> 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.

It's also worth noting that you can pledge for an execed process using the second argument to pledge. This is for when you know what the child is expected to do on your behalf.

> Pledge/unveil are manifestly more successful, being used pervasively (though not really by programs which do not primarily target openbsd).

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.

It really doesn't make much sense to compare pledge/unveil to seccomp and capsicum or any other sandboxing solution.

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.

A major limitation of seccomp is that you can't turn off inheritance. With pledge inheritance is opt-in.

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.

> Freebsd has capsicum which, like seccomp (mentioned else-thread) is much more complex and flexible.

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.

I said you could get them from a cooperating uncapsicumed process. But, it's not simple, and what are you going to write that loophole process in, and why does it get access to the filesystem if it only needed sockets, etc.

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.

It is pretty simple - implementing that for irssi(1) took a dozen or two lines of C, IIRC. Sure, it could be simpler, and hopefully libcasper(3) will make it happen, but it's not much of a difference.