How a 20 year old bug in GTA San Andreas surfaced in Windows 11 24H2
(cookieplmonster.github.io)1321 points by yett a day ago
1321 points by yett a day ago
Raymond is a wizard. Read his blogs for many years and love his style and knowledge.
He's a total legend, yet apparently he's never met Bill Gates in person from what he said in an interview in the Dave's Garage YouTube channel a few years ago. You'd think that someone who's been that prominent for so long in the company would have been invited to a company dinner where he was present or something.
He has stories on his blog about windows 2 iirc, so there was an overlap from a time where they were still relatively small. So I think it's a bit odd they never talked or met.
Small thing but I love the effort he puts into actually coding up his examples instead of screenshots. For example: https://devblogs.microsoft.com/oldnewthing/20250414-00/?p=11...
He has many better ones but that's the latest one I've seen
Raymond knows everything. From microcode bugs on Alpha AXP to template meta programming to UI.
I wonder how many times a Deloitte, PwC, KPMG, Bain, EY, McKinsey, or BCG consultant naively tried putting him on a shortlist for being “impacted” over the years because he was in the Top X of a spreadsheet sorted on Y.
"Look this guy's job seems to be mainly writing blog posts. We could replace that with AI and get it to regularly pitch the new Visual Enshitify 2.0 product launch as a bonus. Win win win!"
IMHO, if something isn’t part of the contract, it should be randomized. Eg if iteration order of maps isn’t guaranteed in your language, then your language should go out of its way to randomize it. Otherwise, you end up with brittle code: code that works fine until it doesn’t.
There are various compiler options like -ftrivial-auto-var-init to initialize uninitialized variables to specific (or random) values in some situations, but overall, randomizing (or zeroing) the full content of the stack in each function call would be a horrendous performance regression and isn't done for this reason.
There are fast instructions (e.g., REP STOSx, AVX zero stores, dc zva) and tricks (MTE, zero pages), but no magic CPU instruction exists that transparently and efficiently randomizes or zeros the stack on function calls. You think there would be one and I bet there are on some specialized high-security systems, but I'm not sure even where you would find such a product. Telecom certainly isn't it.
There are proposed cpu architectures that work that way, like the Mill <https://millcomputing.com/>. Where most cpus support multiple calling conventions the Mill enforces a single calling convention in hardware. There is a hardware `call` instruction that does all the work directly, along with a corresponding `ret` instruction for returning from a function call. It also uses its equivalent of the TLB to ensure that each function is only granted permission to read from that portion of the stack which contains its arguments; any attempt to read outside that region would result in a permission error that causes the read to return a NaR (Not a Result, akin to a floating point NaN).
As an additional protection, new stack frames are implicitly zeroed as they are created. I assume this is done by filling the CPU cache with zeros for those addresses before continuing to execute the called function. No need to wait for actual zeros to be written to main memory.
This is really interesting—how do stack references work in this design?
You couldn't do random, but with a predictable performance hit to memory, cache and write-line use stack addresses COULD be isolated for a program, for a library, etc.
It'd be expensive though; every context switch would require it's own stack and pushing / restoring one more register. There's GOOD reason programs don't work that way and are supposed to not rely on values outside of properly initialized (and not later clobbered) memory.
CPUs already special case xor reg,reg as zeroing out the register, breaking any data dependency on it. If zeroing bits of the stack were common enough, I'd believe CPUs could be made that handled it efficiently (they already special case the stack; push/pop)
I'm a bit distant from this stuff, but it looks like C++26 will have something like -ftrivial-auto-var-init enabled by default. See the "safe by default" section of [1].
For reference, the actual proposal that was accepted into C++26 is [2]. It discusses performance only in general, and it refers to an earlier analysis [3] for more details. This last reference describes regressions of around 0.5% in time and in code size. Earlier prototypes suggested larger regressions (perhaps even "horrendous") but more emphasis on compiler optimizations has brought the regression down considerably.
Of course one's mileage may vary, and one might also consider a 0.5% regression unacceptable. However, the C++ committee seems to have considered this to be an acceptable tradeoff to remove a frequent cause of undefined behavior from C++.
[1]: https://herbsutter.com/2024/08/07/reader-qa-what-does-it-mea...
[2]: https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2024/p27...
[3]: https://open-std.org/jtc1/sc22/wg21/docs/papers/2023/p2723r1...
Microsoft's Visual C++ compiler has the /Ge compiler option ( see https://learn.microsoft.com/en-us/cpp/build/reference/ge-ena... ) Deprecated since VC2005.
This compiler option causes the compiler to emit a call to a stack probe function to ensure that a sufficient amount of stack space is available.
Rather than just probe once for each stack page used, you can substitute a function that *FILLS* the stack frame with a particular value - something like 0xBAADF00D - one could set the value to anything you wanted at runtime.
This would get you similar behaviour to gcc/clang's -ftrivial-auto-var-init
Windows has started to auto-initialize most stack variables in the Windows kernel and several other areas.
The following types are automatically initialized:
Scalars (arrays, pointers, floats)
Arrays of pointers
Structures (plain-old-data structures)
The following are not automatically initialized:
Volatile variables
Arrays of anything other than pointers (i.e. array of int, array of structures, etc.)
Classes that are not plain-old-data
During initial testing where we forcibly initialized all types of data on the stack we saw performance regressions of over 10% in several key scenarios.
With POD structures only, performance was more reasonable. Compiler optimizations to eliminate redundant stores (both inside basic blocks and between basic blocks) were able to further drop the regression caused by POD structures from observable to noise-level for most tests.
We plan on revisiting zero initializing all types (especially now that our optimizer has more powerful optimizations), we just haven’t gotten to it yet.
Randomization at this level would be too expensive. There are tools that do this for debug purposes, and your stuff runs a lot slower in that mode.
I had to Google to find the tid bit that I read about Perl years ago. I think this will affect iteration order of dicts.
> Nov 22, 2012 — Perl 5.18 will introduce per process hash randomization and almost certainly will feature a new hash function.it probably shouldn’t be a “release” thing. actually, certainly. i do wonder how many bugs would never have seen the light of day, if someone’s “set” actually turned out to be a sequence (i.e. allowed duplicate values) resulting in a debug build raising an assert.
Debug builds are worthless for catching issues. How many people actually run them? Perhaps developers run debug builds of individual binaries they're working on when they're trying to repro a bug, but my experience at every company of every size and position in the stack (including the Windows team) is that no one does their general purpose use on a debug build.
Regarding contracts, there's an additional lesson here, quoting from the source:
> This is an interesting lesson in compatibility: even changes to the stack layout of the internal implementations can have compatibility implications if an application is bugged and unintentionally relies on a specific behavior.
I suppose this is why Linux kernel maintainers insist on never breaking user space.
Nope. You have to remember https://www.hyrumslaw.com/
With a sufficient number of users of an API,
it does not matter what you promise in the contract:
all observable behaviors of your system
will be depended on by somebody.
And then you can never remove it!
Semi-related: this type of thing is actually covered in the Site Reliability Engineering book by Google. They highlighted a case of a system that outperformed its SLO, so people depended on it having 100% uptime. They "fixed" this by injecting errors to go closer to their SLA, forcing downstream engineers to deal with the fact that the dependent services would sometimes fail for no reason.
I know it's easier said than done everywhere, just found it to be an interesting parallel.
> If you promise randomization
You don't. You say the order is undefined.
You can pay for those features in debug mode or in chaos monkey mode. It's okay to continue to not pay for them in release mode. Heck, Rust has this approach when it comes to handling integer overflow - fully checked in debug mode, silent wraparound in release mode.
In Ada you can pay for integer overflow checks (runtime) if you want to. With Ada SPARK you can prove that your code does not contain integer overflows so that you don't need runtime checks.
And you can disable these checks with a flag when it comes to Ada, and yeah, with SPARK, none of it happens at runtime.
Check the table at https://docs.adacore.com/spark2014-docs/html/ug/en/usage_sce..., look for "SPARK builds on the strengths of Ada to provide even more guarantees statically rather than dynamically.".
More reading:
https://docs.adacore.com/spark2014-docs/html/ug/en/tutorial....
https://learn.adacore.com (many books for learning Ada and SPARK) available in PDF, EPUB, and HTML format.
However, the compiler does not tell you this. We're back to the problem that it's possible to have a "working" C program that relies on UB and will therefore break at some point, but the tools will not yell at you for doing this. Whereas in Java or C# you get warnings or errors for using maybe-uninitialized variables.
Also, scanf should be deprecated. Terrible API. Never use scanf or sscanf etc. We managed to get "gets()" deprecated, time to spread that to other parts of the API.
atoi() or atof() etc. work OK, but really you need a parser.
Not really the ethos of C(++), though of course this particular bug would be easily caught by running a debug build (even 20 years ago). However, this being a game "true" debug builds were probably too slow to be usable. That was at least my experience doing gamedev in that timeframe. Then again code holding up for 20 years in that line of biz is more than sufficient anyway :)
I once updated a little shy of 1mloc of Perl 5.8 code to run on Perl 5.32 (ish). There were, overall, remarkably few issues that cropped up. One of these issues (that showed itself a few times) was more or less exactly this: the iteration order through a hash is not defined. It has never been defined, but in Perl 5.8 it was consistent: for the same insertion order of the same set of keys, a hash would always iterate in the same way. In a later Perl it was deliberately randomised, not just once, but on every iteration through the hash.
It turned out there a few places that had assumed a predictable - not just stable, but deterministic - hash key iteration order. Mostly this showed up as tests that failed 50% of the time, which suggested to me a rough measure of how annoying an error is to track down is inversely correlated with how often the error appears in tests.
(Other issues were mostly due to the fact that Perl 5 is all but abandoned by its former community: a few CPAN modules are just gone, some are so far out of date that they can't be coerced to still work with other modules that have been updated over time. )
Aren't you just creating another contract? Users might write code that depends on it being random.
Maybe it would be good to change all non promised things between releases. So that such unwritten rules never become something users rely upon.
Then you are wasting runtime clock cycles randomizing lists.
Any sane language would design a list iterator to follow the order of the list. No, the difference is when you're iterating over orderless hash-based sets or maps/dictionaries. Many languages choose to leave the iteration order undefined. I think Python did that up to a point, but afterward they defined dictionaries (but not sets) to be iterated over in the order that keys were added. Also, some languages intentionally randomize the order per program run, to avoid things like users intentionally stuffing hash tables with colliding keys.
> Also, some languages intentionally randomize the order per program run, to avoid things like users intentionally stuffing hash tables with colliding keys.
Most modern langages do that as part of hashdos mitigation, Python did that until it switched to a naturally ordered hashmap, then made insertion order part of the spec. Importantly iteration order remains consistent with a process (possibly on a per-hashmap basis).
Notably, Go will randomise the starting point of hashmap iteration on each iteration.
I am pretty sure there are trivial impls of sets with guaranteed iteration order in Python that use an underlying ordered map and a dummy value in each entry.
Yep, and then you get crash reports you can’t reproduce.
Same can be said about pointer addresses (random for each run). But ASLR exists for a specific reason.
> Not ignore the compilation warnings – this code most likely threw a warning in the original code that was either ignored or disabled!
What compiler error would you expect here? Maybe not checking the return value from scanf to make sure it matches the number of parameters? Otherwise this seems like a data file error that the compiler would have no clue about.
Undefined behavior to access the uninitialized memory. A sanitizer would have flagged that.
The compiler has no way of knowing that the memory would be undefined, not unless it somehow can verify the data file. The most I think it can do is flag the program for not checking the return value of scanf, but even that is unlikely to be true since the program probably was checking for end of file which is also in the return value. It was failing to check the number of matched parameters. This is the kind of error that is easy to miss given the semantics of scanf.
> The compiler has no way of knowing that the memory would be undefined
Yes it would. -fsanitize=address does a bunch of instrumentation - it allocates shadow memory to keep track of what main memory is defined, and it checks every read and write address against the shadow memory. It is a combination of compile-time instrumentation and run-time checking. And yes, it is expensive, so it should be used for debugging and not the final release.
https://clang.llvm.org/docs/AddressSanitizer.html , https://learn.microsoft.com/en-us/cpp/sanitizers/asan?view=m...
Yeah, the debugging here is great, but the actual cause is super mild.
Good point. When reading, I kind of just assumed the "use of initialised memory" warning would pick this up.
But because the whole line is parsed in a single sscanf call, the compiler's static analysis is forced to assume they have now initialised. There doesn't seem to be any generic static analysis approach that can catch this bug.
Though... you could make a specialised warning just for scanf that forced you to either pass in pre-initilized values or check the return result.
I don't think they will get more rare; there will always be a top % of engineers that do deep dives. I hope anyway.
But AI won't replace them, nor did the past 50+ years of software development innovation. There's millions (tens of millions?) of higher programming language developers that don't know the difference between stack or heap besides maybe some theory they half remember from school but they don't care because they don't have to think about it for their day job.
If your whole career will be using higher order languages with very little data stored on stack (vs heap), why should those programmers care? It seems like normal progression of more abstraction in the tools that we use. Similarly, I have programmed a lot of C and C++ in my career and I never once need assembly language. (I am expecting someone to pop in the convo here and tell me about how I am a terrible C/C++ programmer because I don't know any assembly.)
We have been seeing this shift for a while, where "software engineers" graduate from 3 month bootcamps. Except now most likely they will not be earning 500k making crud apps.
What about the incredible front end Devs that only know JS/CSS/HTML? They can still be true craftspeople in their art, be it cross-browser/platform issues or performance tweaking.
Compare python devs of today to fortran devs of the 60s. Something like that distance. Maybe more. But the trend isnt new.
When I worked at Microsoft and I had downtime I would sometimes read the code for app compatibility shims out of pure curiosity.
Win9x video games that made bad assumptions about the stack were a theme I saw. One of the differences between win9x and NT based windows is that kernel32 (later kernelbase) is a now user mode wrapper atop ntdll, whereas in the olden days kernel32 would trap directly into the kernel. This means that kernel32 uses more user mode stack space in NT. A badly behaving app that stored data to the left of the stack pointer and called into kernel32 might see its data structures clobbered in NT and not in 9x. So there were compatibility hacks that temporarily moved the stack pointer for certain apps.
For anyone with access issues
https://web.archive.org/web/20250423144746/https://cookieplm...
> all these findings prove that the bug is NOT an issue with Windows 11 24H2, as things like the way the stack is used by internal WinAPI functions are not contractual and they may change at any time, with no prior notice.
This reminds me of an excellent article I read a while back, the gist of it was that, given sufficient success, there's no such thing as a private API.
Could you please find this article and link it here. I'm curious about the arguments.
Am I the only one to be annoyed by this...?
while (this->m_fBladeAngle > 6.2831855) { this->m_fBladeAngle = this->m_fBladeAngle - 6.2831855; }
Like, "let's just write a while loop that could turn into an infinite loop coz I'm too lazy to do a division"
I want to assume that the GTA developers did this hack because it was faster than floating point division on the Playstation 2 or something.
But knowing they were able to they were able to blow up loading GTA5 by 5 minutes by just parsing json with sscanf, I don't have much hope.
IIRC the whole parsing performance issue was because the original code was written for the SP campaign of GTA5 that only had a handful of objects to parse data for. That was barely a blip in terms of performance impact and AFAIK was written years before GTAOnline was made (where it became an issue - and even then only became an issue much after GTAOnline was first made).
Writing some simple code that works with the data you expect to have without bothering with optimizations is fine, if anything it is one of the actual cases of "premature optimization": even with profiling no real time is spent on that code, your data wont make it spend any time and you should avoid wild guesses since chances are you'll be wrong (even if in this case it could be a correct guess, it'd be like a broken clock guessing the time is always 13:37).
The actual issue with that code was that, after they reused it for GTAOnline and started becoming a performance issue after some time as they added more objects, nobody thought to try and see what is wrong.
They were not the only one to make that mistake e.g. rapidjson had to fix the same error, few people expect parsing one token out of sscanf to strlen the entire input (not only that but there are c++ APIs which call sscanf under the hood).
The second error of deduplicating values by linear scanning an array was way more egregious.
The real, systemic error is that dozens(?) of engineers worked on that product, supposedly often testing the online component and experiencing that wait time first hand; and none thought "wait, parsing JSON doesn't take that long, computers are fast! what's going on?"
I think someone estimated that error cost them millions in revenue? I'm pretty sure a fraction of that could afford an engineer who knows how fast computers ought to be.
GTA was never my wheelhouse, but from what I gathered GTA Online didn't have that much support, and since it was only the initial loading time, and it would have increased over time as the shop content increased, and a very fast machine (e.g. a dev machine) would have had less of an issue, the engineers working on it were probably not that incentivised to dig into it.
Like, even though it's pretty critical to initial user experience initial loading time is generally what gets disregarded the most.
> I'm pretty sure a fraction of that could afford an engineer who knows how fast computers ought to be.
It can, if someone cares enough or realises it's an issue, and then someone is motivated enough to dig into it, or has the time to.
I'm willing to bet it was was done for performance reasons, subtraction is cheaper than float point division. Probably the compiler also has some tricks to optimize this further.
There is absolutely no way this could turn into an infinite loop. It could underflow, but for that to happen angle would have to be less than the 2*pi, therefore exiting the loop.
The article discusses how that turns into an infinite loop and causes a hang.
When you subtract a small float from a very large float, the value doesn't change. This is because the "steps" between float values increase with the size of the value (i.e. floats have coarser resolution for larger magnitudes)
To see this in action, try running the following in a JavaScript interpreter:
console.log(1_000_000_000_000_000_000 - 1);
Long shot, but maybe if the value is small, then this loop could be faster than division.
Sure the lower bound is nicer here. But when the tradeoff includes an unlimited upper bound it's not a very attractive option.
I guess the most robust code handling both performance and unexpected input would be one iteration of this (leveraging the assumption that angles are either always within the bounds, or had one frame of going out of bounds by a small amount); followed by a fmod if that assumption is just totally off.
Much love to Silent, who’s been improving my favorite game for over... a decade now?
My takeaway, speaking as someone who leans towards functional programming and immutability, is "this is yet another example of a mutability problem that could never happen in a functional context"
(so, for example, this bug would have never been created by Rust unless it was deeply misused)
This is more of a problem of the C/C++ standard that it allows uninitialized variables but doesn't give them defined values, considering it "undefined behavior" to read from an uninitialized variable. Java, for example, doesn't have this particular problem because it does specify default values for variables.
But it's this and many other features of C/C++ that make it faster than Java. C/C++ developers really don't want to "pay" for something for safety.
Though, I really like the _mm_undefined_ps() intrinsics for SSE that make it clear that you're purposefully not initialising a variable. Something like that for ints and floats would be pretty sweet.
It is definitely not the case that magically safer is slower. IMO too often the attitude from WG21 (the c++ language committee) has been "Some fast things are unsafe, therefore if we make our language more unsafe it will go faster" which... that's not how implication works.
As a very high level example, take sorting. Rust's standard library provides you both a stable and unstable sort, as does your C++ standard library.
The C++ standard promises these sorts have O(n log n) performance, it's unclear in modern C++ if having a nonsensical ordering† is Undefined Behaviour (as it was in older versions) or outright IFNDR (much worse than UB) but the real world effect will be similar anyway
Rust promises that these sorts work as expected, if you provide nonsensical ordering, obviously it can't very well "sort" things the way you asked, but we don't need to kill your neighbour's cats and wipe the hard disk either, so, it will either give you back the same things in... some order or it will report the fatal error in your software.
The Rust option here is clearly much safer right? So, how much performance is this costing? Actually, it's faster. So C++ is choosing slower and worse. What's the upside?
† For example what about if I insist that Red < Green, but also Green < Red, and furthermore Red == Green is true, but so is Red != Green, however neither Green == Red nor Green != Red are true!
Statically proving the variables get initialized wouldn't change the performance except by making sure you check the return value of sscanf, or turning refusal to check into a couple register wipes. Either way, that's a negligible increase to a hefty function call. It wouldn't require default initializing variables in all circumstances.
When I think of the "no runtime cost" mentality of C/C++ I don't think that normally extends to ignoring errors in I/O functions.
And yet, there is a good chance that C++ will start doing exactly this [1]. Because [2]:
> The performance impact is negligible (less that 0.5% regression) to slightly positive (that is, some code gets faster by up to 1%). The code size impact is negligible (smaller than 0.5%). Compile-time regressions are negligible. Were overheads to matter for particular coding patterns, compilers would be able to obviate most of them.
> The only significant performance/code regressions are when code has very large automatic storage duration objects. We provide an attribute to opt-out of zero-initialization of objects of automatic storage duration. We then expect that programmer can audit their code for this attribute, and ensure that the unsafe subset of C++ is used in a safe manner.
> This change was not possible 30 years ago because optimizations simply were not as good as they are today, and the costs were too high. The costs are now negligible.
[1] https://github.com/cplusplus/papers/issues/1401
[2] https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2023/p27...
Thanks for the references - that was interesting reading, particularly that initialisation can be good for instruction pipelining.
A trick we were using with SSE was something like
__m128 zero = _mm_undefined_ps(); zero = _mm_xor_ps(zero, zero);
Now we were really careful with viewing our ops as data dependencies to reason about pipelining efficiency. But our profiling tools were not measuring this.
We did avoid _mm_set_ps(0.0f) which was actually showing up as cache misses.
I wonder if we were actually slower because cache misses are something we can measure?!
I think the response to that would be: yes but the game would simply not have been made if it wasn't written in C++. That's not to say you couldn't or that you can't make something like GTA:SA in Rust in 2025 or in a safer different language in the early 2000s. It just would take a great deal more time and expense as you'd have needed to construct a lot of tooling and do a lot of training to ensure all of the employees were up to speed before getting started. C++ was, and I think to some extent still is, the lingua franca of the gaming industry - there are some fun exceptions (Naughty Dog implementing much of Crash Bandicoot in a home-grown LISP, and presumably dozens or hundreds of DSLs and other little bespoke scripting languages in use at other studios).
And that's not to mention the uncomfortable truth that while doing this correctly in something like Rust may very well take less effort overall than in C++, that is not the bar we are aiming to clear. They wanted to implement something that was correct-enough, and given that this bug wasn't hit for 20+ years and that the game was a roaring success on all the major platforms - I think that was the right decision.
We don't have enough information to claim it's the "right decision" only that this choice did work, not that other choices couldn't have been better.
In video games you can go back and try another option but life isn't like that and so we can only suppose what might have happened.
Well what happened was that despite being based on an aging Renderware engine and programmed using a language with many potential footguns, the game was stable enough across multiple platforms, architectures and OSes that it was both a critical and commercial success.
I know what you’re saying - you can’t really know what might have been in an alternate reality. But in that alternate reality they’d have had to come up with something truly monumental to outdo themselves here.
I think you’re just being a wee bit picky about me using the words “the right decision”. If we’re honest with ourselves there probably wasn’t a Rust-like language in the conversation when they set out to build GTA3, Vice City or San Andreas so this is all kind of moot unless we're suggesting that Rockstar should have started out by building that language...
I'd actually say that Rust is a third option between "everything is immutable" and "mutable soup". Rust is more of "one mutator at a time". Because, Rust really embraces being able to mutate stuff (so not functional in that sense), it just makes sure that it's in a controlled way.
There'd be a lot less Rust evangelism on this site if there were less UB bug outcomes on this site.
If your attitude is just "I'm not going to use abc because too many people say it's good", without even just trying that out first hand to verify those claims, I don't think you can go very far in your technical skills.
The best engineers I know are open to everything and played with almost every tool/language/whatever to form (sorry) informed opinions about them. They often know what they are talking about, and they choose the best tool for the job.
I think I can articulate what the comment means in a way that may make you rethink what you've said a little bit. I'm not wanting to make you think Rust is bad (I personally think it is good) I'm just trying to show you why this person may not be as backwards as you think they are.
So the person in question is irritated at an interesting blog post about a 20+ year old game being used as another opportunity to push Rust. So for starters Rust obviously wasn't around at the time the game was developed so it's not like Rockstar made the wrong call in implementing this using C++. But more importantly I don't think Rust is currently in a state where studios can justify using it to develop AAA games. They'd need big teams of developers with Rust experience who are well-versed in the sort of problems encountered during game development. You'd need battle-tested build/deployment processes that allow you to produce the binaries for Playstation/Xbox (not too dissimilar CPU/GPU wise, but each with their own platform-level quirks no doubt) and Switch hardware - potentially across multiple generations. You'd need various platforms' OS hooks and network-service APIs available. Additionally you'd need to convince the guys with the money that instead of spending $projected on a game, you'd need to spend $projected+$mystery_number when they take the plunge and write their first game in Rust with new tools etc rather than C++ and everything they currently use. The gaming industry is nothing if not ruthless at making money, if it made financial sense they'd be moving to Rust already - if it will make sense in the future, they'll be planning to do it.
You've been charitable in your read of the original comment, taking it as "this family of problem does not exist in Rust" - and for what it's worth I agree and really value this. However this other commenter has presumably seen it as a bit more naive and missing the bigger picture, and in combination with other similar experiences is questioning the value of these of glowing testimonies.
In addition, a lot of people saying "this is great, this is the future!" doesn't necessarily make something good automatically. For about 5+ years here on HN we had legions of people responding "blockchains will fix this" to almost every problem and very confidently declaring the rest of us are luddites for not getting it. I'm obviously not saying Rust is the same, I'm just trying to show that not following the crowd doesn't automatically mean you're the kind who will always fall behind.
As for how to avoid this? I dunno if you can undo the zillions of RIIR comments that have been floating around since Rust appeared on the scene, but if I was evangelising or even just strongly recommending it I'd just keep in mind that my target audience is maybe sick of seeing the same kinds of comments and would be a bit more creative and/or sensitive in approaching the topic.
While they did mention rust, the actual suggestion was "functional programming and immutability", which to me suggests several other languages first and makes it not really rust evangelism.
FWIW I think a linter or other similar code quality checker would have caught this as well. From a practical perspective (e.g., how do you prevent this from happening again in your game studio's multi-million line code base) that would have been the right thing to do here.
The code would have failed because you can't use an uninitialized variable, so you would have had to set it to a default. You don't just get random garbage from the stack.
You can write a genuine uninitialized local variable in Rust, it's just that you wouldn't do it out of laziness because while in C that's the default in Rust it's a lot of extra work to say "No, I really don't want to initialize this variable" and Rust is like "I mean, if you insist, all I can do is warn you that's a terrible idea".
int k; // C makes an uninitialized variable named k - probably bad idea
let k: i32 = unsafe { MaybeUninit::uninit().assume_init() }; // Rust, same bad idea
But to actually make a variable and not initialize it, as we saw above, is a lot of extra work in Rust because like... that's a bad idea, why would you be setting out to do that?
This is such a bad idea that Rust's unsafe std::mem::uninitialized, which is how they did this before MaybeUninit existed, was de-fanged (giving it poor performance by actually writing a pattern to RAM every time) and deprecated so you get a warning if you try to use it even though it was already marked unsafe. See, people (and I'm sure many C programmers are like this) tend to imagine it's OK for say an integer to be uninitialized because surely any possible value is OK, right ? Nope. Your operating system knows that data was never written, and so it feels entitled to fuck you about if you expect it to stay unchanged, because it never promised that will work - as a result rarely but sometimes you get kicked in the head by the OS and you get a seemingly impossible bug.
It's more that functional languages just happen to be stricter in various ways that would've mitigated against this. You could quite happily design a functional language that has an unsafe equivalent to sscanf in its stdlib, or has big parts of the spec which are "undefined behaviour" that may differ depending on the underlying OS/compiler/runtime/stdlib in use. But the more popular functional languages have gained traction in part because they tend to have a "if you model the types correctly, the program basically works" philosophy around them. I don't think things like Haskell, Ocaml or F# would allow this if you wrote idiomatic code, you'd probably need to do something a little hacky or sketchy.
I've never heard of a functional language that would allow you to initialize a value to whatever value the system memory already had in that memory location. In languages that allow nil, it would at least be nil; in languages that don't, you would have gotten an error about an uninitialized and undefaulted value. In any typed language, you would have also gotten an error.
It's true that C may be unique-ish in this regard though- this bug also couldn't happen in Ruby, which is not a functional language, but Ruby certainly still makes undefined behaviors much more possible than in other languages like Elixir.
Knowing C/C++, I more or less guessed what's happening (uninitialized variable) early in the blog post.
It blows my mind that the languages allow you to leave variables uninitialized which has caused countless bugs (including production bugs that I have seen first hand), and you often need to rely on additional compiler flags or static analysis tools/valgrind etc to catch them. Even though newer languages often use a different solution (default zero value or must initialize a variable before use), people still go back to C/C++ all the time.
> all these findings prove that the bug is NOT an issue with Windows 11 24H2, as things like the way the stack is used by internal WinAPI functions are not contractual and they may change at any time, with no prior notice. The real issue here is the game relying on undefined behavior (uninitialized local variables), and to be honest, I’m shocked that the game didn’t hit this bug on so many OS versions, although as I pointed out earlier, it was extremely close
This sentence is the real takeaway point of the article. Undefined behavior is extremely insidious and can lull you into the belief that you were right, when you already made a mistake 1000 steps ago but it only got triggered now.
I emphasized this point in my article from years ago (but after the game was released):
> When a C or C++ program triggers undefined behavior, anything is allowed to happen in the program execution. And by anything, I really mean anything: The program can crash with an error message, it can silently corrupt data, it can morph into a colorful video game, or it can even give the right result.
> If you’re lucky, the program triggering UB will show an appropriate error message and/or crash, making you immediately aware that something went wrong. If you’re unlucky, the program will quietly mangle data, and by the time you notice the problem (via effects such as crashes or incorrect output) the root cause has been buried in the past execution history. And if you’re very unlucky, the program will do exactly what you hoped it should do, until you change some unrelated code / compiler versions / compiler vendors / operating systems / hardware platforms – and then a new bug becomes visible, and you have no clue why seemingly correct code now fails to work properly.
-- https://www.nayuki.io/page/undefined-behavior-in-c-and-cplus...
As I wrote in my article, this point really got hammered into me when a coworker showed me a patch that he made - which added a couple of innocuous, totally correct print statements to an existing C++ program - and that triggered a crash. But without his print statements, there was no crash. It turned out that there was a preexisting out-of-bounds array write, and the layout of the stack/heap somehow masked that problem before, and his unlucky prints unmasked the problem.
Okay so then, how can we do better as developers today?
0) Read, understand, and memorize what actions in C or C++ are undefined behavior. Avoid them in your code at all costs. Also obey the preconditions of any API you use, whether in the standard library, operating system, etc.
1) Compile your application in Debug mode and compare its behavior to Release mode. If they differ by anything other than speed, then you have a serious problem on your hands.
2) Compile and run with sanitizers like -fsanitize=undefined,address to catch undefined behavior at runtime.
3) Use managed languages like Java, C#, Python, etc. where you basically don't have to worry about UB in normal day-to-day code. Or use very well-designed low-level languages like Rust that are safe by default and minimize your exposure to UB when you really need to do advanced things. Whereas C and C++ have been a bonanza of UB like we have never seen before in any other language.
Other than C#, there is no reason to use those other languages for game dev. Unless the game is fairly simple, or you want to risk a fairly long project by employing a language that hasn't been proven in tge space yet (Rust). No shade at any of those languages, I don't even like C#, just being pragmatic.
The most successful videogame of all time was written for Java as an applet in the browser.
Muggsy Bogues had a very successful NBA career at just 5 foot 3.
I would add: code defensively. Initialize your variables (either to a sensible value, or an outrageously wrong value) before passing pointers to them, even when you "know" that the value will be overwritten. Check for errors. Always consider what happens when things go wrong, not just when things go right. Any time you find yourself thinking, "condition X is guaranteed to hold, so I don't need to check for it" consider checking it anyway just in case you're wrong about that, or it changes later.
My only issue with defensive codding is that often it doesn't play nice with code coverage requirements. I've been in situations where I would like to add defensive coding just in case, but then the PR doesn't pass the coverage checks. The best is when you can ensure via th compiler (e.g. via the type system) that a case is impossible, but C++ (in my case) isn't perfect for this.
I learned this lesson many moons ago, on a Fortran code I wrote for a university assignment. It was a basic genetic algorithm, and for some reason it was converging much more slowly than expected. So I was sprinkling some WRITEs to debug, and suddenly the code converged a hundred times faster.
All this is true. Note also that the C++ folks are putting a serious effort into reducing UB. See the "safe by default" section of this writeup [1]. See also my other comment [2] regarding the performance impact of this sort of change. Short answer: with sufficient optimization, smaller than one might think.
[1]: https://herbsutter.com/2024/08/07/reader-qa-what-does-it-mea...
Once this category of error is raised to your attention, you start to notice it more and more.
A little piece of technology made sense in the original context, but then it got moved to a different context without realizing that move broke the contract. Specifically in this case a flying boat became an airplane.
---
I recently worked a bug that feels very similar:
A linux cups printer would not print to the selected tray, instead it always requested manual feed.
Ok. Try a bunch of command line options, same issue.
Ok. Make the selection directly in the PPD (postscript printer definition) file. Same issue.
Ok! Decompile the PXL file. Wrong tray is set in pxl file... why?
Check Debug2 log level for cups - Wrong MediaPosition is being sent to ghostscript (which compiles the printer options into the print job) by a cups filter... why?
Cups filter is translating the MediaPosition from the PPD file... because the philosophy of cups is to do what the user intended. The intention inferred from MediaPosition in the PPD file (postscript printer definition) is that the MediaPosition corresponds to the PWG (Printer Working Group) MediaPosition, NOT the vendor MediaPosition (or local equivalent - in this case MediaSource).
AHA!! My PPD file had been copied from a previous generation of server, from a time when that cups filter did NOT translate the MediaPosition, so the VENDOR MediaSource numbers were used. Historically, this makes sense. The vendor tray number is set in the vendor ppd file because cups didn't know how to translate that.
Fast forward to a new execution context, and cups filters have gotten better at translating user intention, now it's translating a number that doesn't need to be translated, and silently selecting the wrong tray.
TLDR; There is no such thing as a printer command, only printer suggestions.
Use a debugger folks. A 10x dev cited this story to me about the ills of not using one.
I always wonder, why not write these games on top of a virtual machine like Carmack started doing in Quake, a usage he then later extended to quake 2 and 3 [1].
I'm ignorant about game development, virtual machines and system programming but from the little I understand it seems a sensible choice to make.
While there is an initial price to pay modeling 99% of the game to be implemented on a user-implemented stack seems a sensible approach to me.
This is a game; I don't think a debug configuration (with checks for things like this enabled) would run fast enough to be playable on contemporary hardware.
That's not accurate.
Generally, game console "debug" configurations aren't "true" debug like most people think of -- optimizations are still globally enabled, but the build generally has a number of debug systems enabled that naturally require the use of a devkit. Devkits, especially back then, generally had 2-3x as much memory as retail systems -- so you'd happily sacrifice framerate during feature development to have those systems enabled.
Debugging was (and still is) generally done on optimized builds and, once you know the general area of the problem, you simply disable optimizations for that file or subsystem if you can't pinpoint the issue in an optimized build.
The biggest performance hit, in general, comes from disabling optimizations in the compiler. I say "in general" because there are systems that might be used to find this kind of thing that DO make a game wholly unplayable, such as a stomp allocator. Of course, you wouldn't generally enable a stomp allocator across all your allocations unless you're desperate, so you could still have that enabled to find this kind of bug and end up with a playable game.
The more likely reason here is that no one noticed or cared. GTA:SA is 21 years old and this bug doesn't affect the Xbox or other versions.
From GP:
> (with checks for things like this enabled)
You can (and could) easily compile an optimized build with debug symbols to track down sources of issues, but catching a bug like this would likely take a dynamic checker like Valgrind or MSan, which do not allow for any optimizations if you want to avoid false negatives, and add even more overhead on top of that. (Valgrind with its full processor-level virtualization, and MSan with its shadow state on every access. But MSan didn't exist at the time, and Valgrind barely existed.)
At minimum, fine-grained stack randomization might have exposed the issue, but only if it happened to be spotted in playtests on the debug build.
Problem with valgrind/asan/msan is that you have to start using these tools early in the development process. It can't be a "checklist" item before launch, or you'll have an insurmountable number of bugs, often with them baked in such that fixing the bug causes additional changes that introduce unrelated bugs.
20 years and still nobody has realized what the back of this sign is about. IYKYK.
https://static.wikia.nocookie.net/gta-myths/images/f/f1/Egg_...
fixed link (the above did not work for me) https://static.wikia.nocookie.net/gta-myths/images/f/f1/Egg_...
On Windows 11 24H2, more stack space was modified by a new implementation of Critical Sections.
IMHO this shows the downfall of Microsoft. Why did they do that? Critical sections have been there for many decades and should be basically bug-free by now. My best guess is someone thought they'd "improve" things and rewrote it, then made some microbenchmark that maybe showed the dubious improvement.
The other comment here mentions Raymond Chen, who wrote this article about why backwards-compatibility is very important (and arguably what got Microsoft into the position it's in today):
https://devblogs.microsoft.com/oldnewthing/20031224-00/?p=41...
and also this memorable case: https://news.ycombinator.com/item?id=2281932
User has working software. User updates operating system. User has broken software.
That's a problem for the party trying to sell operating system updates.
The core problem is some compilers initialising memory to zero in Debug mode, masking behaviour of unitialised data, since in most cases zero is a legit value. In Release mode, this zeroing doesn’t happen.
Devs need to be aware that the following C++ initisliser exists which zeros data structures for you:
MyStruct s = { };
Apparently the planes are buggy in all sorts of ways, although I don't remember them being this bad... https://youtu.be/hrJ0eVY5ACw
Yes, it would have made it out of the original programmer's system for that initial commit.
FTA:
I have a likely explanation for why Rockstar made this specific mistake in the data to begin with – in Vice City, Skimmer was defined as a boat, and therefore did not have those values defined by design! When in San Andreas they changed Skimmer’s vehicle type to a plane, someone forgot to add those now-required extra parameters. Since this game seldom verifies the completeness of its data, this mistake simply slipped under the radar.
Someone changed the Skimmer object from a boat to a plane for GTA San Andreas, BUT they DID NOT update the object to have the REQUIRED wheel values for a plane object.
Now the GTA code is expecting more values than it gets.
The vehicles.ide wasn't validated for correctness after the Skimmer object change to plane. Maybe there are more gotchas in that file...
At least users can fix the problem with a text editor instead of waiting and hoping that RockStar would fix the problem and release an update.
It has always been too easy to read & write beyond the stack. This should fail, plain and simple.
Mitigations exist - ASLR, NX pages, stack-smashing protection etc. but nothing comprehensively stops reads of stale data beyond the stack.
Thought experiment for a moment. What if the hardware ensures the unused part of a stack region cannot be read or written.
There are many ways to skin this cat, here’s one based around tracking each stack’s start address A, size S, and current depth D
1. Add an instruction to inform the CPU there is a stack at address A of size S. Its depth D is initially 0.
2. Add a jump instruction which reserves N bytes on the stack at address A, growing depth D to (D+N). Maybe this can be its own “reserve” instruction so as not to need a new jump instruction.
3. Give existing return instructions stack awareness. If returning to an address inside a stack, un-reserve the bytes reserved by the most recent jump, making the new depth (D-N).
4. Fail reads or writes to the stack region beyond its current depth. In other words fail all reads and writes between A+S-D and A+S.
5. The arithmetic is reversed on architectures whose stacks grow downwards.
Downsides I can see:
It cements one calling convention. The CPU memory manager will need a lot of state per stack, of which there are many per process: address A, size S, current depth D, plus a reservation stack - ie. sizes of each frame’s stack memory. That’s a lot of bookkeeping! It’s far from zero cost. The limits of how much bookkeeping the CPU can do impose limits on how deep a stack can go and how many stacks are supported - so when there are too many stacks or one goes too deep, either the CPU needs to signal failure or engage a fallback mode and revert to behaving as CPUs do today. And of course fallback puts things back to the start. It’d therefore only mitigate situations in which an attacker cannot control the depth of the stack / a bug always happens inside the max depth the CPU can bookkeep for.
That said, stacks are ubiquitous! Hardware stack awareness opens up all kinds of new mitigations.
Why isn’t this a common idea? Has it been tried?
This bug wasn't caused by a read beyond the current bounds of the stack, but a stale value from a prior call to the same function at the exact same location on the stack. Buffer-overflow protections like you describe wouldn't help here.
tl;dr of the explanation: the Skimmer vehicle is missing a wheel scale definition, so its wheel scale gets read from uninitialized memory. On previous versions of Windows, this happened to be the wheel scale of the previously-loaded vehicle, so things happened to work fine. Starting on Windows 11 24H2, LeaveCriticalSection (which gets called between loading vehicles) uses more stack space than before, so it now overwrites that memory with a gigantic value, resulting in the Skimmer spawning so high up that it may as well not exist at all.
SilentPatch (for GTAs, at least) specifically is a code-only mod, such that the single .asi file can be removed to uninstall it & all it's changes.
A real update should fix both (note: I don't believe the later releases did, they also just added defaults to the parser) but for SilentPatch: a mod is not a real update, and being as simple as possible to remove & reducing conflicts with other mods is more important here than a fix that digs as deep as possible.
I hope someone can figure out the Red Dead Redemption 2 bug where random animals and characters disappear silently if you have too many texture mods installed.
I spent hours looking for a badger.
I love it how many bugs go from "why doesn't this work?!" to "how on Earth did this work previously?!"
How does a bug from 20 years ago even still work today?
funnily enough, there was an infamous bug in GTA5 for a long time that was related to using JSON : https://nee.lv/2021/02/28/How-I-cut-GTA-Online-loading-times...
I remember that one too :)
They wrote a JSON “parser” using sscanf. sscanf is not bulletproof! Just use an open source library instead of writing something yourself. You will still be a real programmer, but you will finish your game sooner and you won't have embarrassing stories written about you.
Yes, but now it's in the realm of ~3 minutes, and not ~8 minutes even on a top-spec PC, right? I really liked the game, but waiting 8 minutes to load just to get griefed by hackers within seconds of walking outside... I don't understand how that game makes any money.
Putting the (very valid) reasons for not having human-readable game saves aside, are you sure it's worse than using a 3rd party library that's built to accept semi-valid input values, possibly evaluates user input in some way and has difficult to debug bugs that occur only under certain inputs? I agree that writing a stable and safe parser for a binary data file isn't easy, but there's less things that can go wrong when you can hardcode it to reject any remotely suspicious input. Third party XML/JSON libraries OTOH try to interpret as much as possible, even when the values are bogus. Also no need to deal with different text encoding bugs, line endings...
You misunderstood. Game developers should use a _good_ third–party library, not a _bad_ one. At a minimum they should be able to read the source code so that they know it is good. Thus open source libraries should be at the top of the list.
If you don't know what “good” looks like, take a look at [Serde](https://serde.rs/). It’s for Rust, but its features and overall design are something you should attempt to approach no matter what language you’re writing in.
> Putting the (very valid) reasons for not having human-readable game saves aside,
I don't follow. What would the reasons be?
After finishing the article I immediately did ctrl+f "rust" and was disappointed to not see any of the results I wanted, but actually this comment is more hilarious than anyone saying "why didnt rockstar use rust in 2004!!!1111!!???" it's a bit more of a sophisticated joke since there's an IYKYK factor but it is no less hilarious. Bravo sir, bravo.
To u/db48x whose post got flagged and doesn't reappear despite me vouching for it as I think they have a point (at least for modern games): GTA San Andreas was released in 2004. Back then, YAML was in its infancy (2001) and JSON was only standardized informally in 2006, and XML wasn't something widely used outside of the Java world.
On top of that, the hardware requirements (256MB of system RAM, and the PlayStation 2 only had 32MB) made it enough of a challenge to get the game running at all. Throwing in a heavyweight parsing library for either of these three languages was out of the question.
The comment reappeared, and while you're right about using proper libraries to handle data, it doesn't excuse the "undefined behavior (uninitialized local variables)" that I still see all the time despite all the warning and error flags that can be fed to the compiler.
Most of the time, the programmers who do this do not follow the simple rule that Stroustrup said which is to define or initialize a variable where you declare it (i.e. declare it before using it), and which would solve a lot of bugs in C++.
While it doesn't excuse the bad habits, we do have to keep in mind C++98 (or whatever more ancient was used back then) didn't have the simple initializers we now take for granted. You couldn't just do 'Type myStruct = {};' to null-initialize it, you had to manually NULL all nested fields. God forbid you change the order of the variables in the struct if you're nesting them and forget to update it everywhere. It was just considerably more practical to do 'Type myStruct;' then set the fields when needed.
> it doesn't excuse the "undefined behavior (uninitialized local variables)" that I still see all the time despite all the warning and error flags that you can feed to the compiler.
Yeah but we're talking about a 2004 game that was pretty rushed after 2002's Vice City (and I wouldn't be surprised if the bug in the ingestion code didn't exist there as well, just wasn't triggered due to the lack of planes except that darn RC Chopper and RC plane from that bombing run mission). Back then, the tooling to spot UB and code smell didn't even exist or, if at all, it was very rudimentary, or the warnings that did come up were just ignored because everything seemed to work.
JSON didn't save them: This is the same studio that handrolled a JSON parser with accidentally quadratic time complexity, making most players wait 3 to 10 minutes to load GTA Online, for 7 years, until a player got tired and found the root cause.
https://nee.lv/2021/02/28/How-I-cut-GTA-Online-loading-times...
You’re not entirely wrong, but a library doesn’t have to be “heavyweight” in order to be bulletproof. And you can load the library during startup and then unload it after; it doesn’t have to stick around for the whole run time of the game. Modern OSes will reclaim the pages after you stop using them, if there is memory pressure. Of course the PS2 didn’t do that I am sure.
Meanwhile, in a certain modern OS, unloading a library is too broken to the point that people are discouraged to do so... Try to unload GLib [0] from your process :p
Unloading C libraries is fundamentally fraught with peril. It's incredibly difficult to ensure that no dangling pointers to the library remain when it's unloaded. It's really fun to debug, too. The code responsible for the crash literally is not present in the process at the time of the crash!
Definitely, and architectures back then were far less standardized. The Xbox 360 was a big-endian PowerPC CPU, the PS2 had a custom RISC-based CPU. On the desktop, this was still the era of PowerPC-based Macs. Far easier (and I would argue safer) to use a standard, portable sscanf-like function with some ascii text, than figure out how to bake your binaries into every memory and CPU layout combination you might care about.
Easier for internal development. Non- or less technical team members can tweak values without having to rebuild these binary files. Possibly also easier for lightweight modding externally as well.
This isn't that uncommon - look at something like Diablo 2 which has a huge amount of game data defined from text files (I think these are encoded to binary when shipped but it was clearly useful to give the game a mode where it'd load them all from text on startup).
Video games are made by a lot of non-programmers who will be much more comfortable adjusting values in a text file than they are hex editing something.
Besides, the complaint about not having a heavyweight parser here is weird. This is supposed to be "trusted data", you shouldn't have to treat the file as a threat, so a single line sscanf that's just dumping parsed csv attributes into memory is pretty great IMO.
Definitely initialize variables when it comes to C though.
Wow I had no idea YAML was that old. I always thought it was created some time around when CI/CD became popular. Now I'm really curious how it ended up as a superset of JSON.
The flaw isn't the language. The issue is a 0.5x programmer not knowing to avoid sscanf() and failing to default and validate the results. This could be handled competently with strtok() parsing the lines without needing a more complicated file format.
Worked fine on the target machines and the "0.5x programmer" got to see their family for winter holiday. Or are you saying they should have defensively programmed around a bug manifesting 21 years later and skip seeing their family during crunch time?
To be honest, I just don't like how you disparaged the programmer out-of-context. Talk is cheap.
I’ll be the first to defend the greybeards I’ve befriended and learned from in AAA, but having seen codebases of that age and earlier, the “meta” around game development was different back then. I think the internet really changed things for the better.
Your average hire for the time might have been self-taught with the occasional C89 tutorial book and two years of Digipen. Today’s graduates going into games have fallen asleep to YouTube lectures of Scott Meyers and memorized all the literature on a fixed timestep.
This is the kind of thing I'd expect from Raymond Chen - which is extremely high praise!
I'm glad they tracked it down even further to figure out exactly why.