1dd4ba0186
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
55 lines
2.2 KiB
C
55 lines
2.2 KiB
C
#pragma once
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#include "types.h"
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struct sbuf;
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// Asking the kernel a question.
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//
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// docs/plan/debugger.md milestone D2. The thing this exists to end is the
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// ninety-second loop: add a cprintf, rebuild, boot, read the serial log, and
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// learn the answer to exactly the one question you thought of before the
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// rebuild.
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//
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// The interesting part is not the views. It is that a subsystem publishes its
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// own state with **one line at the bottom of the file that owns the data**, and
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// nothing central has to be edited to add one:
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//
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// dbg_register("dma", dma_dump, "the DMA arena's free extents");
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//
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// That is the difference between something that stays current and something
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// that does not. /proc/sysrq is the version that does not -- a switch in one
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// file with a letter per action, which is why after two years it holds three
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// self-tests and no state at all.
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//
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// Views run in ordinary process context, from a write to /dev/dbg or from
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// sysrq, so they may take locks. **They must not be called from panic().**
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// The argument is whatever followed the view's name on the command line, or
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// "" when there was none. Most views ignore it; the ones that can produce a
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// hundred lines use it to narrow -- `vma 4` rather than `vma`. It is also the
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// shape milestone D3 needs, since `whose <addr>` is nothing but an argument.
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typedef void (*dbg_view_fn)(struct sbuf *out, const char *arg);
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// Publish a view. `what` is one line, shown by `help`.
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//
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// Safe to call from any subsystem's init, in any order: the table is static
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// and needs no initialisation of its own.
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void dbg_register(const char *name, dbg_view_fn fn, const char *what);
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// Run one command line and render the answer. Returns the number of bytes
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// written into `out`.
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//
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// The protocol is lines of text -- see docs/plan/debugger.md for why that and
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// not a typed binary format. Commands:
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//
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// help what this kernel can be asked
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// all every view, in registration order
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// <name> [arg] that view, optionally narrowed
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int dbg_command(const char *line, struct sbuf *out);
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// Every view, straight to the console. This is what sysrq 'g' runs, and it is
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// the level-0 path from the plan: no process, no filesystem, no client.
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void dbg_dump_all(void);
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#define DBG_MAJOR 10
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