| Filename | Latest commit message | Latest commit date |
|---|---|---|
3185685997
One to four processes died per boot with a fault they could not have caused:
`rip 0x0` -- an instruction fetch at zero, which a process does not branch to
-- or a dereference of `0xffffffffffffffea`, which is `-EINVAL`. It hit shells
and schinitd alike and had been carried for a long time as "zsh sometimes jumps
into .data".
`syscall_entry` has to swap FS_BASE from the user's TLS to the kernel's before
it can touch any `%fs:` data, and `wrmsr` destroys rax, rcx and rdx. Those have
to be parked somewhere first -- not on the user stack, which is untrusted and
has not been switched away from yet. They were parked in three globals:
// Single-CPU scratch for syscall_entry FS swap
// (safe while IF cleared by SYSCALL/SFMASK)
uint64 _sc_rax, _sc_rcx, _sc_rdx;
IF being clear answers for an interrupt arriving on *this* CPU. It says nothing
whatever about a second CPU executing the same three stores at the same
instant, and with more than one processor entering syscalls that is not a rare
interleaving -- it is the ordinary one.
**%rcx is the user return address.** SYSCALL puts the user's rip there and
SYSRET takes it back from there. So the CPU that lost the race returned its
process into *another process's* instruction stream, with %rax -- the syscall
number on the way in, the return value on the way out -- swapped alongside it.
Both symptoms fall straight out of that: a zero handed over and followed, and
an errno used as a pointer.
Per-CPU words at %gs:32..48 now. %gs is usable there, before the FS swap, for
exactly the reason RESTORE_KERNEL_FS can already read %gs:0: GS base is per-CPU
in hardware, seginit() sets it once, and userspace cannot change it --
sys_arch_prctl() implements ARCH_SET_FS and ARCH_GET_FS and rejects the rest.
Measured rather than argued. Both variants built once, `build/esp` saved for
each, the same pristine schfs.img restored before every boot, -smp 4:
three globals 10 boots 10 faults, in five of the ten
three per-CPU 18 boots 0 faults
A _Static_assert pins all seven per-CPU offsets, because trapasm.S spells them
as literals and cannot include a header -- a slot that moves would leave a
register quietly holding somebody else's value, which is the hardest possible
thing to find.
**This is the second bug of this exact shape in this file.** The first was
`kernel_fs_base`, a single global every CPU wrote during its own seginit(), so
the last writer won and every CPU restored one CPU's per-CPU block;
RESTORE_KERNEL_FS reads %gs:0 for that reason. The lesson generalises and is
now written down in both places: **on the entry path, a global is a bug.**
Closes the row added to Gate 0 four commits ago. It was the right row to put
first -- a compositor built on this would have failed in ways indistinguishable
from its own bugs.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
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| .. | ||
| 1.0.md | ||
| foundations.md | ||
| hybrid.md | ||
| README.md | ||
| roadmap.md | ||
| schapi.md | ||
| wayland.md | ||
What happens next
Everything in this directory is unbuilt. When a piece of it is finished it leaves — deleted from here, rewritten as a description in ../architecture/. See the rule.
Read 1.0.md first.
It draws a finish line and puts most of the work below on the far side of it. Anything not named there is not being worked on, and the documents in this directory describe more than 1.0 contains.
It also holds the register of unfinished work, which is the one place anything half-built is listed. No new capability starts until that register is empty, and a row leaves it only by being finished or by being deleted from the tree.
The three current decisions
Taken together, because they turn out to need the same foundation.
| wayland.md | The GUI stack becomes Wayland. SchWM and SchDE are deleted — actually gone from the tree, not marked; Schcom survives and changes its wire format. |
| Built. All four mechanisms exist, the GPU driver runs outside the kernel, it survives being killed, and the display mode can be changed from a process. H4 (USB) was withdrawn rather than left open — USB is the bus peripherals arrive on, not a peripheral. | |
| Built. PID 1 is now a role the kernel refills; the description moved to ../architecture/init.md. |
And the one they all wait on:
| foundations.md | Descriptor passing, shared memory, sockets in the filesystem. The critical path. Item 1 is built — sendmsg/recvmsg with SCM_RIGHTS. |
And the one that says which parts of the above are actually being worked on:
| 1.0.md | The finish line. Four gates, and everything else explicitly deferred. |
And the one the hybrid mechanisms were built for:
Built. The Linux Driver Layer for SchOS: someone else's drivers, in a process, reached through /dev/pci. ping works over 8139too and over e1000. The description moved to ../architecture/ldls.md. |
|
Built, all of it, including the network transport. A debugger that observes rather than halts and answers in this kernel's own vocabulary: symbols, a log that outlives the kernel, /dev/dbg, whose, hardware watchpoints, and schdbgd. The description moved to ../architecture/debugging.md. |
Two longer-horizon documents sit behind these: schapi.md, the API layering and the compatibility promise, and roadmap.md, which holds everything still open that is not one of the three above.
The order
Set by 1.0.md, which is the authority. In short:
0. Fix what is already wrong — first
Not a document of its own; it is Gate 0 of 1.0.md, assembled from the What it does not do sections in ../architecture/.
Before any new capability, and the reason is the one that put
netdev_selftest() before the TCP/IP stack: a compositor built on a kernel with
an unexplained hang and a socket layer that reports success for things it did not
do will fail in ways indistinguishable from its own bugs.
1. foundations.md — the critical path
Both the Wayland work and the hybrid work stop here. Descriptor passing and shared memory are the substrate for handing a buffer to a compositor and for handing a device to a driver process. Doing it once for both is the entire reason these are one plan rather than three.
2. init.md — before the desktop, not after — done
A compositor being brought up for the first time crashes constantly, and something that restarts it and says why is worth far more during that work than at the end of it. Built; see ../architecture/init.md.
3. hybrid.md — moved up, and the reason is worth recording
This document used to put the hybrid work after 1.0, deliberately last, on this argument:
designing a userspace driver interface with no driver consuming it produces an interface that is wrong in ways nobody can see
The argument was sound and the conclusion drawn from it was wrong. What forced the change was the driver situation: SchOS cannot write a driver for every device that exists, no project can, and the only route to the hardware support an established OS has is a compatibility layer with the foreign drivers kept outside the kernel. That layer is this plan's mechanisms. It is not optional and it is not late work.
The original argument still holds, and it is answered rather than ignored:
SchOS's own drivers are the consumers. vgpud is a real driver, with real
requirements, and building the interface against it is what keeps it a general
mechanism rather than a hole shaped like something foreign. The compositor is
still a demanding consumer — it is no longer the only one available.
Four mechanisms, all built. One eviction done, the second withdrawn with its reasons recorded, and the foreign driver host they were for is ../architecture/ldls.md — built, and carrying packets.
4. wayland.md — with the GPU driver either side of the seam
W1–W3 and a terminal. W4 and W5 are post-1.0. Item 1 of foundations.md —
descriptor passing — was the thing it could not start without, and it now
exists.
In one sentence
Fix what is broken, build the descriptor-passing and shared-memory substrate, make init survivable, move the drivers out through an interface SchOS's own drivers have to live with first — and put a terminal on the screen.
What this plan does not address
Stated here so it is not mistaken for covered:
- No IOMMU, so the hybrid work isolates against bugs and not against a hostile driver.
- No page cache, so
foundations.mddelivers amemfdand not a general answer to shared file mappings. The general answer is still owed. - No garbage collector for descriptors in flight, so an
AF_UNIXsocket passed over itself and never received leaks. Bounded at thirty-two sockets, and nothing on this plan does it. - No dynamic linking, so no Mesa, no accelerated clients, and no third party
shipping a
.sofor anything. - A new compatibility surface between PID 1 and PID 2, created by a plan whose own schapi.md §3 spends a section warning about those.
- The three open networking items —
getsockname, routing, and interrupt-driven receive — are untouched by any of this and still open. They are in roadmap.md.