Re: [PATCH 0/3] util/userfaultfd: Support /dev/userfaultfd

["Michal Prívozník" <mprivozn@redhat.com>]

On 1/26/23 16:25, Peter Xu wrote:
> On Thu, Jan 26, 2023 at 02:15:11PM +0000, Dr. David Alan Gilbert wrote:
>> * Michal Prívozník (mprivozn@redhat.com) wrote:
>>> On 1/25/23 23:40, Peter Xu wrote:
>>>> The new /dev/userfaultfd handle is superior to the system call with a
>>>> better permission control and also works for a restricted seccomp
>>>> environment.
>>>>
>>>> The new device was only introduced in v6.1 so we need a header update.
>>>>
>>>> Please have a look, thanks.
>>>
>>> I was wondering whether it would make sense/be possible for mgmt app
>>> (libvirt) to pass FD for /dev/userfaultfd instead of QEMU opening it
>>> itself. But looking into the code, libvirt would need to do that when
>>> spawning QEMU because that's when QEMU itself initializes internal state
>>> and queries userfaultfd caps.
>>
>> You also have to be careful about what the userfaultfd semantics are; I
>> can't remember them - but if you open it in one process and pass it to
>> another process, which processes address space are you trying to
>> monitor?
> 
> Yes it's a problem.  The kernel always fetches the current mm_struct* which
> represents the current context of virtual address space when creating the
> uffd handle (for either the syscall or the ioctl() approach).

Ah, I did not realize that.

> 
> It works only if Libvirt will invoke QEMU as a thread and they'll share the
> same address space.
> 
> Why libvirt would like to do so?

Well, we tend to pass files as FD more and more, because it allows us to
give access to "privileged" files to unprivileged process. What I did
not realize is that userfaultfd is different, not yet another file.

Michal