About Archive Tags RSS Feed


Entries posted in October 2020

Writing an assembler.

3 October 2020 13:00

Recently I've been writing a couple of simple compilers, which take input in a particular format and generate assembly language output. This output can then be piped through gcc to generate a native executable.

Public examples include this trivial math compiler and my brainfuck compiler.

Of course there's always the nagging thought that relying upon gcc (or nasm) is a bit of a cheat. So I wondered how hard is it to write an assembler? Something that would take assembly-language program and generate a native (ELF) binary?

And the answer is "It isn't hard, it is just tedious".

I found some code to generate an ELF binary, and after that assembling simple instructions was pretty simple. I remember from my assembly-language days that the encoding of instructions can be pretty much handled by tables, but I've not yet gone into that.

(Specifically there are instructions like "add rax, rcx", and the encoding specifies the source/destination registers - with different forms for various sized immediates.)

Anyway I hacked up a simple assembler, it can compile a.out from this input:

.hello   DB "Hello, world\n"
.goodbye DB "Goodbye, world\n"

        mov rdx, 13        ;; write this many characters
        mov rcx, hello     ;; starting at the string
        mov rbx, 1         ;; output is STDOUT
        mov rax, 4         ;; sys_write
        int 0x80           ;; syscall

        mov rdx, 15        ;; write this many characters
        mov rcx, goodbye   ;; starting at the string
        mov rax, 4         ;; sys_write
        mov rbx, 1         ;; output is STDOUT
        int 0x80           ;; syscall

        xor rbx, rbx       ;; exit-code is 0
        xor rax, rax       ;; syscall will be 1 - so set to xero, then increase
        inc rax            ;;
        int 0x80           ;; syscall

The obvious omission is support for "JMP", "JMP_NZ", etc. That's painful because jumps are encoded with relative offsets. For the moment if you want to jump:

        push foo     ; "jmp foo" - indirectly.

        nop          ; Nothing happens
        mov rbx,33   ; first syscall argument: exit code
        mov rax,1    ; system call number (sys_exit)
        int 0x80     ; call kernel

        push bar     ; "jmp bar" - indirectly.

I'll update to add some more instructions, and see if I can use it to handle the output I generate from a couple of other tools. If so that's a win, if not then it was a fun learning experience:



Offsite-monitoring, from my desktop.

20 October 2020 13:00

For the past few years I've had a bunch of virtual machines hosting websites, services, and servers. Of course I want them to be available - especially since I charge people money to access at some of them (for example my dns-hosting service) - and that means I want to know when they're not.

The way I've gone about this is to have a bunch of machines running stuff, and then dedicate an entirely separate machine solely for monitoring and alerting. Sure you can run local monitoring, testing that services are available, the root-disk isn't full, and that kind of thing. But only by testing externally can you see if the machine is actually available to end-users, customers, or friends.

A local-agent might decide "I'm fine", but if the hosting-company goes dark due to a fibre cut you're screwed.

I've been hosting my services with Hetzner (cloud) recently, and their service is generally pretty good. Unfortunately I've started to see an increasing number of false-alarms. I'd have a server in Germany, with the monitoring machine in Helsinki (coincidentally where I live!). For the past month I've started to get pinged with a failure every three/four days on average, "service down - dns failed", or "service down - timeout". When the notice would wake me up I'd go check and it would be fine, it was a very transient failure.

To be honest the reason for this is my monitoring is just too damn aggressive, I like to be alerted immediately in case something is wrong. That means if a single test fails I get an alert, as rather than only if a test failed for something more reasonable like three+ consecutive failures.

I'm experimenting with monitoring in a less aggressive fashion, from my home desktop. Since my monitoring tool is a single self-contained golang binary, and it is already packaged as a docker-based container deployment was trivial. I did a little work writing an agent to receive failure-notices, and ping me via telegram - instead of the previous approach where I had an online status-page which I could view via my mobile, and alerts via pushover.

So far it looks good. I've tweaked the monitoring to setup a timeout of 15 seconds, instead of 5, and I've configured it to only alert me if there is an outage which lasts for >= 2 consecutive failures. I guess the TLDR is I now do offsite monitoring .. from my house, rather than from a different region.

The only real reason to write this post was mostly to say that the process of writing a trivial "notify me" gateway to interface with telegram was nice and straightforward, and to remind myself that transient failures are way more common than we expect.

I'll leave things alone for a moment, but it was a fun experiment. I'll keep the two systems in parallel for a while, but I guess I can already predict the outcome:

  • The desktop monitoring will report transient outages now and again, because home broadband isn't 100% available.
  • The heztner-based monitoring, in a different region, will report transient problems, because even hosting companies are not 100% available.
    • Especially at the cheap prices I'm paying.
  • The way to avoid being woken up by transient outages/errors is to be less agressive.
    • I think my paying users will be OK if I find out a services is offline after 5 minutes, rather than after 30 seconds.
    • If they're not we'll have to talk about budgets ..

| No comments