# Finding out what the kernel is doing Every bug in this project used to be 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. What ended it was a real bug. A DMA block was on two allocators' free lists at once, and finding that took several rounds of that loop, because the kernel could not be asked whether two allocators owned the same page. It could not be asked anything. > **The kernel had no way to answer a question it was not already compiled to > answer.** Three things close that, and none of them is a debugger in the stepping sense. --- ## The rule the shape follows **It observes; it does not control.** No stepping, no software breakpoints, no stopping the world. That is not a scope compromise. Half the bugs this kernel actually produces are *timing*: an interrupt storm at 77,000 a second, a level-triggered line nobody acknowledges, a driver that faults only on its second start, a supervisor that restarts a job fifteen times in forty-five seconds. **Every one of those disappears when you halt the machine to look at it.** And the layering rule, which is the circularity rule the display already has, pointed somewhere else: > 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. So the kernel's half is the minimum that works when nothing else does, and everything convenient is a process. --- ## Naming an address `tools/ksyms.pl` turns the kernel's own symbol table into a table the kernel carries, and `ksym_lookup()` resolves an address through it. A crash chain used to be nine hex numbers: ``` Call chain #0 ffff80000010a9d9 panic+0x5f #1 ffff80000011e07f proc_sysrq_cmd+0x99 #2 ffff80000011e205 proc_write_sysrq+0x6e #3 ffff800000130159 procfs_write+0x59 #4 ffff80000012e566 writei+0x45 #5 ffff800000128a96 filewrite+0x2ec #6 ffff80000016c0fa sys_writev+0x12b #7 ffff80000016986e syscall+0xf6 #8 ffff800000100d2e syscall_trapret ``` ### Two link passes, and why two is enough The kernel is linked twice. Pass one exists to be read; pass two carries the table built from it. The obvious objection is that adding forty kilobytes to the image moves everything the table describes — which is why Linux's `kallsyms` needs three passes and a convergence check. **`.ksym` is placed after `.text` and `.rodata`,** so it cannot move a single address the table records. Two passes, and the second is stable by construction. Not by assumption, though: the build lists the text symbols of both passes and **fails** if they differ. A symbol table that is subtly wrong for every address is worse than none, because it produces confident, false answers. `ksym.c` reads the table through **weak** symbols, so pass one — which has no table — still links, and a tree with the generator removed still boots and simply cannot name anything. --- ## A log that outlives the kernel `cprintf` used to go to a serial line and a screen and then be gone. `panic.c` said so in its own header: *"there is no core dump and no log to go back to."* There is now. Every character that passes through `consputc()` — which is every kernel message, including the whole PSOGM report — is also appended to a byte ring with a timestamp per line. It is readable as **`/proc/kmsg`**. The ring lives in a region carved out before the page allocator is given anything, and it has a magic header, so **a kernel that reboots finds the previous kernel's log still there**: ``` --- boot 1 --- [ 0.00] e820: 1018 MB usable in 7 regions, top 1016 MB (19 entries) ... [ 7.18] PSOGM -- Purple Screen of Guru Meditation [ 7.18] Call chain [ 7.18] #0 ffff80000010a9d9 panic+0x5f ... --- boot 2 --- [ 0.00] klog: 128 KB at ffff80000053e000, 2162 bytes carried over from the previous boot ``` ### Where it lives, and why not where you would put it The obvious place is the top of RAM, beside the DMA arena. That was the first attempt, and on the reboot the magic came back as ASCII — because **the top of usable memory is exactly where the firmware puts its own allocations** before it hands the machine over. It sits immediately above the kernel image instead. `end` is the same address on every boot of the same build, which is the property a region has to have to be found again, and the bytes just past the kernel are the ones the loader has already committed to leaving alone. No hole has to be punched in the allocator: `kinit1`'s lower bound is a parameter. The magic and a header checksum mean a region something else has used is re-initialised rather than printed as if it were a log. **A reboot that does not preserve DRAM loses the log, and says so** rather than showing garbage. Two properties of the writer are deliberate. It **takes no lock** — it runs inside `consputc()`, and a lock there would be a way for the logger to deadlock the thing it exists to report on. And it **buffers before it exists**: the region is carved after `e820_init()`, so the handful of lines before that go into a static array and are flushed in, because a log that is blank for the earliest part of the boot is blank for the part that is hardest to debug any other way. --- ## Asking a question `/dev/dbg` is a conversation: write a line, read the answer. ``` # schdbg dma arena 0xffff80003e8ed000-0xffff80003f8ed000 16384 KB total, 12295 KB free in 3 extent(s), 4088 KB out # base phys bytes 0 0xffff80003e8f3000 0x000000003e8f3000 4091904 1 0xffff80003ecea200 0x000000003ecea200 3584 2 0xffff80003f0d3000 0x000000003f0d3000 8495104 ``` It is text, not a typed binary protocol, and the reason is that the same bytes have to work at every level — including the one where nothing can allocate, frame a record, or wait for a client to acknowledge anything. A protocol you cannot drive by hand is a protocol you cannot debug. Everything `schdbg` does is also: ``` # echo dma > /dev/dbg && cat /dev/dbg ``` ### The registry is the point, not the views A subsystem publishes its state with **one line at the bottom of the file that owns the data**: ```c dbg_register("dma", dma_dump, "the DMA arena's free extents"); ``` Nothing central is edited to add one. That is the difference between something that stays current and something that does not — `/proc/sysrq` is the version that does not, a `switch` in one file with a letter per action, and after two years it holds three self-tests and no state at all. | View | | |---|---| | `dma` | the arena's free extents, in virtual and physical | | `mem` | the page allocator's free list, and whether it has a cycle in it | | `proc` | every process, with state, size and its mmap bump pointer | | `vma` | every mapping a process holds but does not own the pages of | | `claims` | devices, DMA blocks and interrupts held by userspace drivers | | `pci` | the bus, with each device's command register decoded | | `sock` | the socket table, with buffered bytes and descriptors in flight | `all` prints every view; `help` lists them; a view may take an argument, so `vma 4` narrows to one process. Several of those exist because of a specific bug. `vma` is the table that decides whether `exit()` frees a page or leaves it alone, and the bug that made this worth building was a DMA block that was never in it. `claims` shows the three things that go wrong with a userspace driver — a device held by a process that no longer exists, DMA handed out and never returned, an interrupt line left masked with nobody reading it. `pci` decodes the command register because "is bus mastering actually off?" is a question with a wrong answer that looks fine. ### Three ways in, and the one that always works | | Works when | |---|---| | `sysrq g` | always — every view to the console, no process, no filesystem, no client program | | `/dev/dbg` | there is a scheduler and a filesystem | | `schdbg` | there is a shell | Views run in ordinary process context and may take locks. **They must not be called from `panic()`.** --- ## What it does not do - **It does not step, break, or halt.** If a bug genuinely needs a stepper, this is the wrong tool, and that is a deliberate trade rather than an omission. - **No cross-subsystem queries.** *"Who owns physical page `0x3eceb000`?"* — the question that would have ended the investigation that prompted all this — still cannot be asked. Each view answers for its own subsystem and nothing correlates them. That is `whose`, and it is not built. - **No watchpoints.** DR0–DR3 are untouched and `T_DEBUG` maps to `SIGTRAP`, so "something zeroed my page table, tell me who" has no answer. - **No network transport.** The plan puts a TCP bridge in a userspace daemon, deliberately outside the kernel; it does not exist, so the debugger is local or serial only. - **No memory writes**, and no way to poke anything. Reading is what finds bugs. - **`all` truncates at 16 KB** and says so. On a busy machine, ask for one view at a time. - **One conversation at a time.** The answer buffer is global, so two programs asking at once get one answer each in some order — never half of the other's, but not two independent sessions either. - **No userspace symbolization.** The kernel names kernel symbols. A fault in a process still reports a bare address, and resolving it is a host-side job. - **The stack walk stops where the frame pointer stops.** `getcallerpcs()` walks `rbp` chains, so a trap frame in the middle of a chain ends the walk. - **`sysrq g` costs you log history.** Its output goes through `cprintf`, so sixteen kilobytes of views push the same amount out of a 128 KB ring. - **No tracing.** No static tracepoints, no event ring, no counters. "What happened in the two hundred events before this" is not a question this can answer. What remains is in [../plan/debugger.md](../plan/debugger.md).