Re: do_change_type(): refuse to operate on unmounted/not ours mounts

[Al Viro]

On Thu, Jul 24, 2025 at 01:02:48PM -0700, Andrei Vagin wrote:
> Hi Al and Christian,
> 
> The commit 12f147ddd6de ("do_change_type(): refuse to operate on
> unmounted/not ours mounts") introduced an ABI backward compatibility
> break. CRIU depends on the previous behavior, and users are now
> reporting criu restore failures following the kernel update. This change
> has been propagated to stable kernels. Is this check strictly required?

Yes.

> Would it be possible to check only if the current process has
> CAP_SYS_ADMIN within the mount user namespace?

Not enough, both in terms of permissions *and* in terms of "thou
shalt not bugger the kernel data structures - nobody's priveleged
enough for that".

What the hell is CRIU trying to do there?