Not everything will be rewritten in Rust, so C++ must become safer, and we should all care about C++ becoming safer.
It has become increasingly apparent that not only do many programmers see the benefits of memory safety, but policymakers do as well. The concept of “memory safety” has gone from a technical term used in discussions by the builders and users of programming languages to a term known to Consumer Reports and the White House. The key contention is that software weaknesses and vulnerabilities have important societal impacts — software systems play critical roles in nearly every part of our lives and society — and so making software more secure matters, and improving memory safety has been identified as a high-leverage means to do so.
Policymakers believe it’s high leverage because of quality reporting from a number of software companies, including Microsoft, Apple, and Google, showing that a large portion of the vulnerabilities they track for their own software are derived from memory unsafety. This reporting was then aggregated by Alex Gaynor, Yael Grauer, and others to make a compelling case that you could achieve meaningful reduction in software vulnerabilities, and importantly in the severity of vulnerabilities, by improving memory safety.
It’s important to pause here and note that many existing popular languages are already, at least partially, memory safe. In fact, the US government’s “Case for Memory Safe Roadmaps,” published in December of 2023, lists C♯, Go, Java, Python, and Swift as memory safe languages, all garbage collected languages which cover an enormous number of devices today. C♯ is an extremely popular language for Windows machines with the .NET runtime, Go is used for many web servers and pieces of “cloud native” software run by popular cloud hosts, Java is the language of Android and a great deal of widely-deployed software used by large enterprises, Python is the most popular language today for many data science applications, and Swift is the recommended language for all iOS applications today (though Objective-C remains supported and widely used as well). All of these languages are already delivering meaningful memory safety, though many still present at least some memory safety–related issues like permitting dereferencing of null pointers.
The one other language on the list from the “Case for Memory Safe Roadmaps” is Rust, and Rust is unique in that list for being the only memory safe language without garbage collection. I believe this fact is part of why so much movement is happening around memory safety now. Rust has enabled memory safety in contexts where garbage collection is not generally accepted, and therefore languages which require it do not generally see adoption. In those spaces, the overwhelming majority of existing code is written in C or C++.
It is this code to which Rust presents an intruiguing possibility: could we rewrite that code from C or C++ into Rust, and thereby gain the benefits of memory safety where they were not previously feasible? It has become a common joke both inside and outside of the Rust community that, for any project, people should Rewrite It In Rust, and that the Rust Evangelism Strike Force stands ready to make this recommendation to any project it observes, especially when memory safety–related vulnerabilities arise.
Despite the jokes, not everything will be rewritten in Rust, and in fact, many things that are in C and C++ today should not be rewritten in Rust. This is also the position of many major tech corporations and foundations, including…
“We expect that rewriting large, existing unsafe codebases will o ften be impractical. and recommend that old, unsafe codebases be updated gradually via interoperability or by enforcement of safer coding pa tterns rather than entirely rewri tten.”
“[R]efactoring large existing code bases into a new language is typically non-trivial. It can introduce new bugs and vulnerabilities, and in some cases it is difficult to redeploy new object code (e.g., in OT/ICS devices). We encourage new development to occur in memory-safe languages where practical. Organizations should take a risk-based approach for existing code, focusing their refactoring efforts where it will be the most beneficial; it would be too costly and risky to try to rewrite all software currently in memory-unsafe languages. Focusing on rewriting the riskiest components of the most important software is a practical approach for existing software.”
“Unfortunately, the majority of OSS that is still used, maintained, and extended for critical web and operating system infrastructure predates widespread adoption of memory safe programming languages. Over 65% of websites are served by an open source server written in an unsafe programming language like C or C++. The cost (both monetary and logistically) of migrating all or any significant portion of OSS to memory safe programming languages is substantial. A cost-benefit analysis is therefore necessary.
When performing this analysis, it is important to observe that pre-existing testing should be considered a security property and as important as any core feature: rewriting a piece of software in Rust can actually introduce security problems if, for example, the previous implementation’s tests are not included in the rewrite. Thus, these rewrites need to be performed carefully and holistically, with consideration for existing tests and undocumented invariants.”
“Supporting rewrites of the most critical open-source software components in memory safe languages can produce valuable experience for developers and funders. If rewritten components are adopted at scale, they will reduce the vulnerability of the software ecosystem. However, rewrites are also acknowledged by developers to be very expensive, bear the risk of introducing new non-memory safety errors, and are, ultimately, a long-term project. Therefore, additional strategies should also be prioritized, both to foster the adoption of memory safe programming languages for new code and to increase the efficiency and impact of increasing the safety of existing components, including through rewrites.”
To understand, we’ll need to do a cost-benefit analysis of rewrites like this.
The benefits of rewriting software from a memory unsafe language like C or C++ into a memory safe language like Rust seem clear: you introduce memory safety, and eliminate vulnerabilities related to it! Since we have the reporting from Apple, Google, Microsoft, and others showing that many of their vulnerabilities are related to memory safety, and in fact the percentages of vulnerabilities they’ve observed which are memory safety–related seem stable across organizations, we should expect that moving to memory safety should in general result in similar reductions in vulnerabilities!
However, let’s step back: the reported proportions of vulnerabilities assigned to memory safety issues are for actively developed projects, which means these are projects which are being maintained and for which new lines of code are being written.
Software defects, including weaknesses and vulnerabilities, can be thought of in relation to the lines of code produced at a given time. Per 1,000 lines of code, you have an expected “defect rate.” Some of those defects may be weaknesses, which are not exploitable, while others may be vulnerabilities, which are exploitable. Either way, they are “born” with the introduction of new code. While changes over time may convert weaknesses to vulnerabilities, by making something exploitable which previously wasn’t exploitable, and new interactions between systems may make assumptions in existing code no longer valid and thus introduce vulnerabilities, in both cases over time we can assume that defect rates decrease because we expect the rate of fixing defects in a fixed segment of code to outpace the introduction of new vulnerabilities from changing system interactions.
To put it more plainly: if you don’t rewrite the code substantially, and you periodically fix bugs, over time the number of vulnerabilities in the code falls.
If you’re familiar with the idea of “systems thinking” and more specifically of modeling “stocks and flows,” that’s basically what we’re doing here. In this context, the “stocks” are amount of code and number of detects, and the “flows” are introduction of new code, and discovery of defects. I am not going to do a more detailed stocks and flows breakdown here, but this kind of thinking is a good approach for modeling the considerations here.
Any project can use assurance techniques like code review, testing, code analysis, defensive programming, and more to reduce the defect rate or increase the rate that defects are identified and fixed.
Daniel Stenberg, the creator of curl, the popular “command line tool and library for transferring data with URLs” (to quote the project), covers this idea quite nicely in his 2022 blog post “Increased CVE Activity in curl?”. In the “Finding vs. Introducing” section, he notes that over time, the curl project has reduced the rate at which new vulnerabilities are introduced. 75% of the CVEs reported against the project at the time the article was written were from before March 2014, when the project was much smaller in terms of code size. While the introduction of new code has stayed relatively linear over time, the rate of vulnerabilities per 1,000 lines of code has decreased through the use of assurance techniques like rigorous testing.
For any software project, we should expect that over time, if the code doesn’t change substantially and the project receives some rate of bugfixes, the number of vulnerabilities latent in the codebase will decrease.
For inactively developed projects, which we should not expect to be introducing new vulnerabilities at substantial rates, a rewrite may actually be counter- productive. While a rewrite to a new language could eliminate memory safety vulnerabilities, it also is likely to introduce many new vulnerabilities of other types. If the expected rate of latent memory safety vulnerabilities in the code is low, this tradeoff may not be worthwhile.
In this analysis I’ve so far ignored the economic considerations, but those are also important. Code rewrites are expensive, because (as any employer can tell you), staff time is expensive, especially for programmers who tend to fetch high pay relative to many other professions. For legacy software which is currently in maintenance mode with minimal staff support, and thus cheap to run, the cost/value proposition of a rewrite may not make sense, and may in fact change the calculus around the legacy systems entirely. Often a system becomes a legacy system because it is not valuable enough to maintain, but too costly to decomission and replace. If a cheap-to-maintain legacy system is faced with the proposition of an expensive rewrite, it may instead be eliminated. The externalities of this kind of change are difficult to consider in advance and in general.
However, this does not mean we ought to throw in the towel for codebases already written in C or C++ which may not be worth rewriting in Rust. Those codebases matter, or else they would not be preserved in operation, and that means these are codebases whose function in some way impacts people’s lives, and that vulnerabilities in them may result in real harm. Throwing up our collective hands for these languages when there exists the possibility of reducing harm is unacceptable.
For the remainder of this post, I’ll be focusing only on C++. I believe we should want both C and C++ to become safer languages, but I think there is more appetite to pursue this in the C++ community and standards group that there is for C, and that C++ already has a number of language affordances which prime it to be able to introduce safety-providing abstractions which would likely be more difficult in C. My general recommendation in favor of advancing safety applies to both, but I am going to focus on C++ here.
So, what should we do? To quote Steve Klabnik on this issue:
[I]t seems (to me) like there are now four camps in the “how do we make C++ safer” category. I’m listing these in an order, I’ll explain that after:
- contracts
- profiles
- successor languages
- borrow checking
This tweet thread is in response to a vote from a recent C++ standards committee meeting about further exploration of opportunities for memory safety in C++, and the rest of the thread goes on to explain Steve’s understanding of the current state of each of the four approaches.
These are being pursued, to varying degrees, by people in the C++ standards process, and outside of it. Herb Sutter, Chair of the ISO C++ Standards Committee, published an article in March of this year titled “C++ Safety, in Context,” (disclosure: I was a pre-release reviewer for that article, for which I provided feedback on its descriptions of Rust’s guarantees and the landscape of memory safety today). Herb’s article outlines his own view of what C++ can and should pursue for memory safety, and falls in the “contacts” and “profiles” camps of Steve’s list above.
“Profiles” are also the path recommended by Bjarne Stroustrup, creator of C++, and the rest of the “C++ Directions Group” in their writing on the topic. In the “profiles” approach, C++ would support the activation of predefined opt-in sets of additional static analyses which would restrict what C++ programs can do, but improve their safety, while enabling interoperation with C++ written and compiled without the same profiles.
That said, there are critics of the “profiles” approach, and it’s not clear that this approach has yet borne fruit of producing a meaningfully safe profile of C++ which approaches the memory safety of Rust, or indeed of less memory-safe languages like Java.
Indeed, Sean Baxter, who proposed the vote described above on the C++ standards committee pursuing memory, is also working on one of multiple successor-language projects for C++: Circle. This is item 3 in Steve Klabnik’s list, which includes (known to me, at least):
Circle, Sean’s language, is “a subset of a superset” of C++. This means you can intermix valid existing C++ code with code written in the safe extension of C++ which is like-C++ but with limits placed to enforce memory safety.
Carbon, an “experimental successor to C++,” aims for interoperation with C++ with more safety (though explicitly aiming for fewer safety guarantees than Rust). Code written in Carbon is not valid C++ code, but code written in Carbon is intended to be able to easily call C++, and be easily called by C++ code.
cppfront, being developed by Herb Sutter, is an experimental syntax for C++ which compiles to “real” C++, and essentially is intended to make it easier / the default to write safer C++ code with fewer footguns to remember not to fire.
As I am not a C++ programmer, I can’t comment on which of these is the right approach, and I have no idea which of these will persist or how this effort will shake out over time. My point here is in fact not to make a case for or against any of these approaches, but rather to say that anyone with an interest in advancing memory safety and software security more broadly should care about these efforts and what happens with C++ long-term.
Which leads me to the crux of the whole point here: C++ isn’t going anywhere. As good as I think memory safety is (and I’ve been using and involved in Rust projects since 2014, so it’s 10 years of my career that I’ve committed to this cause), it will not be universally achieved through rewriting existing code in Rust.
The economic costs of these rewrites would be substantial, and in fact such rewrites may be counterproductive in terms of vulnerability rates even if pursued in some contexts (code which changes minimally over time and has been assured well through alternative techniques already). We can’t, and shouldn’t, even want a near-term future where Rust rewrites are pursued across all or most software.
Using memory safe languages for new code is, of course, ideal. I do hope that over time, the proportion of new code written in non-memory-safe languages decreases. Yet code has a tendency to last a long time, far longer than we often anticipate. We won’t be getting rid of C++ any time soon.
So we should care enormously about what happens to the memory safety of C++ as a language, and what opportunities may arise to pursue lower-cost and lower-risk paths to improved memory safety besides full rewrites of existing code into memory safe languages. Diversifying paths to memory safety through profiles, contracts, and successor languages are all important to enable viable options in more contexts.
We should also not shame people working with and sustaining existing C and C++ codebases, nor chide them to pursue rewrites in Rust. This does nothing to help Rust, and in fact makes it look terrible as a community, and ignores the often very real constraints holding back rewrites from starting.
Thank you to everyone involved in the C++ community who is working to make the language safer. You are doing the hard and often thankless work of securing something that’s no longer new and shiny. I am sure as you do that you are also dealing with challenging attitudes within the C++ community, from people who may feel defensive or uncertain about a memory safe future and C++’s place in it.
I hope we can all have a little more empathy, a little more nuance, and a little less of an Evangelism Strike Force in the future.