linux-kernel - Re: [RFC v2 10/16] luo: luo

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Message-ID: <aGqHFkPWOrD6whv6@kernel.org>
Date: Sun, 6 Jul 2025 17:24:22 +0300
From: Mike Rapoport <rppt@...nel.org>
To: Christian Brauner <brauner@...nel.org>
Cc: Pasha Tatashin <pasha.tatashin@...een.com>, pratyush@...nel.org,
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Subject: Re: [RFC v2 10/16] luo: luo_ioctl: add ioctl interface

On Tue, Jun 24, 2025 at 11:50:49AM +0200, Christian Brauner wrote:
> On Thu, May 15, 2025 at 06:23:14PM +0000, Pasha Tatashin wrote:
> > Introduce the user-space interface for the Live Update Orchestrator
> > via ioctl commands, enabling external control over the live update
> > process and management of preserved resources.
> > 
> > Create a misc character device at /dev/liveupdate. Access
> > to this device requires the CAP_SYS_ADMIN capability.
> > 
> > A new UAPI header, <uapi/linux/liveupdate.h>, defines the necessary
> > structures. The magic number is registered in
> > Documentation/userspace-api/ioctl/ioctl-number.rst.
> > 
> > Signed-off-by: Pasha Tatashin <pasha.tatashin@...een.com>
> > ---
> >  .../userspace-api/ioctl/ioctl-number.rst      |   1 +
> >  drivers/misc/liveupdate/Makefile              |   1 +
> >  drivers/misc/liveupdate/luo_ioctl.c           | 199 ++++++++++++
> >  include/linux/liveupdate.h                    |  34 +-
> >  include/uapi/linux/liveupdate.h               | 300 ++++++++++++++++++
> >  5 files changed, 502 insertions(+), 33 deletions(-)
> >  create mode 100644 drivers/misc/liveupdate/luo_ioctl.c
> >  create mode 100644 include/uapi/linux/liveupdate.h

...

> > +static const struct file_operations fops = {
> > +	.owner          = THIS_MODULE,
> > +	.open           = luo_open,
> > +	.unlocked_ioctl = luo_ioctl,
> > +};
> > +
> > +static struct miscdevice liveupdate_miscdev = {
> > +	.minor = MISC_DYNAMIC_MINOR,
> > +	.name  = "liveupdate",
> > +	.fops  = &fops,
> > +};
> 
> I'm not sure why people are so in love with character device based apis.
> It's terrible. It glues everything to devtmpfs which isn't namespacable
> in any way. It's terrible to delegate and extremely restrictive in terms
> of extensiblity if you need additional device entries (aka the loop
> driver folly).
> 
> One stupid question: I probably have asked this before and just swapped
> out that I a) asked this already and b) received an explanation. But why
> isn't this a singleton simple in-memory filesystem with a flat
> hierarchy?
> 
> mount -t kexecfs kexecfs /kexecfs
> 
> So userspace mounts kexecfs (or the kernel does it automagically) and
> then to add fds into that thing you do the following:
> 
> linkat(fd_my_anon_inode_memfd, "", -EBADF, "kexecfs/my_serialized_memfd", AT_EMPTY_PATH)

Having an ability to link a file descriptor to kexecfs would have been
nice. We could even create a dependency hierarchy there, e.g.

mkdir -p kexecfs/vm1/kvm/{iommu,memfd}

linkat(kvmfd, "", -EBADF, "kexecfs/vm1/kvm/kvmfd", AT_EMPTY_PATH)
linkat(iommufd, "", -EBADF, "kexecfs/vm1/kvm/iommu/iommufd", AT_EMPTY_PATH)
linkat(memfd, "", -EBADF, "kexecfs/vm1/kvm/memfd/memfd", AT_EMPTY_PATH)

But unfortunately this won't work because VFS checks that new and old paths
are on the same mount. And even if cross-mount links were allowed, VFS does
not pass the file objects to link* APIs, so preserving a file backed by
anon_inode is another issue.

> which will serialize the fd_my_anon_inode_memfd. You can also do this
> with ioctls on the kexecfs filesystem of course.

ioctls seem to be the only option, but I agree they don't have to be bound
to a miscdev.

-- 
Sincerely yours,
Mike.