SchOS/docs/plan/debugger.md

A debugger for this kernel

Every milestone on this plan is built. Symbols, a log that outlives the kernel, the introspection registry behind /dev/dbg, the whose ownership query and hardware watchpoints are all described in ../architecture/debugging.md.

What is left here is level 2, the network transport — a userspace daemon bridging TCP to /dev/dbg, deliberately outside the kernel. Nothing else.

Every bug in this project so far has been found the same way: add a cprintf, rebuild, boot, read the serial log, guess again. That loop is about ninety seconds long and it answers exactly one question per turn — the question you thought to ask before you rebuilt.

The DMA ownership bug in do_dma() is what made the case. Finding it took several rounds of that loop, and the thing that eventually found it was not a print at all; it was reading freevm() and noticing what PHYSTOP meant for the arena. The kernel could not be asked whether two allocators owned the same page, because nothing in it can be asked anything.

The kernel has no way to answer a question it was not already compiled to answer. That is the gap.

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What there was, and what was missing

The state this plan was written against. D0 to D2 have since closed the first three rows; they are kept because they are the argument.

cprintf	goes to UART and console and is then gone. There is no log. panic.c says so itself: "there is no core dump and no log to go back to."
PSOGM	collects a great deal at the moment of a panic — CPU, process, call chain, load address — and paints it. It is the only artefact of a crash, and it does not survive being looked away from.
getcallerpcs()	walks frames and prints raw addresses. Turning one into a function name is a manual objdump session.
/proc/sysrq	a hardcoded switch with one letter per action. xhci_dump, usb_storage_dump, display_selftest — the right instinct, in the wrong shape: they are self-tests, not state, and adding one means editing a central file.
DR0–DR3	unused. T_DEBUG maps to SIGTRAP and nothing ever arms a hardware watchpoint.

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The two rules

It observes; it does not control

No stepping. No software breakpoints. No stopping the world.

This is the decision everything else follows from, and it is not a scope compromise. Half the bugs this project actually hits are timing: an interrupt storm at 77,000 a second, a level-triggered line nobody acknowledges, a driver that faults only on the second start, a supervisor that restarts a job fifteen times in forty-five seconds. Every one of those disappears when you stop the world to look at it. A debugger that can only work by halting the machine cannot see the class of bug this kernel produces.

So it answers questions, and it watches. Both of those work on a machine that is still running.

It must not depend on what it is debugging

The same circularity rule the display already has, pointed at a different subsystem:

A debugger that needs lwIP cannot debug lwIP. A debugger that needs a process cannot debug the scheduler. A debugger that needs to allocate cannot debug the allocator.

That rule decides the layering below rather than taste deciding it, which is the only reason to write a rule down.

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Why not GDB

The licence is not the reason — a stub written here from the published protocol would be our own Apache-2.0 code, the same clean-room argument as ../architecture/devices.md makes for Linux drivers. Two technical reasons, and they are both about what a generic debugger structurally cannot do.

It only knows registers and memory. GDB can tell you that 0x3eceb000 contains zeroes. It cannot tell you that the page is in the DMA arena's free list and on kalloc's free list and mapped into pid 7 — because those are this kernel's abstractions, and a debugger that knows about them has to be written against them. That query is the one that would have ended the session described above in one line.

It stops the world, per the rule above.

There is a third, smaller reason: a gdbstub is a protocol you implement and then debug, and its failure mode is an unresponsive stub with no way to ask it why. The thing below can always be driven by hand over a serial line.

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The layers

Three transports, and the rule decides which gets what. The kernel's half is the minimum that works when nothing else does; everything convenient lives in a process — which is the same sentence as hybrid.md's, about a different thing.

	Works when
0. Console and UART	always, including panic context, with interrupts off, no allocation, no scheduler, no lock taken	sysrq-driven. If the machine is dying this is the one you have.
1. /dev/dbg	there is a scheduler and a process	A character device. The local client is an ordinary program — exactly the /dev/pci discipline, for exactly the same reason.
2. The network	there is a network	A userspace daemon bridging TCP to /dev/dbg. Not in the kernel. lwIP must stay out of the debugger's path, or the debugger cannot be pointed at lwIP.

Level 2 is deliberately last, and it is not in the first cut.

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The protocol is lines of text

A request is a line. A response is lines, ending in a blank one.

> whose 0x3eceb000
phys 0x3eceb000
  dma    arena extent 2, FREE (4096000 bytes @ 0x3eceb000)
  kalloc free list, position 41
  pid 7  vgpud   va 0x4000d000  rw-u  vma[3] no_free
  *** three owners -- this page is claimed twice

> view dma
arena  0x3e000000-0x3f000000  16384 KB, 12284 KB free
  free[0]  0x3e0a4000  8192 KB
  free[1]  0x3ecec000  4092 KB
  in use   4096 KB in 3 blocks

Text, and not a typed binary protocol, for three reasons that all turn out to be the same reason:

• It works at level 0. Panic context cannot allocate, cannot frame records, cannot wait for a client to acknowledge anything. It can put characters on a serial line. A binary protocol needs a text fallback anyway, and then there are two protocols.
• It is usable before a client exists. cat, echo, a serial terminal, the guest's own shell. A protocol that needs its client built first is a protocol nobody can debug.
• It survives the thing it is debugging. When the answer arrives garbled, you can still read most of it.

A richer client can be layered on later by parsing this, and if that ever happens the kernel does not change.

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Milestones

D0	Symbols. A compact sorted table emitted from nm at link time and linked into the image; ksym_lookup(addr, &off).	done
D1	A log that survives. A ring behind cprintf, timestamped, in a region the allocator never sees — so a panic's last words are readable on the next boot. /proc/kmsg.	done
D2	The introspection registry. dbg_register("dma", dma_dump). Seven views, /dev/dbg, schdbg, and sysrq g.	done
D3	whose <addr> — the cross-subsystem ownership query above.	done
D4	Watchpoints, DR0–DR3, break-on-write, reporting rip and call chain and continuing rather than stopping.	done
D5	The network transport, level 2 above: a userspace daemon bridging TCP to /dev/dbg. Off unless a boot flag turns it on, loopback by default.

Why D0 and D1 come before the interesting parts

Neither is clever. Both multiply everything after them: a view that prints an address is worth less than one that prints a name, and a query you can only run after you have noticed something is worth less than a log that was already running when it happened.

D1 in particular changes the job rather than improving it. Today a triple fault leaves nothing at all — the screen is gone, the serial buffer is gone, and the only evidence is whatever you happened to be watching. A log that outlives the boot turns that into a file.

What D1 turned out to cost

One measurement changed the design. The log was first placed at the top of RAM beside the DMA arena, which is the obvious spot for a region nothing should allocate — and on the first reboot its magic came back as ASCII, because the top of usable memory is exactly where the firmware makes its own allocations before handing the machine over. It sits above the kernel image instead, which is both a stable address across boots and memory the loader has already committed to. Recorded because the reasoning that picked the wrong place was sound, and only the test was not.

What D2 is really for

Not the views themselves — the registry. A subsystem publishes its state with one line at the bottom of the file that owns the data, and nothing central has to be edited. That is the only version of this that stays current; the sysrq switch is the version that does not, and it is already three self-tests and no state.

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The security note, for the parts not yet built

A debugger that reads arbitrary physical memory and a TCP listener are, together, a remote root hole. That is true of every kernel debugger ever written and it is not a reason not to build one, but it has to be decided rather than discovered:

• Level 2 is off unless a boot flag turns it on, and binds loopback by default.
• No memory writes in the first cut. Reading is what finds bugs; writing is what a later milestone can argue for on its own.

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What it will still not do

• It will not step, break, or halt, per the rule. If a bug genuinely needs a stepper, this is the wrong tool and that is a deliberate trade.
• No stack unwinding through interrupt frames. getcallerpcs() walks rbp chains and stops where the frame pointer stops; a trap frame in the middle of a chain ends the walk.
• No userspace symbolization. The kernel will name kernel symbols. A fault in a process reports the binary and the offset, and resolving that stays a host-side job.
• No profiling. No sampling, no counters, no flame anything. A timer-driven sampler is a different tool that happens to share a transport.
• Watchpoints are limited to four, by the hardware, and to 1/2/4/8 bytes. There is no software watchpoint fallback and there should not be one.
• It is not a tracer. Static tracepoints and an event ring are the obvious next thing after D4 and are not on this plan.

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On the freeze

1.0.md says no new capability starts until the register is empty, and the register is not empty. Two things about that.

The bookkeeping answer is that the rule is "adding a row requires closing one", and A released PCI device closed when H2 landed. There is a slot.

The better answer is that the register is a list of capabilities of the OS, and this is not one — nothing in it ships to a user or appears in the acceptance test. It is a tool, and the case for spending time on a tool is that it makes everything still on the register cheaper to finish. Gate 0 of 1.0.md is "fix what is already wrong", assembled from every What it does not do section in ../architecture/. That gate is a list of bugs to find in a kernel that cannot currently be asked anything.

Build the instrument before the survey, and then the freeze holds for the rest of it.