Why use OpenBSD?

But only in very small sandboxes, right? Yes, seccomp could potentially be used for your JIT/interpreter sandbox. And because it inherently executes untrusted input, that's definitely the most important place.

But compare how many applications execute untrusted remote programs to how many programs that have had security vulnerabilities. Or indeed, how much code.

What percentage of code runs in chrome/firefox's sandbox? 0.0001%?

Have you tried to create a seccomp ruleset for a real program? I have. There are too many variations between machines and code paths that you'll necessarily need to leave wide open doors through your policy. Sure, the more you disable the "luck" you manufacture in case of a bug, preventing exploitation. But no, it's not fit for purpose outside these extremely niche use cases.

> So even if one uses Cosmopolitan libc, if you link to some other library that library may also do direct syscalls. And which syscalls is does, and under which circumstances, is generally not part of the ABI promise. So this can still break between semver patch version upgrades.

Well but isn't that a more general problem with pledge? I can link to libfoo, drop rpath privileges, and it'll work fine until libfoo starts lazily loading /etc/fooconf (etc.)

A nice thing about pledge is that it's modularized well enough so such problems don't occur very often, but I'd argue it's not less common of an issue than "libfoo started doing raw syscalls." The solution is also the same: a) ask libfoo not to do it, or b) isolate libfoo in an auxiliary process, or c) switch to libbar.

> And at the very least you'll also be linking to libseccomp. :-)

libseccomp proponents won't tell you this, but you can in fact use seccomp without libseccomp, as does Cosmopolitan libc. All libseccomp does is abstract away CPU architecture differences, which a libc already has to do by itself anyway.

(In my project, I got annoyed enough by the kernel header dependency that I just replaced libseccomp with a shell script: https://codeberg.org/bptato/chawan/src/commit/cad5664fc0aa10... although this might have gotten me a place reserved in hell.)

> 1) pledge() lets you give high level "I just want to do I/O on what I already have", and it doesn't matter if new syscalls "openat2" (should be blocked) or "getrandom" (should be allowed) are created. (see the `newfstatat` example on printf).

You can do this with seccomp if you're libc. A new syscall is of no consequence for the seccomp filter unless libc starts using it, in which case libc can just add it to the filter. (Of course the filter has to be an allow-list.)

> And 2) OpenBSD limits syscalls to be done from libc, and libc & kernel are released together. Other libs need to go through libc.

That avoids one failure mode, but I think you assign too much importance to it. If your dependency uses a raw syscall (and let's be honest this isn't that common), you'll see your program SIGSYS and add it manually.

If you have so many constantly changing dependencies that you can't tell/test which ones use raw syscalls and when, you have no hope of successfully using pledge either.

> But I don't think that we need to throw out the baby with that bathwater.

We agree here, just not on which baby :)

> And it's not just about libfoo doing raw syscalls. `unveil()` allows blocking off the filesystem.

You're right, seccomp is unsuitable for implementing unveil because it can't inspect contents of pointers. I believe Cosmopolitan uses Landlock for it.

> Though another problem is that it doesn't help child processes with a statically compiled newer libc

If you're trying to pledge a program written by somebody else, expect problems on OBSD too because pledge was not designed for that. (It can work in many cases, but that's kind of incidental.)

If it's your own program, fine, but that means you're compiling your binaries with different libcs and then wat.

> So yeah, because they mandate syscalls from libc, ironically OpenBSD should have been able to make pledge/unveil a libc feature using a seccomp-like API, or hell, implemented entirely in user space. But Linux, which has that API, kinda can't.

My take is "it can, with caveats that don't matter in 99% the cases pledge is useful in." (Entirely in user space no, with seccomp yes.)