Weeknote 2021-07

Well. That’s it! Missed a week. Had to happen sooner or later. It’s like scratching a new laptop. After carefully protecting the innocence of a pristine new thing, the first ding has been made and it’s all downhill from here!

Flippancy aside, I’m going to try and maintain a regular cadence with these weeknotes. I’ve enjoyed writing them, the reflection has been useful and I’m looking forward to reading back on them in a years time so see where I am and where I’ve come from.

Anyway. Moving on with this bumper two week update:

I have changed my mind about the snow. I mean, it’s still cold and grim and I hate falling over, but we spent most of the weekend teaching Phoebe how to sled down hills and that was hella fun. Liz managed to find seemingly the last sled in stock anywhere near us, about 10 minutes before closing time on Friday, at our local sports store.

I’m sure they had marked the thing up by about 20% but even still it was worth every penny for how much we used it. It really made our weekend.

We’re slowing getting used to the new mattress. Having a hybrid memory foam mattress is still pretty jarring for me, as when I sit on it I expect it to behave the same as our old sprung mattress and it just doesn’t. That being said, I have started to sleep much better which is a real win.

It was my friend Matt’s birthday. He’s been doing the Lockdown Open Mic Club for a year now, and they did a session for his birthday. They’d been sending a physical birthday card around the country for each of the members to sign, which Matt opened on the night, and they put together a really lovely version of one of his songs.

It was such a lovely night, and I’m so impressed at the Lockdown Open Mic community, they’re a lovely bunch of people from around the country (and a couple in the US I believe).

It almost brought a tear to my eye.

In completely unrelated news, I dusted off and tuned my guitar and started putting a playlist together of songs that I’d enjoy having a crack at. Not that I need more hobbies at this point.

I need to remember what strings are on my acoustic so I can buy some more.

I found this set of Octonauts themed cookie cutters on Thingiverse. Phoebe was very excited to print them with me and we used them together to make Octonaut themed Easter biscuits - they came out alright, even if my brain was so foggy I forgot to put any of the spices in (and later on I found the sugar in the fridge)!

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Went drinking with the lads on Friday Got drunk alone at home in front of a Google Hangout.

The other people who were in the hangout are, however, some of the loveliest people I know and the ensuing laughter cleansed my lockdown darkened soul. Also I learned a new beer pouring trick but I can’t find a link to the Tiktok so y’all gonna have to just trust me.

In work news. I’m currently chasing down a really interesting memory corruption issue.

I may have mentioned this before by the memory layout in Ruby’s GC consists of a region of consecutive 40 byte chunks (called “slots”). I have one slot that contains the object in question and the following three slots contain metadata for the object - we’ll call these chunks collectively ‘the payload’. At the start of the payload region we have a 16 byte area for metadata about the payload, it’s length and so on - and then we go straight into the arbitrary payload data.

We can see this here: this is the main object:

(lldb) p obj
(VALUE) $2 = 0x00000001018137e0
(lldb) rp obj
bits [LM    ]
T_CLASS: [PROMOTED] (struct RClass) $4 = {
  basic = (flags = 0x0000000000001062, klass = 0x00000001018799a0)
  super = 0x0000000101879860
  ptr = 0x0000000101813818
  class_serial = 526
}

This is what a Ruby class looks like. The struct member ptr points to a struct of type rb_classext_t that looks like this:

(lldb) p *((struct RClass *)obj)->ptr
(rb_classext_struct) $5 = {
  iv_index_tbl = 0x0000000000000000
  iv_tbl = 0x0000000100a63340
  m_tbl = 0x0000000100a605c0
  const_tbl = 0x0000000000000060
  callable_m_tbl = 0x0000000000000000
  cc_tbl = 0x0000000000000000
  subclasses = 0x0000000000000000
  parent_subclasses = 0x00000001018798c8
  module_subclasses = 0x0000000000000060
  origin_ = 0x00000001018137e0
  refined_class = 0x0000000000000008
  allocator = 0x0000000000000000
  includer = 0x0000000000000000
}

You can see that the address of obj is 0x1018137e0 and the address of the rb_classext_t struct pointed to by ptr is 0x101813818, if we subtract the address of obj from the ptr address we get 0x38 or 56 bytes. This fits with our knowledge of the memory layout that each slot is 40 bytes wide and the Payload section contains a 16 byte header object.

We can verify this by grabbing the obj and adding 40 bytes and we should get hold of a Payload header object.

(lldb) rp obj + 40
bits [      ]
Not-handled type 0x17
(RBasic *) $10 = 0x0000000101813808

Type 0x17 is the type for the Payload head, I promise. It’s only unhandled because I haven’t made the debugger helper scripts support it yet!

So this is all good. But what if we look at the rb_classext_t struct a little more closely? That 0x60 memory address in const_tbl looks a little bit low compared to all the other memory locations we’ve been dealing with so far.

And sure enough: the crash I’m seeing is a SIGSEGV (memory access violation) when trying to access the const_tbl member at address 0x60.

So what’s setting that value to 0x60? This behaviour is only reproduceable on my branch, but I’m not doing anything with the content of rb_classext_t, just it’s position.

The interesting thing about this bug is that if you count the bytes from the start of the payload slot to the broken memory address you have 16 for the header, + 24, for iv_index_tbl, iv_tbl and m_tbl respectively. These are all unsigned long which on my machine is 8 bytes.

This makes 40 bytes - which is suspicious as it’s exactly the same size as a slot. And, if we then jump forward another 40 bytes we get to the module_subclasses member, and that has also been set to 0x60.

This is all super sus and almost certainly an offset bug, where we’re taking the address at the head of the payload and naively finding the next slot by just adding 40 bytes, and then setting a value there.

The only annoying thing is that I can’t find it, despite almost certainly introducing it.

tl;dr I made the mistake of thinking for a minute, that I could write C. The computer decided to remind me that that was not the case.

Bumper keyboard update this time around! First up, the Nibble kit, which I took a day off to build.

It was a really fun build, although the diodes took ages to get straight. Thoroughly enjoyed the whole process - except lubing and filming the switches, which I normally don’t mind doing but I definitely should not have tried to do in the same day.

It takes ages, and I normally like to be quite slow and thorough with the process, but knowing that I had all those diodes to do while I was doing it really made it feel like mission impossible.

Lesson for next time is definitely split the tasks out and lube and build on different days.

As for the keyboard - I like it, although I’m back to using my HHKB right now. There are a couple of things I’m not sure about:

1. It’s got some pretty gross quality DSA keycaps on it while I wait for my Group buy sets to come in, and I have decided that I don’t like DSA on flat, row staggered boards. It’s just about bearable with an angle, but othat means I need to built/print some bumpers to raise up the board.

2. I think 62g linears are still a bit too heavy for me, although I’m not ruling out the idea that I might like them a lot better with better keycaps on. I’ve got some Cherry MX Browns (eww) in a different board that I might spring swap if I still don’t like them with the new keycaps.

   I’m probably going to find some tactile switches for my next build though. I’m veering towards ergo clears or maybe something panda-esque.

3. I’ve been using this for a week and I’m still not used to a column of keys (and an encoder) to the left of the main block. It turns out I have been anchoring with the left shift and the escape (or `) key for years and didn’t realise it.

Anyway: Here it is in all it’s glory.

After a flash of inspiration I put together a 45% keyboard design and PCB. It’s based on a ProMicro. An all-in-one AVR based development board so it’s easy mode for PCB design. No worrying about differential signalling down the USB bus or worrying about whether I can place the crystal close enough to the MCU. All I have to do with a ProMicro is route the matrix, which took a couple of hours at most. I’m pleased with how it came out.

I also wanted to experiment with silkscreen artwork so I dropped in a jpg of Megumin, probably my favourite character so far from very silly anime Konosuba (which I’m idly watching a few episodes at a time whenever I find myself doing the ironing).

Final Version I sent off to JLCPCB looks like this:

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I originally wanted it matte black, just with the image in red, but JLCPCB only support white silkscreen printing, so I just YOLO’d the whole PCB in red instead. I hope it comes out a reasonable colour.

And the layout it uses is this:

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This is basically a meme by keyboard community standards. A Winkeyless, big bar, weeb-themed 40%, with a top-right Kyuu style blocker, and I’m here for it!

No case design yet. I’m thinking something 3D printed, or maybe clear acrylic layered to show of the garish PCB to it’s full glory. Who knows maybe V2 will even have RGB ;)

While I was at JLCPCB I also got together some gerbers and ordered the latest version of the CRKBD Light, a minimalist version of the fantastic CRKBD.

I’ve used a CRKBD for a while - it was my first build and I’ve wanted to build a Corne Light since foostan posted this tweet so it seemed rude not to while I was at JLCPCB and everything.

Who knows when I’m going to get round to building either of these. But maybe next week I can at least get some switches ordered.

The final cherry on top of the last two weeks is that the weather has finally started warming up again! Spring is on it’s way.

Warm enough that that on Sunday, I spent most of the day in the garden, playing with Phoebe, and neither of us wore coats. We went bug hunting, planted some seeds, played a lot of hide and seek and spent time on her swing and slide.

It’s hard for me to communicate just how much of a blessing it is to be able to watch your kid play, uninhibited and unrestricted, in a safe outside space. That together with the warmth of the early spring sun has really lifted my spirits.

I’ve always been grumpy about gardens. I have never liked gardening, or anything that required outdoor dirty work, and have always viewed them as way too much work for not enough reward.

But I will admit that after a year of severely restricted travel and opportunity, and a winter where even the garden was unusable, that I love my garden. I’m very lucky to have it - and I’m actually excited by the thought of getting out there and doing the work to make it nice for the kids growing up.

I still haven’t done the OpenGraph thing.