Pledge/unveil are openbsd-specific. Freebsd has capsicum which, like seccomp (mentioned else-thread) is much more complex and flexible.
Pledge/unveil are manifestly more successful, being used pervasively (though not really by programs which do not primarily target openbsd). Seccomp and capsicum are barely used at all (likely due to their higher complexity), and capsicum usage was even removed from a few freebsd utilities. At the same time, there have been some valid criticisms of pledge/unveil by capsicum people; IIRC something to do with its restrictions not persisting following an exec?
The validity of that criticism is so-so. It's a common complaint from people who are trying to build an externally imposed sandbox that dictates what a program can do.
But that's not what pledge and unveil are. They're more of an internally imposed set of constraints: the program just announces what it's going to do. After that, if it breaks the contract (due to a bug or malicious intervention), the system has license to kill it.
The program knows what it is going to do, so it can write the contract for itself. But it doesn't know what some other program is going to do, so it doesn't make sense for these restrictions to persist after exec. The program-to-be-exec'd should have its own pledges.
In reality it's even more complicated than that: programs often need to perform some "privileged" operations before they are ready to put on their straight jacket. It'd be very hard for program A to say that program B starts with privileges X, Y, Z, and then after instruction Q drops Y and Z. And program B might require more privileges than what A had when it called exec, so again it's just not going to work for this at all. If A's privileges were to persist, then it would have to have some way to elevate them or it'd never drop them in the first place. Both seem like a bad deal.
It's just a completely different mechanism, but people think sandbox sandbox sandbox and if that's all one can think of, pledge and unveil might seem like a terrible tool for that. They're not a tool for sandboxing untrusted programs.
Because of their simplicity, they are easily added as a patch to a ported application. There are probably more ports using them than programs that target openbsd.
Pledge and Unveil are really part of the program's specification. They are much closer in practice to asserts, pre-conditions, or contracts depending on the programming language you have used. They basically make sure that the program you are writing doesn't do anything stupid with bad inputs from the outside world. Its part of the development process, and not very hard to add to an existing program.
This has major consequences because it makes it possible for a process to secure itself without affecting its ability to spawn child processes. You could apply seccomp in-process or use a spawn-helper before securing it. But more realistically it means that one applies an external seccomp profile to a group of processes (e.g. a container) that contains the union of all needed syscalls.
So while seccomp is more flexible in some sense (being able to run BPF filters on every syscall) its architecture leads to it being applied in a less flexible way.
Capsicum may be more flexible in some ways, but it's also less flexible in others.
After you a process entera capsicum mode, it can't open new sockets, except by accepting on an existing listen socket or by receiving them on a unix socket, sent by a cooperating non-capsicum process. This means you can't capsicum a TLS proxy like hitch, which would be a great thing to capsicum since the operation is pretty simple and OpenSSL is scary.
Capsicum is simply not flexible in this way. Maybe if there was a way to open a new socket with a capability you setup earlier, that would be flexible enough.
https://awesomekling.github.io/pledge-and-unveil-in-Serenity...
(There are other mechanisms like containers or freebsd' jail that try to accomplish the same thing, but those tend to be "lots of functionality inside the sandbox" solutions, whereas openbsd is mostly aiming for "allow nothing besides the minimum in the sandbox".
(edited for clarity, thanks for the nudge ghoward).
> ...they are mostly hopeless to try to apply to portable software after the fact if the devs aren't constantly testing with it.
They don't have to be hopeless. They were pretty easy ([1], [2]) to add to my bc.
[1]: https://git.yzena.com/gavin/bc/commit/3e8cd345de9d5b65ac7c33...
[2]: https://git.yzena.com/gavin/bc/commit/b6c65bb44c910c054bedfa...
I was referring to the OpenBSD devs trying to hack pledges into other people's software by trying to guess what capabilities it needs while porting. That's not a good thing to try to do -- see also all the various attempts at guessing seccomp profiles in the Linux world, which has the same problem.
grep the ports tree for pledge patches.
That is both the benefit and the downside of non-commercial projects: you get to do whatever you want.
It’s an openbsd feature they built for themselves and deployed throughout the base system.