kernel.yama.ptrace_scope = 1
> If you can run gdb on that distribution, then you can definitely do it, even if it's just because they whitelist gdb in selinux/aa (in which case you can just script gdb).
No, you can't. Try it.
> And I rather doubt it is a default, since ptrace-based sandboxing is a thing (even Firefox was using it).
You can trace child processes, but not any other process.
EDIT: Apparently Ubuntu, but not Debian, SuSE, Arch, etc. Well, TIL.
> No, you can't. Try it.
You definitely can, it's just that that as you said, gdb just can't attach to anything but a child process; making gdb only work with a process it spawned itself.
Anyway, in addition to changing .profile, you still can do practically everything including modifying SHM segments, writing to pipes, sockets, etc. You could start closing feature by feature (e.g. remove /proc like Android), but for some reason it doesn't seem like the right approach.
The approach being used in containerization is namespaces. You can put new processes into a new IPC / user / PID / network / time / etc. namespace, which isolates them from the parent namespace. Once that's done and you can't mess with other processes via the filesystem / kernel, the remaining hole is servers with inadequate security models, such as X11.