From a57f2c25de23f682c3faee35aea4fe084151bddd Mon Sep 17 00:00:00 2001 From: antifallobst Date: Sun, 12 Feb 2023 10:22:25 +0100 Subject: [PATCH] docs: documented Panic screen output --- .wiki/Kernel-documentation.md | 56 +++++++++++++++++++++++++++++++++++ 1 file changed, 56 insertions(+) diff --git a/.wiki/Kernel-documentation.md b/.wiki/Kernel-documentation.md index d904f9e..8b18b4a 100644 --- a/.wiki/Kernel-documentation.md +++ b/.wiki/Kernel-documentation.md @@ -45,6 +45,62 @@ When the Computer is in paging mode, only mapped pages are accessible. Now every Process gets its own page table and tada: we have successfully isolated the processes from each other, because every process can only access the data that it needs to access. +## Panic screen +When a fatal / not recoverable error occurs, the kernel panics. It logs panic information and then halts forever. +Such a panic log can look like the following one: +``` +[ Error ] !=====[ KERNEL PANIC ]=====! +Interrupt ID: 0x0E +Error Code: 0b00000000000000000000000000000010 +Error Message: Page Fault + +Paging Info: + Page Map: 0x000000000FB0A000 + +CPU Flags: + Parity + Sign + +CPU Registers: + RIP: 0xFFFFFFFF8000027A RAX: 0x0000100000079838 RBX: 0x0000000000000000 + RCX: 0xFFFFFFFFFFFFFFD8 RDX: 0x0000000000000000 RSI: 0x0000000000000000 + RDI: 0x0000100000079838 RBP: 0xFFFF80000FB1AF10 RSP: 0xFFFF80000FB1AEF0 + +[ Warning ] !=====[ HALTING SYSTEM ]=====! +``` +but what does it say? + +In most cases, a panic occurs while handling an interrupt. +If this is the case, we will have the state of the cpu while it was interrupted. +This cpu state provides us very much information. + +`Interrup ID` tells us, which interrupt caused the panic. +In this case the ID is `0x0E`, a `Page Fault Exception`. + +`Error Code` is a binary representation of the 32 least significant bits of the error code pushed by some interrupts. +If an interrupt pushes no error code, this will be just zeros. +In our example the code tells us, that the error happened because of a write attempt to a not present page. + +`Error Message` tells us, what happened. + +`Paging Info` contains all information about paging. +At the moment, this is just the physical address of the loaded page map. + +`CPU Flags` contains information about which bits are set in the CPU status register. +If this block doesn't appear, there are no bits set. + +`CPU Registers` contains the data, in the main cpu registers. +This is probably the most interesting block, because you get very detailed information out of here, +if you know what each of these registers does in the cpu. + +### Panic without interrupt +If the panic wasn't caused by an interrupt, it has no cpu_state, and because of that it has no detailed info about the execution state. +In this rare case, you will get the following message: +``` +No detailed Information available (cpu_state null reference) +``` +The `Error Message` could still be helpful, but good luck finding that bug. + ## Format strings Format strings are strings that are formatted at runtime. They are created by defining a pattern, like the following one: