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@xxxxxxxxxx>
> > ---
> > .../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.
