Race conditions are a big ugly can of worms whose exploitability could probably be the basis for a long, tedious debate.
When people talk about Zig being unsafe, they're mostly reacting to the fact that UAFs are still viable in it.
As you know, buffer bleeds like Heartbleed and Cloudbleed can happen even in a memory safe language, they're hard to defend against (padding is everywhere in most formats!), easier to pull off than a UAF, often remotely accessible, difficult to detect, remain latent for a long time, and the impact is devastating. All your RAM are belong to us.
For me, this can of worms is the one that sits on top of the dusty shelf, it gets the least attention, and memory safe languages can be all the more vulnerable as they lull one into a false sense of safety.
It caught different banking/investment systems written in memory safe languages leaking server RAM. You could sometimes see the whole intranet web page, that the teller or broker used to generate and send the statement, leaking through.
Bleeds terrify me, no matter the language. The thing with bleeds is that they're as simple as a buffer underflow, or forgetting to zero padding. Not even the borrow checker can provide safety against that.
The tool is called Pure [1]. It was originally written in JavaScript and open-sourced, then rewritten for Microsoft in C at their request for performance (running sandboxed) after it also detected David Fifield's “A Better Zip Bomb” as a zero day.
I'd love to rewrite it in Zig to benefit from the checked arithmetic, explicit control flow and spatial safety—there are no temporal issues for this domain since it's all run-to-completion single-threaded.
Got to admit I'm a little embarrassed it's still in C!
I say this as someone who is doing a lot of unsafe for graphics programming - I want to be able to pass a buffer to a shader without necessarily having zeroed out all the memory, in the common case I'm only using some of that buffer to store the scene data etc. I have a safe-ish abstraction for this (BufWriter in piet-gpu, for the curious), but it's still possible for unsafe shaders to do bad things.
For example, another way to think of this is that you have a buffer of initialized memory, containing a view onto some piece of data, from which you serve a subset to the user, but you get the format of the subset wrong, so that parts of the view leak through. That's a bleed.
Depending on the context, the bleed may be enough or it might be less severe, but the slightest semantic gap can be chained and built up into something major. Even if it takes 6 chained hoops to jump through, that's a low bar for a determined attacker.
If there's full initialization then this is just a logic error, no? Apart from some kind of capability typing over ranges of bytes (not very ergonomic), this would be a very difficult subtype of "bleed" to statically describe, much less prevent.
I think you mentioned WUFFS before the edit; I find that approach very promising!
The SafeDocs program and approach looks incredible. Installing tools like this at border gateways for SMTP servers, or as a front line defense before vulnerable AV engine parsers (as Pure is intended to be used), could make such a massive dent against malware and zero days.
"I would consider that type of logic error more or less impossible to defend at the language level, but I can see how analysis tools can be helpful."
This is how I see it too, as a logic error that exposes sensitive memory in a way that can be as unsafe as a UAF. It's a great leveler of languages, and why I get excited about enabling checked arithmetic by default in safe builds, explicit control flow, and minimizing complexity, abstractions and dependencies—this all helps to reduce the probability of semantic gaps and leaks.
Uninitialised memory is considered unsafe. There aren't any loopholes even for "just bytes". You can't accidentally make an uninitialised buffer, and you can't expose one without explicit `unsafe{}`.
Sych unsafe code is typically wrapped in safe interfaces, so the risk is limited to implementation side, and not spread to every usage site.
For filling buffers the language pushes towards use of growable Vec with capacity that tracks how much has been initialised, or to collect straight from an iterator, which is foolproof.
Custom buffers are not even used that often, because things like zip writing or compression use io::Write streaming interface.
let mut buf = vec![0; DEFAULT_BUFFER_SIZE];
let database_frame_len = database_socket.read(&mut buf)?; // use 1
let database_result = Database::parse(&buf[..database_frame_len])?;
let html_template = HtmlTemplate::new(database_result);
let html_len = html_template.write(&mut buf)?; // use 2
socket.write(&buf[..html_len]);
The only advice I can really give to avoid this kind of bug is: don't reuse buffers unless it's actually a bottleneck.
html_template.pipe_to(socket);