──────┴─────────┴─────────┴──────┴──────┴────────┤
│ SchFS │ lwIP │ vt/gfx │ buffer cache │
├─────────┴────────┴──────────┴───────────────────┤
│ device model — class vtables, bus binding │ ← the driver seam
├─────────────────────────────────────────────────┤
│ /dev/pci — MMIO, DMA and IRQs to a process │ ← the eviction seam
├─────────────────────────────────────────────────┤
│ drivers: xHCI e1000 virtio-gpu VBE IDE PS/2│
├─────────────────────────────────────────────────┤
│ scheduler │ paging │ traps │ IRQ │ SMP │ DMA │ SchIFT core
└─────────────────────────────────────────────────┘
▲
UEFI loader (boot/uefi)
Three seams matter. Above the syscall table, software is ordinary and
portable — musl and toybox are unmodified. Below the device model, a driver
never names its consumer and a consumer never names a driver. And at
/dev/pci, a driver stops having to be in the kernel at all: a process is
handed a device's registers, DMA memory and interrupts, and the in-kernel driver
that held it releases it cleanly. vgpud is the virtio-gpu driver living out
there today. See ../plan/hybrid.md.
Where it came from
SchOS began as xv6 and is now mostly something else. What survives from xv6 is the shape of the process table, the sleep/wakeup discipline, and the buffer cache. What has been replaced: the scheduler and trap path (rewritten for x86-64 and for more than one processor), the memory manager, the entire filesystem, the syscall layer, and every driver.
That inheritance is worth knowing when reading the code, because the parts that still look like xv6 follow xv6's conventions and the parts that do not, do not.
The documents
Read in this order if you are new:
| boot.md | Firmware to the first userspace process. The shortest path through the whole system. |
| kernel.md | Memory, processes, scheduling, traps, signals, SMP. |
| syscalls.md | The one table everything is generated from, and how a musl binary runs unmodified. |
| filesystem.md | The VFS, the buffer cache, mounts, procfs. |
| schfs.md | The copy-on-write root filesystem. |
| devices.md | Buses, classes, binding, and the vtables consumers talk to. |
| usb.md | xHCI, the USB core, and the three class drivers. |
| graphics.md | Framebuffer, text rendering, the console, and PSOGM. |
| terminals.md | The six virtual terminals, and the chord that switches them. |
| networking.md | NIC drivers, lwIP, and both socket families. |
| init.md | schinit, PID 1 as a role, and the job manifests. |
| debugging.md | Symbols, a log that outlives the kernel, and asking the kernel a question. |
| ldls.md | Running somebody else's drivers: a Linux .ko, in a process, reaching hardware through /dev/pci. |
Three rules that hold across all of it
The kernel and the host tools compile the same SchFS source. mkschfs
cannot disagree with the kernel about the on-disk format, because there is only
one implementation. Format bugs are found on the host, where a test takes a
second instead of a boot.
Syscalls are declared in exactly one place. kernel/sys/table.inc.
The constants, the dispatch table and the Linux translation map are all
generated from it, so a syscall cannot be half-registered.
A list you can read beats a list assembled by side effects. Drivers are
written out by hand in kernel/drivers/registry.c rather than collected by a
linker section, and platform devices are registered in one function rather than
inside each driver's init. The tradeoff is deliberate: self-registration is
nicer to write, and it means the list of what is in your kernel is not written
down anywhere you can read.