I don't understand your comment - dereferencing a null pointer is unsafe, in the sense that it does not reliably crash but may do other things, as we saw in the kernel case we're talking about. Yes that particular case was only exploitable if you mapped the zero page, but given how all-bets-are-off a situation it created (where extremely experienced programmers thought they knew what the code did, thought it was safe, and were wrong), I would not want to count on all cases not being exploitable without mapping the zero page.
We are talking about an extremely simple straightforward API with an obvious contract. It's good enough for this function to reliably surface almost all wrong uses with a segfault immediately. Wrong use will result in segfaults and otherwise bugs and crashes. The goal is not to work when used wrong but to work when used right. You cannot save the world from scratch in every little function. You still have a job to get done, and you have to move on.
> You cannot save the world from scratch in every little function. You still have a job to get done, and you have to move on.
Or you can take all of 10 minutes to put sanity-check assertions at the start of all your public-facing API functions, eliminating a source of security bugs, get on with your life, and worry about the performance implications as and when it becomes a problem (hint: it's never going to become a problem).
You can try and do this if it's a relatively narrow public facing API, but otherwise this is a theoretic ideal. In practice, if you add an assertion for every pointer argument to every little function, you'll go insane, and it is completely pointless, and the code will not be readable anymore.
There are so many other interesting and relevant invariants that are usually in an API contract that are much harder or impossible to check upfront (let alone express formally in a type system), and even violations may be impossible to diagnose when they happen.
People focus on NULL because that's the only way they can apply their silly limited type systems. But NULL checks give very little return for investment. In practice, you'll see templated Option<T> types and whatnot, and when I have to look at or even work with such code I want to kill myself because it's so painful.
No, people focus on a handful of things like null, buffer overrun, and use-after-free because they still make up the majority of security vulnerabilities that we see exploited in the wild. You may imagine that subtle logic errors are more common, but the data doesn't bear that out; also FWIW I've never seen one of these detailed invariants be impossible to express in a type system if you spend 5 minutes actually trying.
Given a, b, c input parameters to my func, it must hold that that a->m->t == b->t. c->mutex must be held, and c->cond is the condition variable that goes with c->mutex and will release any waiters on the buffer contained in a.
Or: Integer x is representable using 12 bits only, Integer y should be a multiple of N and I have a integer s is used as a bit-shift that should be less than 8.
Or: I need to guarantee that no locks have to be taken and no allocations have to be made on this complicated looking codepath. While holding a lock, we must not do any syscalls (syscall a, b, c are ok though), and surely not make any logging calls.
I know only one system that can express this, it's called STRAIGHTFORWARD CODE, and it requires doing engineering and casual logic out-of-band, and yes it does include making mistakes and repairing them incrementally.
I don't know a type system that would let me explain these things to me and tell me where I was wrong. But maybe you can show me, with 5 minutes of actually trying?
So define a wrapper type that represents that invariant (it's not going to take up space at runtime), where the only constructor enforces it?
> Integer x is representable using 12 bits only, Integer y should be a multiple of N and I have a integer s is used as a bit-shift that should be less than 8.
Those are all standard things that already exist?
> Or: I need to guarantee that no locks have to be taken and no allocations have to be made on this complicated looking codepath. While holding a lock, we must not do any syscalls (syscall a, b, c are ok though), and surely not make any logging calls.
Sounds like a pretty standard free monad case? Define a command algebra in which the "ok" syscalls are a subtype, and then require that the thing you want to only use the ok calls to have a type that reflects that?
Please, go ahead and type the example. I think you are trolling.
> Define a command algebra in which the "ok" syscalls are a subtype
Dude, it's clear you're not doing any actual work. You are living in an ivory tower, and you underestimate the complexity and detail and volatility of real world applications by at least 3 orders of magnitude. You don't understand how to modularize and contain complexity.
You _cannot_ complete a project with this attitude.
You are ignorant of the fact that a type system is necessarily a blunt simplification of the real complexity. Therefore, use of types must be pragmatic, and actual logic must be coded in normal code (which should be obvious but it isn't to type theory weirdos). Otherwise, you require dependent typing or whatever, and you will have to write your code twice, once in a usable programming language and once in a very unusable programming language. Much more than only twice actually, given that all the implicit detail should apprently go explicitly formalized at the type level.
Just to make sure I'm not entirely talking out of my arse because I'm so incredibly annoyed by your otherworldly proposition, I asked an AI about the sel4 microkernel. It consists of 10,000 lines of C code (that says a lot about its practical utility, which is very limited), and of 1,3 million lines of manually written proof code (which says a lot about the practicality of proving).
It takes a lot longer to figure out if it'll be a problem than to just add the check. And you don't have to ponder whether it's possible for a null to get there, because now it's fine if it does.
Are you talking about extending the API contract to allow for NULL? That is often the path to madness, especially if it requires complicating the signature (return value etc). Better to just assert/crash.
Because no one is expecting it to work if a null is passed. Your total range of behaviours left are crashes, doesn't crash and is silently ok, or doesn't crash and causes something worse (data corruption, you get your product in a CVE, that area).
My proposition is that "it's silently ok" isn't likely enough, which is in line with your position on "don't extend the contract to accept null". So what's left is crash, or something worse.
So if those are your choices, don't waste time justifying that a null can't get there, just add a check to ensure you get the better behaviour. It takes seconds.
If you follow that line of reasoning, you will end up testing almost every pointer before accessing it. The reason is that you are extending your valid state space massively since you aren't able to specify "this subset of 7 trillion distinct states is invalid, if it was the case we would have failed before".
You are requiring yourself to find a valid outcome for an input that doesn't make _any_ sense in the context of what your application is meant to achieve. How is that not a Sysiphean task?
You're not "extending" the valid state space. That null value being passed to that function is already a potential state of your program.
You're actually pruning the valid state space; before, when the null value is passed to the function, there are more operations performed that have uncertain consequences. If you assert-and-fail when you get the null input, you've pruned those states.
So if I understand correctly now, you _do_ proclaim to put asserts, not write code that somehow copes with the "possiblity" of NULL.
"Because no one is expecting it to work if a null is passed", so you can do whatever. If you write an assert for every pointer passed to every function, that will be a lot of asserts, for pretty much the same outcome in practice. Asserts are just marginally more ergonomic when they trigger, but are a nuisance in the code often. So my position is to use them judiciously, but not overdo it, be instead focused on the actual task.
When the lack of non-null assertions is an actual problem during development, you have much larger structural issues.
Yes. The work to assert each pointer passed into a function isn't "high"; it's purely mechanical, it could almost be refactored automatically. But most of all, the effort required to prove you don't need to is _way_ higher.
I thought so, too, at some point, and I put it to practice, actually did that for a long time. (I'm not _that_ much of a talker, I can back up what I'm saying with real experience having erred on the opposite side for a long time).
I thought so too, for many other things, and did so too in practice. Then I was doing thousands of little tasks in order to improve "security" and "correctness" and "proving everything", and never got to actually do anything useful, and my code became bloated to the point it didn't get to do much useful work.
Now I'm more considerate, trying to weigh everything in the larger context, is it REALLY required to get my job done?
