With just a month left until its seventh birthday, everything curl has now surpassed this amazing milestone. The book now contains more than 100,000 words. Distributed over 883 sections. All written in glorious markdown.
Two years ago when we celebrated its 5th birthday, it was still this measly thin “pamphlet” of 72,000 words. It has grown by almost 40% over the last two years.
The average word length in the book is now 5.25 characters and all this is spread out over 14,900 lines (in the source markdowns).
63 individuals have had their commits merged. I have great help from people to polish off weird language and wrong English.
My ambition with this book remains the same: to document everything there is to tell about curl and libcurl from every aspect. Code, use, development, project, background, future, philosophy and more.
CI
News from the last year for everything curl is that we have several CI jobs now that verify new contributions to make sure we don’t degrade too much. They check that:
we avoid some words (contractions and some other things) – basically my most common mistakes
spellcheck
markdown heading level sanity check
The spellcheck part can of course be a bit tedious for such a technical document but I realized that since I am such a sloppy writer I need that check. This has really reduced the inflow of PRs with spelling fixes.
These CI jobs makes the quality of the book much better even though it is a highly moving target.
Keeping up
As curl is a constantly evolving project that adds new features and changes things every now and then, there is also a constant stream of new things to add or update in the book.
Since I also want the book to work for readers that may very well run curl versions from several years ago, we need to keep that in mind and make sure to keep “old behavior” and details around for a while.
In the curl project we do timed releases and we try to do them planned and scheduled long in advance. The dates are planned. The content is not.
I have been the release manager for every single curl release done. 209 releases at current count. Having this role means I make sure things are in decent shape for releases and I do the actual mechanic act of running the release scripts etc on the release days.
Since 2014, we make releases on Wednesdays, every eight weeks. We sometimes adjust the date slightly because of personal events (meaning: if I have a vacation when the release is about to happen, we can move it), and we have done several patch releases within a shorter time when the previous release proved to have a serious enough problem to warrant an out-of-schedule release. Even more specifically, I make the releases available at or around 8 am (central euro time) on the release days.
The main objective is to stick to the 56 day interval.
It probably goes without saying but let me be clear: curl is a software project that quite evidently will never be done or complete. It will keep getting fixes, improvements and features for as long as it lives, and as long as it lives we keep making new releases.
Timed releases
We decided to go with a fixed eight weeks release frequency quite arbitrarily (based on past release history and what felt “right”) but it has over time proved to work well. It is short enough for everyone to never have to wait very long for the next release, and yet it is long enough to give us time for both merging new features and having a period of stabilization.
Timed releases means that we ship releases on the predetermined release date with all the existing features and bugfixes that have landed in the master branch in time. If a change is not done in time and it is not merged before the release, it will simply not be included in the release but will get a new opportunity to get included and shipped in the next release. Forever and ever.
I am a proponent of timed releases compared to feature based ones for projects like curl. For the simplicity of managing the releases, for long term planning, for user communication and more.
The cycle has three windows
Within the eight week release cycle, there are three distinct and different windows or phases.
1 – release margin
The cycle can be said to start at the day of a release. The release is created straight from the master git branch. It gets signed, uploaded, blogged about and the news of it is shouted across the globe.
This day also starts the “release margin window”. In this window we still accept and merge bugfixes into the master branch, but we do not merge changes or new features.
The five day margin this speaks of, is that this window gives us a few calendar days to assess and get a feel for how the previous release is being received. Is there a serious bug reported? Did we somehow royally mess up? If we did an important enough snafu, we may decide to do a follow-up patch release soon and if we do, we are in a better position if we have not merged any changes yet into the release branch.
If we decide on doing a patch release, we skip the feature window and go directly to feature freeze.
2 – feature window
If we survived the margin window without anything alarming happening, we open the feature window. On the Monday following the preceding release.
This is the phase during which we merge changes and new features in addition to the regular normal bugfixes (assuming there are any of course). Things that warrant the minor version number for the next release to get bumped.
The feature window is open 23 days. Ideally, people have already been working and polishing on their pull-requests for a while before this window opens and then the work can get merged fairly quickly – and presumably painless.
What exactly can be considered a change and what is a bugfix can of course become a matter of opinion and discussion, but we tend to take the safer approach when in doubt.
3 – feature freeze
In the 28 days before the pending release we only merge bugfixes into the master branch. All features and changes are queued up and will have to wait until the feature window opens again. (Exceptions might be made for experimental and off-by-default features.)
This phase is of course intended for things to calm down, to smooth out rough edges, to fix any leftover mistakes done in the previous feature merges. With the mindset that the pending next release will be the best, most stable, shiniest, glorious release we ever did. With even fewer bugs than before. The next release is always our best release yet.
The release cycle as an image
(Yes, if you squint and roll your chair away from the screen a bit, it looks like the Chrome logo!)
Release frequency graph
The graph below shows the number of days between all curl releases ever done. You can see the eight week release cycle introduced in 2014 visible in the graph, and you can also easily spot that we have done much quicker releases than eight weeks a number of a times since then – all of them actually a sign of some level of failure since that means we felt urged to do a patch release sooner than we had previously anticipated.
Disclaimer: I work for wolfSSL but I don’t speak for wolfSSL. I state my own opinions and I try to be as honest and transparent as possible. As always.
QUIC API
Back in the summer of 2020 I blogged about QUIC support coming in wolfSSL. That work never actually took off, primarily I believe because the team kept busy with other projects and tasks that had more customer focus and interest and yeah, there was not really any noticeable customer demand for QUIC with wolfSSL.
Time passed.
On July 21 2022, Stefan Eissing submitted his work on introducing a QUIC API and after reviews and updates, it was merged into the wolfSSL master branch on August 9th.
The QUIC API is planned to appear “for real” in a coming wolfSSL release version. Until then, we can play with what is available in git.
Let me be clear here: the good people at wolfSSL has not decided to write a full QUIC implementation, because that would be insane when so many good alternatives are already being worked on. This is just a set of new functions to allow wolfSSL to be used as TLS component when a QUIC stack is created.
Having QUIC support in wolfSSL is just one (but important) step along the way as it makes it possible to use wolfSSL to build a QUIC implementation but there are some more steps needed to turn this baby into full HTTP/3.
ngtcp2
Luckily, ngtcp2 exists and it is an established QUIC implementation that was written to be TLS agnostic from the beginning. This “only” needs adaptions provided to make sure it can be built and used with wolSSL as the TLS provider.
Stefan brought wolfSSL support to ngtcp2 in this PR. Merged on August 13th.
nghttp3
nghttp3 is the HTTP/3 library that uses ngtcp2 for QUIC, so once ngtcp2 supports wolfSSL we can use nghttp3 to do HTTP/3.
curl
curl can (as one of the available options) get built to use nghttp3 for HTTP/3, and if we just make sure we use an underlying ngtcp2 built to use a wolfSSL version with QUIC support, we can now do proper curl HTTP/3 transfers powered by wolfSSL.
Stefan made it possible to build curl with the wolfSSL+ngtcp2 combo in this PR. Merged on August 15th.
Available HTTP/3 components
With this new ecosystem addition, the chart of HTTP/3 components for curl did not get any easier to parse!
If you start by selecting which HTTP/3 library (or maybe I should call it HTTP/3 vertical) to use when building, there are three available options to go with: quiche, msh3 or nghttp3. Depending on that choice, the QUIC library is given. quiche does QUIC as well, but the two other HTTP/3 libraries use dedicated QUIC libraries (msquic and ngtcp2 respectively).
Depending on which QUIC solution you use, there is a limited selection of TLS libraries to use. The image above shows TLS libraries that curl also supports for other protocols, meaning that if you pick one of those you can still use that curl build to for example do HTTPS for HTTP version 1 or 2.
TLS options
If you instead rather pick TLS library first, only quictls and BoringSSL are supported by all QUIC libraries (quictls is an OpenSSL fork with a BoringSSL-like QUIC API patched in). If you rather build curl to use Schannel (that’s the native Windows TLS API), GnuTLS or wolfSSL you have also indirectly chosen which QUIC and HTTP/3 libraries to use.
Picotls
ngtcp2 supports Picotls shown in orange in the image above because that is a TLS 1.3-only library that is not supported for other TLS operations within curl. If you build curl and opt to go with a ngtcp2 build using Picotls for QUIC, you would need to have use an second TLS library for other TLS-using protocols. This is possible, but is rarely what users prefer.
No OpenSSL option
It should probably be especially highlighted that the plain vanilla OpenSSL is not an available option. Primarily because they decided that the already created API was not good enough for them so they will instead work on implementing their own QUIC library to be released at some point in the future. That also implies that if we want to build curl to do HTTP/3 with OpenSSL in the future, we probably need to add support for a forth QUIC library – and someone would also have to write a HTTP/3 library to use OpenSSL for QUIC.
Why wolfSSL adding QUIC is good for HTTP/3
People in general want to build applications and infrastructure using released, official and supported libraries and the sad truth is that there is a clear shortage in such TLS libraries with QUIC support.
In your typical current Linux distribution, quictls and BoringSSL are usually not viable options. The first since it is an OpenSSL fork not many even ship as a package and the second because it is done by Google for Google and they don’t do releases and generally care little for outside-Google users.
For the situations where those two TLS options are out of the game, the image above shows you the grim reality: your HTTP/3 options are limited. On Windows you can go with msh3 since it can use Schannel there, but on non-Windows you can only use ngtcp2/nghttp3 and before this wolfSSL support the only TLS option was GnuTLS.
For many embedded solutions, or even FIPS requirements, wolfSSL is now the only viable option for doing HTTP/3 with curl.
curl, along with every other Internet tool with aspirations, supports proxies. A proxy is a (usually known) middle-man in a network operation; instead of going directly to the remote end server, the client goes via a proxy.
curl has supported proxies since the day it was born.
Which proxy?
Applications that do Internet transfers often would like to automatically be able to do their transfers even when users are trapped in an environment where they use a proxy. To figure out the proxy situation automatically.
Many proxy users have to use their proxy to do Internet transfers.
A library to detect which proxy?
The challenge to figure out the proxy situation is of course even bigger if your applications can run on multiple platforms. macOS, Windows and Linux have completely different ways of storing and accessing the necessary information.
libproxy
libproxy is a well-known library used for exactly this purpose. The first feature request to add support for this into libcurl that I can find, was filed in December 2007 (I also blogged about it) and it has been popping up occasionally over the years since then.
In August 2016, David Woodhouse submitted a patch for curl that implemented support for libproxy (the PR version is here). I was skeptical then primarily because of the lack of tests and docs in libproxy (and that the project seemed totally unresponsive to bug reports). Subsequently we did not merge that pull request.
Almost six years later, in June 2022, Jan Brummer revived David’s previous work and submitted a fresh pull request to add libproxy support in curl. Another try.
The proxy library dream is clearly still very much alive. There are also a fair amount of applications and systems today that are built to use libproxy to figure out the proxy and then tell curl about it.
What is unfortunately also still present, is the unsatisfying state of libproxy. It seems to have changed and improved somewhat since the last time I looked at it (6 years ago), but there several warning signs remaining that make me hesitate.
This is not a dependency I want to encourage curl users to lean and depend upon.
I greatly appreciate the idea of a libproxy but I do not like the (state of the) implementation.
My responsibility
Or rather, the responsibility I think we have as curl maintainers. We ship a product that is used and depended upon by an almost unfathomable amount of users, tools, products and devices.
Our job is to help guide our users so that the entire product, including third party dependencies, become an as safe and secure solution as possible. I am not saying that we can take full responsibility for the security of code outside of our own domain, but I think we should recognize that what we condone and recommend will be used. People read our support for library X as some level of approval.
We should only add support for third party libraries that meets a certain quality threshold. This threshold or bar maybe is (unfortunately) not written down anywhere but is still mostly a soft “gut feeling” based on human reviews of the situation.
What to do
I think the sensible thing to do first, before trying to get curl to use libproxy, is to make sure that there is a library for proxies that we can and want to lean on. That library can be the libproxy of today but it could also be something else.
Some areas in need of attention in libproxy that I recently also highlighted in the curl PR, that would take it closer to meeting the requirements we can ask of a dependency:
Improve the project with docs. We cannot safely rely on a library and its APIs if we don’t know exactly what to expect.
There needs to be at least basic tests that verify its functionality. We cannot fix and improve the library safely if we cannot check that it still works and behaves as expected. Tests is the only reliable way to make sure of this.
Add CI jobs that build the project and run those tests. To help the project better verify that things don’t break along the way.
Consider improving the API. To be fair, it is extremely simple now, but it’s also so simple that that it becomes ineffective and quirky in some use cases.
Allow for external URL parser and URL retrieving etc to avoid blocking, double-parsing and to reuse caches properly. It would be ridiculous for curl to use a library that has its own separate (semi-broken) HTTP transfer and its own (synchronous) name resolving process.
A solid proxy library?
The current libproxy is not even a lot of code, it could perhaps make more sense to just plainly write a new library based on how you would want a library like this to work – and use all the knowledge, magic and experience from libproxy to get the technical parts done correctly. A libproxy-next-generation.
But
This would require that someone really wanted to see this development take place and happen. It would take someone to take lead in the project and push for changes (like perhaps the 5 bullets I listed above). The best would be if someone who would like to use this kind of setup could sponsor a developer half/full time for a while to get a good head start on this.
Based on history, seeing we have had this known use case and this library around for well over a decade and this is the best we have accomplished so far, I am not optimistic that we can turn this ship around. Until then, we simply cannot allow curl to use this dependency.
The InterPlanetary File System (IPFS) is according to the Wikipedia description: “a protocol, hypermedia and file sharing peer-to-peer network for storing and sharing data in a distributed file system.”. It works a little like bittorrent and you typically access content on it using a very long hash in an ipfs:// URL. Like this:
I guess partly because IPFS is a rather new protocol not widely supported by many clients yet, people came up with the concept of IPFS “gateways”. (Others might have called it a proxy, because that is what it really is.)
This gateway is an HTTP server that runs on a machine that also knows how to speak and access IPFS. By sending the right HTTP request to the gateway, that includes the IPFS hash you are interested in, the gateway can respond with the contents from the IPFS hash.
This way, you just need an ordinary HTTP client to access IPFS. I think it is pretty clever. But as always, the devil is in the details.
Rewrite ipfs:// into https://
The quest for IPFS aficionados have seemingly become to add support for IPFS using this gateway approach to multiple widely used applications that know how to speak HTTP(S). Just rewrite the URL internally from IPFS to HTTPS.
The IPFS “URL rewrite” is done in such so that the example IPFS URL is converted into “https://$gateway/$hash”. The transfer is done by the client as if it was a plain old HTTPS transfer.
What if the gateway is not local?
This approach has its biggest benefit of course when you can actually use a remote IPFS gateway. I presume most random ordinary users who want to access IPFS does not actually want to download, install and run an IPFS gateway on their machine to use this new power. They might very well appreciate the idea and convenience of accessing a remote IPFS gateway.
Remote IPFS gateways illustration
After all, the gateways are using HTTPS so at least the transfers are secure, right?
Leading people behind IPFS are even running public IPFS gateways for anyone to use. Both dweb.link and ipfs.io have been mentioned and suggested for default use. Possibly they are the same physical host as they appear to have the same IP addresses (owned by Protocol Labs, one of the big proponents behind IPFS).
In an attempt to make this even easier for users, ffmpeg is made to use a built-in default gateway if none is set by the user.
I would expect very few users to actually have an IPFS gateway set. And even fewer to actually specify a local one.
So, when users want to watch a video using ffmpeg and an ipfs:// URL what happens?
The gateway sees it all
The remote gateway, that is administered by someone, somewhere, gets to see the full incoming request, and all traffic (video, whatever) that is sent back to the client (ffmpeg in this example) goes via the gateway. Sure, the client accesses the gateway via HTTPS so nobody can tamper with the traffic along the way, but the gateway is in full control and can inspect and tamper with the data as much as it likes. And there is no way for the client to know or detect if it is happening.
I am not involved in ffmpeg, nor do I have any insights into how the discussions evolved and were done around this subject, but for curl I have put my foot down and said that we must not blindly use and trust any default remote gateway like this. I believe it is our responsibility to not lure users into this traffic-monitoring setup.
Make sure you trust the gateway you use.
The curl way
curl will probably output an error message if there was no gateway set when an ipfs:// URL is used, and inform the user that it needs a gateway set to function and probably also show a URL for a page that contains more information about what this means and how to specify a gateway.
Bouncing gateways!
During the work of making the IPFS support for curl (which I review and comment on, not written or authored myself) I have also learned that some of the IPFS gateways even do regular HTTP 30x redirects to bounce over the client to another gateway.
Meaning: not only do you use and rely a total rando’s gateway on the Internet for your traffic. That gateway might even, on its own discretion, redirect you over to another host. Possibly run somewhere else, monitored by a separate team.
I have insisted, in the PR for ipfs support to curl, that the IPFS URL handling code should not automatically follow such gateway redirects, as I believe that adds even more risk to the user so if a user wants to allow this operation, it should be opt-in.
Imagine rising use of this
I would imagine that the ones hosting a popular default gateway either becomes overloaded and slow, or they need to scale up. If they scale up, they risk to leak traffic even wider. Scaling up also makes the operation more expensive, leading to incentives to make money somehow to finance it. Will the cookie car in the form of a massive data trove perhaps then be used/sold?
Users of these gateways get no promises, no rights, no contracts.
Other IPFS privacy concerns
Brave, the browser, has actual native IPFS support (not using any gateway) and they have an informative page called How does IPFS Impact my Privacy?
Final verdict
I am not dissing the idea nor implementation of IPFS itself. I think using HTTP gateways to access IPFS is a good idea in general as it makes the network more accessible. It is just a fragile solution that easily misleads users to do things they maybe shouldn’t. So maybe a little too accessible?
I poked the ffmpeg project over Twitter, there was immediate reaction and there is already a proposed patch for ffmpeg that removes the use of a default IPFS gateway. “it’s a security risk”
The preferred method of providing changes to the curl project, be it source code, documentation or web site contents, is by submitting a pull-request. A “PR”. On the curl repository on GitHub.
When a proposed curl change, bugfix or improvement is submitted as a PR on GitHub, it gets built, checked, tested and verified in countless ways and a few hundred developers get a notification about it.
The first thing I do every morning and the last thing I do every night before I go to bed, is eyeing through the list of open PRs, and especially the ones with recent activities. I of course also get back to them during the day if there is activity that might need my attention – not to mention that I also personally submit by own PRs to curl at an average rate of a few per day.
You can rightly say that my day and my work with curl is extremely PR centered. And by extension, also extremely git and GitHub focused.
Merging the work
When all the CI checks run green and all review comments and concerns have been addressed, the PR has become ripe for merge.
As a general rule, we always work in the master branch as the current development branch that is destined to become the next release when the time comes. An approved PR thus gets merged into the master branch and pushed there. We keep the git history linear and clean, meaning we do merge + rebase before pushing.
Commit messages
One of the key properties of a git commit, is the commit message. The text that describes the particular change in the repository.
In the curl project we have a standard style or template for how to write these messages. This makes them consistent and helps making them useful and get populated with informational meta-data that future versions of ourselves might appreciate when we look back at the changes decades into the future.
The PR review UI on GitHub unfortunately totally ignores commit messages, which means that we cannot comment on them or insist or check that users write them in our preferred style. Arguably, it is also just quicker and easier not to do that and instead just gently brush up the message in the commit locally before the commit is pushed.
Adding correct fixes #xxx and closes #xxx information to the message also helps making sure GitHub closes the associated issues/pull-requests and allows us to map git repository commits with GitHub activities at will – like when running statistic scripts etc.
This strictness and way of working makes it impossible to use the “merge” button in the GitHub UI since we simply cannot ensure that the commit is good enough with it. The button is however not possible to remove from the UI with any settings. We just prohibit its use.
They appear as “closed this in”
This way of working makes most PRs appear as “closed this in #[commit hash]” in the GitHub UI instead of saying “merged by” that same hash. Because in GitHub’s view, the PR was not merged.
GitHub could fix this with a merges keyword, which has been suggested since forever, but clearly they do not see this as a problem.
Occasional users of course get surprised or even puzzled by us closing the PR “instead of merging”, because that is what it looks like. When it really is not.
Ultimately, I consider the state and contents of our git repository and history more important than how the PRs appear on GitHub so I stick to our way of working.
Signed commits
As a little side-note, I of course also always GPG sign my commits so you can verify on GitHub and with git that my commits are genuinely done by me.
Stats
To give you an idea of the issue and pull request frequency and management in curl (the source code repository). Snapshots from the curl dashboard from August 8, 2022.
Update
Several people have mentioned to me after I posted this article that I could squash and edit the commits and force-push the work back to the user’s branch used for the PR before using the “merge” button or auto-merge feature of GitHub.
Sure, I could do it that way, but I think that is an even worse solution and a poor work-around. It is very intrusive method that would make me force-push in every user’s PR branch and I am certain a certain amount of users will be surprised by that and some even downright upset because it removes/overwrites their work. For the sake of working around a GitHub quirk. Nope, I will not do that.
My name is Emma Irwin, and I am a program manager at Microsoft, specifically working in the open source program’s office (OSPO). One of the programs I lead is the FOSS fund.
This may sound a bit odd, but normally I send an email notice to project winners, yet someone pointed out that I have missed notifying curl of their won (and I could not find any history of reaching out). As a result, I offer my sincere apologies now! – I will happily provide notice now, although the payments themselves have already started!
The Free and Open Source Software Fund (FOSS Fund) is a way for our employees to collectively select open source projects to receive $10,000 sponsorship awards throughout the year.
Microsoft’s engineers select projects they are super passionate about. Only employees who contribute to open source projects can participate in the selection process.
curl was selected in January for $10, 000.00 provided one month, for ten months through GitHub Sponsors.
Thanks very much and congratulations!
-Emma Irwin
Emma Irwin (she/her) Senior Program Manager Open Source Programs Office Microsoft
When I (in spite of knowing better) talk to ordinary people about what I do for a living and the project I work on, one of the details about it that people have the hardest time to comprehend, is the fact that I really and truly don’t know a lot about who uses my code. (Or where. Or what particular features they use.)
I work on curl full-time and we ship releases frequently. Users download the curl source code from us, build curl and put it to use. Most of “my” users never tell me or anyone else in the curl project that they use curl or libcurl. This is of course perfectly fine and I probably could not even handle the flood if every user would tell me.
This not-knowing is a most common situation for Open Source authors and projects. It is not unique for me.
The not knowing your users is otherwise unusual in a world of products and software, and quite frankly, sometimes it is an obstacle for us as well since we lack a good way to communicate with users about plans, changes or ideas. It also makes it really hard to estimate our own success and the always-recurring question: how many users do you have?
To be fair: I do know quite a lot of users as well. But I don’t know how representative they are, nor how big fraction of the totals they consist of etc.
A user of my project runs into an issue and asks for help in a project forum – without hiding where they come from or what product they make.
While vanity-searching I find my project’s Open Source license mentioned for or bundled with a product or tool or device.
A user emails me asking funny questions about a product I never heard of, and then I realize they found my email because that product uses my code and the license has my email in it.
Today on July 20 2022, curl turns exactly 8888 days old. It was born on March 20, 1998 when curl 4.0 was shipped.
The number 8 is considered a lucky number in several Asian cultures and I figure we can view this as a prequel to the planned curl version 8 release we intend to ship on curl’s 25th birthday.
Of course the precursors to curl existed well over a year before curl’s birthday, but let’s not get into that right now.
On July 1st 2017, exactly five years ago today, the OSS-Fuzz project “adopted” curl into their program and started running fuzz tests against it.
OSS-Fuzz is a project run by Google and they do fuzzing on a large amount of open source projects: OSS-Fuzz aims to make common open source software more secure and stable by combining modern fuzzing techniques with scalable, distributed execution.
That initial adoption of curl into OSS-Fuzz was done entirely by Google themselves and its fuzzing integration was rough and not ideal but it certainly got the ball rolling.
Later in in the fall of 2017, Max Dymond stepped up and seriously improved the curl-fuzzer so that it would better test protocols and libcurl options deeper and to a higher degree. (Max subsequently got a grant from Google for his work.)
Fuzzing curl
Fuzzing is really the next-level thing all projects should have a go at when there is no compiler warning left to fix and the static code analyzers all report zero defects, because fuzzing has a way to find silly mistakes, off-by-ones etc in a way no analyzers can and that is very hard for human reviewers to match.
curl is a network tool and library written in C, it is meant to handle non-trusted data properly and doing it wrongly can have serious repercussions.
For the curl project, OSS-Fuzz has found several silly things and bad code paths especially after that rework Max put in. We have no less than 6 curl CVEs registered in 2017 and 2018 giving OSS-Fuzz (at least partial) credit for their findings.
In the first year or two, OSS-Fuzz truly kept us on our toes and we fixed numerous other flaws as well that weren’t CVE worthy but still bugs. At least 36 separate ones to date.
The beauty of OSS-Fuzz
For a little Open Source project such as curl, this service is great and in case you never worked with it let me elaborate what makes me enjoy it so much.
Google runs all the servers and keeps the service running. We don’t have to admin it, spend energy on maintaining etc.
They automatically pull the latest curl code from our master branch and keep fuzzing it. All day, every day, non-stop.
When they find a problem, we get a bug submitted and an email sent. The bug report they create contains stack traces and a minimized and reproducible test case. In most cases, that means I can download a small file that is often less than a hundred bytes, and run my locally built fuzzer code with this input and see the same issue happen in my end.
The bug they submit is initially closed for the public and only accessible to select team of curl developers.
90 days after the initial bug was filed, it is made public automatically.
The number of false positives are remarkably low, meaning that almost every time the system emails me, it spotted a genuine problem.
Fuzzing per commit
In 2020, the OSS-Fuzz team introduced CIFuzz, which allows us to run a little bit of fuzzing – for a limited time – on a PR as part of the CI pipeline. Using this, we catch the most obvious mistakes even before we merge the code into our master branch!
After the first few years of us fixing bad code paths, and with the introduction of CIFuzz, we nowadays rarely get reports from OSS-Fuzz anymore. And when we do, we have it complain on code that hasn’t been released yet. The number of CVEs detected in curl by OSS-Fuzz has therefore seemingly stopped at 6.
Taking fuzzing further
After five years of fuzzing, we would benefit from extending and improving the “hooks” where and how the fuzzing is made so that we can help it find and reach code paths with junk that it so far hasn’t reached. There are of course still potentially many undetected flaws in curl remaining because of the limited set of entry point we have for the fuzzer.
I hope OSS-Fuzz continues and lives on for a long time. It certainly helps curl a lot!
