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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
commit 1dd4ba0186
35 changed files with 2296 additions and 83 deletions
Download patch file Download diff file Expand all files Collapse all files

93
kernel/dev/procfs.c
View file

@ -17,6 +17,7 @@
#include "types.h"
#include "defs.h"
#include "klog.h"
#include "irq.h"
#include "x86.h"
#include "device.h"
@ -34,77 +35,14 @@
#include "errno.h"
#include "fsstat.h"
#include "schfs.h"
#include "sbuf.h"
#include "dbg.h"
#define PROC_BUFSZ 2048
// --- tiny formatting helpers (no snprintf in the kernel) -------------------
struct sbuf {
char *p;
int len;
int cap;
};
static void
sb_putc(struct sbuf *sb, char c)
{
if (sb->len < sb->cap - 1)
sb->p[sb->len++] = c;
}
static void
sb_puts(struct sbuf *sb, const char *s)
{
while (*s)
sb_putc(sb, *s++);
}
// A string in a fixed-width column, space-padded. cprintf has no width
// specifier and neither does this buffer, so tables build their own columns.
// Over-long values are allowed to run past their column rather than be
// truncated: a misaligned row is easier to read than a mangled name.
static void
sb_pad(struct sbuf *sb, const char *s, int width)
{
int n = 0;
while (s[n]) {
sb_putc(sb, s[n]);
n++;
}
while (n++ < width)
sb_putc(sb, ' ');
}
// Four lowercase hex digits, zero-padded. PCI and USB identifiers are always
// written this way, and a lookup table keyed on "8086:100e" does not match
// "8086:100E" or "8086:10e".
static void
sb_puthex4(struct sbuf *sb, uint32 v)
{
static const char hex[] = "0123456789abcdef";
for (int i = 3; i >= 0; i--)
sb_putc(sb, hex[(v >> (i * 4)) & 0xF]);
}
static void
sb_putu(struct sbuf *sb, uint64 v)
{
char tmp[24];
int i = 0;
if (v == 0) { sb_putc(sb, '0'); return; }
while (v && i < (int)sizeof(tmp)) { tmp[i++] = '0' + (v % 10); v /= 10; }
while (i--) sb_putc(sb, tmp[i]);
}
// "<label><value> <unit>\n" — the shape almost every /proc line takes.
static void
sb_kv(struct sbuf *sb, const char *label, uint64 v, const char *unit)
{
sb_puts(sb, label);
sb_putu(sb, v);
if (unit && *unit) { sb_putc(sb, ' '); sb_puts(sb, unit); }
sb_putc(sb, '\n');
}
// The formatting helpers these files are built out of now live in
// include/sbuf.h, because the debugger's introspection registry needs exactly
// the same thing and two copies would have drifted. See docs/plan/debugger.md.
// --- the files -------------------------------------------------------------
@ -403,6 +341,7 @@ gen_filesystems(struct sbuf *sb)
#define PROC_DEVICES 13
#define PROC_FSSTAT 14
#define PROC_NETIF 15
#define PROC_KMSG 17
// Render global file `id` (a PROC_* constant) into `buf`. Returns its length.
//
@ -446,6 +385,13 @@ proc_gen_global(int id, char *buf, int cap)
int
procread(struct inode *ip, uint64 off, char *dst, int n)
{
// The log is served straight out of its ring rather than regenerated into a
// scratch buffer like everything else here: it is 128 KB, and this function
// renders the whole file on every read into an array on a 4 KB kernel stack.
// See klog_read().
if (ip->minor == PROC_KMSG)
return klog_read(off, dst, n);
char buf[PROC_BUFSZ];
int len = proc_gen_global(ip->minor, buf, PROC_BUFSZ);
@ -502,6 +448,16 @@ proc_sysrq_cmd(char cmd)
cprintf("sysrq: pid 1 asked to exit -- it should come back\n");
break;
case 'g':
// Every registered view, straight to the console.
//
// This is docs/plan/debugger.md's level 0: no process, no filesystem, no
// client program. If /dev/dbg cannot be opened -- because the filesystem
// is what is broken -- this still works, and on a machine with only a
// serial line it is the whole debugger.
dbg_dump_all();
break;
case 'w': {
// Wedge the supervisor: make schinitd hang, on purpose, so PID 1's
// heartbeat timeout is exercised instead of reasoned about. There is no
@ -518,6 +474,7 @@ proc_sysrq_cmd(char cmd)
cprintf("sysrq: unknown command '%c' "
"(known: c = crash, u = dump USB, s = read the USB disk, "
"d = display self-test, t = text self-test, n = net self-test, "
"g = dump every debug view, "
"w = wedge the init supervisor, i = kill pid 1)\n",
cmd);
break;