Never break userspace [LWN.net]

Never break userspace

Posted Feb 16, 2024 19:03 UTC (Fri) by rahulsundaram (subscriber, #21946)
In reply to: Never break userspace by mb
Parent article: A turning point for CVE numbers

> It's exactly the same thing. Things change. Deal with it like everybody has to deal with systemd.

The difference is that kernel developers have publicly committed to never breaking userspace. Systemd developers haven't. It is the disconnect between the public messaging and reality that's causing the contention. Not the changes themselves necessarily.

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Never break userspace

Posted Feb 16, 2024 19:10 UTC (Fri) by mb (subscriber, #50428) [Link] (3 responses)

>The difference is that kernel developers have publicly committed to never breaking userspace

Things like uevents, tracepoints, sysfs files, etc... were pretty much never part of that claim.
Devs try hard to not make unnecessary breakages, but if a sysfs file disappears/changes or an uevent changes, programs have to deal with it.
Has always been like that.

> It is the disconnect between the public messaging and reality that's causing the contention.

The disconnect between the expectation and the reality is causing the contention.

Never break userspace

Posted Feb 16, 2024 19:21 UTC (Fri) by bluca (subscriber, #118303) [Link] (2 responses)

> Things like uevents, tracepoints, sysfs files, etc... were pretty much never part of that claim.

Citation needed. That is very much not evident from any claim anybody has ever made that I have seen.

Never break userspace

Posted Feb 16, 2024 19:41 UTC (Fri) by mb (subscriber, #50428) [Link] (1 responses)

>Citation needed

Even syscalls have been removed in the past, breaking applications.
BUT these applications always were very limited in count and usually part of the OS itself.

Citation: Look at the sources.

There has never been a thing like a general stability guarantee.
It always has been a matter of common sense.
If a change only breaks udev or systemd and nothing else, it might make sense to do it.

Never break userspace

Posted Feb 16, 2024 20:07 UTC (Fri) by bluca (subscriber, #118303) [Link]

Excellent, you have now demonstrated that my original post, claiming that there is no such thing as 'kernel never breaks userspace', was indeed correct, and thus 'just update to the latest' cannot be suggested as a solution to security issues since public interfaces will be broken left and right with no regard for backward compatibility. Congrats!

> If a change only breaks udev or systemd and nothing else, it might make sense to do it.

I beg to differ

Never break userspace

Posted Feb 16, 2024 19:25 UTC (Fri) by Wol (subscriber, #4433) [Link] (1 responses)

> The difference is that kernel developers have publicly committed to never breaking userspace.

Where?

Okay, I know Linus says "never break user-space", and he is very strict about it. But at the end of the day, shit happens.

And there's plenty of kernel developers who *haven't* signed up to it. They just know that trying to get it past Linus is not a battle worth fighting most of the time.

There's one big example I can think of, that had a rather nasty fall-out, in the raid world. So bad, in fact, that kernels were modified to have an explicit "fail to boot" config, iirc!

Something to do with the fact that raid layout was accidentally changed. So you have pre-change kernels that will trash post-change arrays, pre-discovery kernels that will trash pre-change arrays, and post-discovery kernels that will refuse to access arrays without a "this is a pre/post-layout flag".

Sometimes that's all you can do :-(

Cheers,
Wol

Never break userspace

Posted Feb 16, 2024 20:04 UTC (Fri) by bluca (subscriber, #118303) [Link]

In this very thread you can see such statements