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debugger: the kernel can be asked a question

Browse source 
docs/plan/debugger.md, milestones D0, D1 and D2. What they replace is the
ninety-second loop -- add a cprintf, rebuild, boot, read the serial log --
which answers exactly the one question you thought of before the rebuild.

The rule the shape follows: **it observes, it does not control.** No
stepping, no software breakpoints, no stopping the world. Half the bugs
this kernel produces are timing -- an interrupt storm, a level-triggered
line nobody acknowledges, a driver that faults only on its second start --
and every one of them disappears when you halt the machine to look. And
the layering rule, which is the circularity rule the display already has:
a debugger that needs lwIP cannot debug lwIP.

D0 -- symbols. tools/ksyms.pl turns the first link pass into a table the
second pass carries. Two passes is enough because .ksym is placed after
.text and .rodata, so carrying the table cannot move anything recorded in
it; the build lists both passes' text symbols and refuses to link if they
ever disagree, because a symbol table that is subtly wrong gives confident
false answers. ksym.c reads it through weak symbols, so pass one -- which
has no table -- still links. A crash chain now reads panic+0x5f <-
proc_sysrq_cmd+0x99 <- ... <- syscall+0xf6.

D1 -- a log that outlives the kernel. Every character through consputc()
lands in a byte ring with a timestamp per line, readable as /proc/kmsg.
It sits in a region carved before the page allocator gets anything, with a
magic header, so a kernel finds the previous kernel's log still there --
including the whole PSOGM report. panic.c's own comment said "there is no
core dump and no log to go back to"; that is no longer true.

  The obvious place for it is the top of RAM beside the DMA arena. That
  was the first attempt and the magic came back as ASCII on the reboot,
  because the top of usable memory is exactly where the firmware makes its
  own allocations. It lives above the kernel image instead, which is a
  stable address across boots of the same build.

D2 -- the introspection registry. A subsystem publishes its state with one
line at the bottom of the file that owns the data:

  dbg_register("dma", dma_dump, "the DMA arena's free extents");

Seven views -- dma, mem, proc, vma, claims, pci, sock -- reachable three
ways: sysrq 'g' (always, no process and no filesystem), /dev/dbg (echo a
line, cat the answer), and schdbg. The protocol is lines of text because
the same bytes have to work where nothing can allocate or frame a record,
and because a protocol you cannot drive by hand is one you cannot debug.
struct sbuf moved out of kernel/dev/procfs.c into include/sbuf.h rather
than being written a second time.

Several views exist because of a specific bug. `vma` is the table that
decides whether exit() frees a page, and the DMA ownership bug that made
this worth building was a block that was never in it.

Also fixed, found by building with clang for the first time in a while:
struct iovec was defined in both posix.h and socket.h, and every file that
included both was a redefinition. GCC 15 defaults to C23, which permits an
identical struct redeclaration, so the gcc path never noticed -- while
clang defaults to gnu17 and rejected it, breaking the one toolchain
shell.nix exists to provide and the UEFI loader requires. Both are now
guarded.

D3 (`whose <addr>`, the cross-subsystem ownership query) and D4 (hardware
watchpoints on DR0-DR3) are not built and are described honestly as
missing in the architecture doc's "What it does not do".

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VQ7Qx8E1q1M1bbCq8QLHS2
This commit is contained in:
Nexoniarz 2026-09-14 00:58:45 +02:00
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# A debugger for this kernel
> **D0, D1 and D2 are built.** Symbols, a log that outlives the kernel, and the
> introspection registry behind `/dev/dbg` are described in
> [../architecture/debugging.md](../architecture/debugging.md). What is left
> here is D3, D4, and the network transport.
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.
---
## 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. |
---
## 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.
---
## 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](../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.
---
## 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](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.
---
## 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.
---
## 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. | |
| **D4** | **Watchpoints**, DR0–DR3, break-on-write, reporting rip and call chain and **continuing** rather than stopping. | |
### 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.
---
## 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.
---
## 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.
---
## On the freeze
[1.0.md](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](1.0.md)
is *"fix what is already wrong"*, assembled from every *What it does not do*
section in [../architecture/](../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.