[yencabulator]

Zombie processes are already dead and aborted, there's nothing more a kill -9 would do to them.

The kernel retains minimal state about them because the system has made a promise to report that the process exited to its parent process, and the parent process hasn't gotten around to asking for that yet.

(Don't confuse zombies with uninterruptible I/O sleep, or buggy kernel workers.)

[mjevans]

I might have done that last line, but I do mean generally. Even if it isn't kill -9 (which it should be, somehow, since that's the human's intent when they use it); there should be some mechanism for reaching the 'failed' state and the process itself leaving the accounting tables.

Stuck mounts have a half solution (lazy unmounts) but even _that_ interface really also needs a timeout value after which operations on the target should be assumed to fail rather than return correctly.

Offhand, I wonder if there's currently or previously been a DoS attack based on defunct uninterruptible sleep. Theoretically a system could be exhausted of PIDs which could lead to nasty issues.

[yencabulator]

> 'failed' state and the process itself leaving the accounting tables.

Once again, that cannot be done until the parent process consumes the exit status. That's what the zombie is there for. Zombies don't take up much space.

> Stuck mounts have a half solution (lazy unmounts) but even _that_ interface really also needs a timeout value after which operations on the target should be assumed to fail rather than return correctly.

These days most NFS etc mounts are "soft mounts", that is operations will eventually time out.

Lazy unmount doesn't really apply here, it makes the mountpoint disappear from the global namespace, but all existing open files remain untouched, and the mount lives as long as anything is still using it; it just removes the "entry point" to the mount.

On today's Linux, it's up to each filesystem to provide abort/timeout mechanism. For timeouts, this is the right design, as demonstrated by macOS complications with FUSE. I do wish there was a common way to make things abort.

There was a patch in circulation a long time ago, that could seamlessly switch all open FDs of any given mountpoint into a whole different filesystem named badfs. badfs would just return an error on any operation. As far as I know, that patch never got merged, probably because nobody ever got it working 100%.

That kind of a DoS would require a local attacker, and then the victim to access a mountpoint owned by the attacker. Using FUSE, you could get a lot of processes hanging like that, for sure. I guess you could trap a mail delivery agent, if you still had a system where mail was delivered to users' home directories.

However, forcibly aborting any FUSE mount is a single `echo 1 >/sys/fs/fuse/connection/NNNN/abort`, the only challenge is finding the right ID. (See https://github.com/bazil/fuse/blob/fb710f7dfd05053a3bc9516dd...)