linux-kernel - Re: [PATCH v3] fs: introduce getfsxattrat and setfsxattrat syscalls

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Message-ID: <20250225-testfahrt-seilwinde-64e6f44c01ce@brauner>
Date: Tue, 25 Feb 2025 12:24:08 +0100
From: Christian Brauner <brauner@...nel.org>
To: Arnd Bergmann <arnd@...db.de>
Cc: Amir Goldstein <amir73il@...il.com>, 
	Andrey Albershteyn <aalbersh@...hat.com>, "Darrick J. Wong" <djwong@...nel.org>, 
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	Pali Rohár <pali@...nel.org>, Theodore Ts'o <tytso@....edu>
Subject: Re: [PATCH v3] fs: introduce getfsxattrat and setfsxattrat syscalls

On Tue, Feb 25, 2025 at 11:40:51AM +0100, Arnd Bergmann wrote:
> On Tue, Feb 25, 2025, at 11:22, Christian Brauner wrote:
> > On Tue, Feb 25, 2025 at 09:02:04AM +0100, Arnd Bergmann wrote:
> >> On Mon, Feb 24, 2025, at 12:32, Christian Brauner wrote:
> >> 
> >> The ioctl interface relies on the existing behavior, see
> >> 0a6eab8bd4e0 ("vfs: support FS_XFLAG_COWEXTSIZE and get/set of
> >> CoW extent size hint") for how it was previously extended
> >> with an optional flag/word. I think that is fine for the syscall
> >> as well, but should be properly documented since it is different
> >> from how most syscalls work.
> >
> > If we're doing a new system call I see no reason to limit us to a
> > pre-existing structure or structure layout.
> 
> Obviously we could create a new structure, but I also see no
> reason to do so. The existing ioctl interface was added in
> in 2002 as part of linux-2.5.35 with 16 bytes of padding, half
> of which have been used so far.
> 
> If this structure works for another 23 years before we run out
> of spare bytes, I think that's good enough. Building in an
> incompatible way to handle potential future contents would
> just make it harder to use for any userspace that wants to
> use the new syscalls but still needs a fallback to the
> ioctl version.

The fact that this structure has existed since the dawn of time doesn't
mean it needs to be retained when adding a completely new system call.

People won't mix both. They either switch to the new interface because
they want to get around the limitations of the old interface or they
keep using the old interface and the associated workarounds.

In another thread they keep arguing about new extensions for Windows
that are going to be added to the ioctl interface and how to make it fit
into this. That just shows that it's very hard to predict from the
amount of past changes how many future changes are going to happen. And
if an interface is easy to extend it might well invite new changes that
people didn't want to or couldn't make using the old interface.