From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 46C2B231826; Wed, 9 Jul 2025 21:27:17 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1752096438; cv=none; b=LRcm0YoUCtgIVnkiYcRNsnGvQtrqK1QETna7ZAkiGzAZfg7OlSQ2/tRf4aCzgiH+D7A2TCKYfpOduVYZdV1fzB+ZjZaNfO87akCd2Y7TmTySwcPZgxpTmKEST3S9LPHxaw38icqq95WzRRa1VdfZo6T/NhvtrDPA4Xhqz/sK09I= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1752096438; c=relaxed/simple; bh=di0qI7GEmczq14zBNaBk8AWjgNHZtoR0rPpsB6FrrvM=; h=From:To:Cc:Subject:In-Reply-To:References:Date:Message-ID: MIME-Version:Content-Type; b=cim249bpC8A1gH0HPnZJwGgRZxF7etS9henaecWzsdP23Rp826bFmSoAbYoj1bVfuD1WyxfgopKcNYthdZzAZpJmhVardwao5oBtKb2w5z1Fj/tkfEkQahUfjZuh/MjfGibIV49TUhhrjGwIBnAqfGBjv2nqGO+mP5EQLPlWcSc= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=ldzQw00U; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="ldzQw00U" Received: by smtp.kernel.org (Postfix) with ESMTPSA id F162EC4CEF4; Wed, 9 Jul 2025 21:27:09 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1752096437; bh=di0qI7GEmczq14zBNaBk8AWjgNHZtoR0rPpsB6FrrvM=; h=From:To:Cc:Subject:In-Reply-To:References:Date:From; b=ldzQw00U+OetIKA481nc0vtmKNZA1RskLqjb7X1NOEN1OeisHWun1QCwnCjI5ottS vx4NKF7Pf8e0EPpxQC9AFg2qPBCYZ3d6DbJ21t2lwKjYlgSAvwQRR1yt4EJkrndA1f trzz9UmGeEgappXz0em9JwDLdjx01xkdVQbI3rVE3SIia4ICLQG8cbVWFxlnpTAUKc WQFupUDU4w3ADhfQa1mGwDVdFb//Oc88+StcqBA4tOTf5V4klAd2DKIQiX+U54taBE cwWDg3qNLgwXVZYFjvCKSq8cfutNsrmXNizt1CYW6H3QEfQT+dYgCuQqr6GqUb4mQt RtrsejD0cEWWg== From: Pratyush Yadav To: Mike Rapoport Cc: Christian Brauner , Pasha Tatashin , pratyush@kernel.org, jasonmiu@google.com, graf@amazon.com, changyuanl@google.com, dmatlack@google.com, rientjes@google.com, corbet@lwn.net, rdunlap@infradead.org, ilpo.jarvinen@linux.intel.com, kanie@linux.alibaba.com, ojeda@kernel.org, aliceryhl@google.com, masahiroy@kernel.org, akpm@linux-foundation.org, tj@kernel.org, yoann.congal@smile.fr, mmaurer@google.com, roman.gushchin@linux.dev, chenridong@huawei.com, axboe@kernel.dk, mark.rutland@arm.com, jannh@google.com, vincent.guittot@linaro.org, hannes@cmpxchg.org, dan.j.williams@intel.com, david@redhat.com, joel.granados@kernel.org, rostedt@goodmis.org, anna.schumaker@oracle.com, song@kernel.org, zhangguopeng@kylinos.cn, linux@weissschuh.net, linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org, linux-mm@kvack.org, gregkh@linuxfoundation.org, tglx@linutronix.de, mingo@redhat.com, bp@alien8.de, dave.hansen@linux.intel.com, x86@kernel.org, hpa@zytor.com, rafael@kernel.org, dakr@kernel.org, bartosz.golaszewski@linaro.org, cw00.choi@samsung.com, myungjoo.ham@samsung.com, yesanishhere@gmail.com, Jonathan.Cameron@huawei.com, quic_zijuhu@quicinc.com, aleksander.lobakin@intel.com, ira.weiny@intel.com, andriy.shevchenko@linux.intel.com, leon@kernel.org, lukas@wunner.de, bhelgaas@google.com, wagi@kernel.org, djeffery@redhat.com, stuart.w.hayes@gmail.com Subject: Re: [RFC v2 10/16] luo: luo_ioctl: add ioctl interface In-Reply-To: References: <20250515182322.117840-1-pasha.tatashin@soleen.com> <20250515182322.117840-11-pasha.tatashin@soleen.com> <20250624-akzeptabel-angreifbar-9095f4717ca4@brauner> Date: Wed, 09 Jul 2025 23:27:08 +0200 Message-ID: User-Agent: Gnus/5.13 (Gnus v5.13) Precedence: bulk X-Mailing-List: linux-doc@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain 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, , defines the necessary >> > structures. The magic number is registered in >> > Documentation/userspace-api/ioctl/ioctl-number.rst. >> > >> > Signed-off-by: Pasha Tatashin >> > --- >> > .../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... -- Regards, Pratyush Yadav