Message-ID: <aG9rNQ277weSR_dl@kernel.org>
Date: Thu, 10 Jul 2025 10:26:45 +0300
From: Mike Rapoport <rppt@...nel.org>
To: Pratyush Yadav <pratyush@...nel.org>
Cc: Christian Brauner <brauner@...nel.org>,
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Subject: Re: [RFC v2 10/16] luo: luo_ioctl: add ioctl interface

On Wed, Jul 09, 2025 at 11:27:08PM +0200, Pratyush Yadav wrote:
> On Sun, Jul 06 2025, Mike Rapoport wrote:
> 
> > 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.
> 
> Yep, I was poking around the VFS code last week and saw the same
> problem.
> 
> >
> >> 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.
> 
> I suppose you can have a special file, say "preserve_fd", where you can
> write() the FD number.
> 
> This is in some ways similar to how you would write it to the ioctl()
> via the arg buffer/struct. And I suppose you can have other special
> files to do the things that other ioctls would do.
> 
> That is one way to do it, although I dunno if it classifies as a
> "proper" use of the VFS APIs...

IIUC Christian's point was mostly not about using VFS APIs (i.e.
read/write) but about using a special pseudo fs rather than devtmpfs to
drive ioctls.
 
So instead of 

	fd = open("/dev/liveupdate", ...);
	ioctl(fd, ...);

we'd use

	fd = open("/sys/fs/kexec/control", ...);
	ioctl(fd, ...);

> -- 
> Regards,
> Pratyush Yadav

-- 
Sincerely yours,
Mike.

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