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>
sendmsg/recvmsg with SCM_RIGHTS. Item 1 of docs/plan/foundations.md and
mechanism 1 of docs/plan/hybrid.md, which are the same primitive seen from two
directions: Wayland's wl_shm_create_pool passes a file descriptor with no
integer-shaped alternative, and handing a driver process the device it is
allowed to touch is the same act. Both plans stopped here.
Linux's structures byte for byte -- msghdr, cmsghdr, the CMSG_ macros -- because
the programs that need this are not ours. libwayland and toybox both call
recvmsg with Linux's headers and would not notice a friendlier door.
The delivery rule is the whole design. A batch of descriptors is pinned to the
stream position of the first byte of the message that carried it, and that has
two halves: a reader arriving at that position is handed them, and a reader that
has not reached it is stopped SHORT of it. Leave out the second half and a
program asking for 4096 bytes swallows three messages and is handed one
message's descriptors, with nothing in the result saying which. socktest checks
exactly that: two one-byte messages, an eight-byte read, one byte back.
The kernel holds a reference to every descriptor in flight, so a sender may
close its own copy the moment sendmsg returns. Giving that reference back is the
part that leaks if it is not written down, and there are three ways a message
ends:
- received with room installed in the receiver's table
- received without room, or by
plain read(2) closed; MSG_CTRUNC for the first case
- never received, socket closed closed by sockclose
The middle row looks like a choice and is not: holding kernel references for a
receiver that has proved it is not looking is a leak with extra steps.
No garbage collector. A socket descriptor sent over that same socket and never
received holds a reference to the socket that holds the reference to it, and
neither close reaches zero. Linux carries several hundred lines for this.
Nothing on either plan passes a socket over a socket -- Wayland passes memory,
the driver work passes devices -- so it is a bounded leak that is documented
rather than a silent one.
Fourteen checks in cmd/socktest, and every one moves data through the descriptor
rather than inspecting it. The descriptor passed is always a pipe end, because a
pipe announces its own lifetime: a read of a pipe whose writers are all gone
returns 0, which turns "was the kernel's reference released" -- otherwise
invisible from userspace -- into a value a test can compare.
Two bugs fell out, neither in the new code:
- sockpair() never set the socket's domain, so every socketpair() socket
claimed domain 0. Invisible while the data paths asked only whether a socket
was AF_INET or AF_NETLINK, since zero answers "neither" correctly.
SCM_RIGHTS asks "is this AF_UNIX", and zero is wrong for that one.
- pipe_poll() reports POLLHUP but not POLLIN at end of file, so a non-blocking
read of a pipe whose writers had all gone returned EAGAIN forever instead of
0 -- a program that polls and reads spins waiting for an end that already
happened. sockreadable() always had the equivalent condition; the pipe path
did not. Found because the test helper for "is this descriptor closed" is a
non-blocking read of a pipe.
projects/schcom/README.md said SCM_RIGHTS was done for a long time while nothing
in the kernel implemented it, and an entire GUI plan was drawn on that line.
It is true now, which is not the same as it having been right, and the README
says so.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VQ7Qx8E1q1M1bbCq8QLHS2
docs/ was ten files where a plan, a description, a guide and a bug report all
sat next to each other. It is now architecture/ (what SchOS is), plan/ (what it
will be), guides/ (how to work on it) and history/ (why it is like this), with
an index that states the one rule holding it together: when something is
finished it moves from plan/ to architecture/ and is rewritten as a description
of the thing rather than a record of building it.
A ticked checkbox says the work happened. It does not say what the work
*produced*, and six months later the checkbox is all that is left. The network
stack proved the point: finished, verified end to end, and for a month the only
description of it anywhere was a checkbox four levels of indentation deep.
So roadmap.md went from 534 lines to 181 -- nothing deleted, the done work
graduated. Seven subsystems that were finished and undescribed now have
descriptions: boot, kernel, syscalls, filesystem, networking, graphics, usb,
init. Every one ends with "What it does not do", which is the section that pays;
a document listing only strengths is marketing, and the reader who needs it most
is the one about to trip over a limitation nobody wrote down.
schfs.md still opened with "phases 1 through 3 1/2 are done, phase 4 is next"
long after all five had landed. It is a description now, with the phase record
and the hard-won bugs moved to history/.
Three corrections found on the way, all load-bearing:
* projects/schcom/README.md claimed SCM_RIGHTS was done. There is no
sendmsg/recvmsg in the kernel at all, and an entire GUI plan rested on it.
* The root README said "No TCP/IP" months after the stack landed.
* history/kernel.md sec.7 was marked "in progress" on a problem that process
groups solved.
Also adds plan/1.0.md, which draws a finish line so the open list stops being
infinite, and marks schwm/ and schde/ superseded: in Wayland the compositor is
the window manager, so that split dissolves rather than being abandoned.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VQ7Qx8E1q1M1bbCq8QLHS2
Nexoniarz