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

["Darrick J. Wong" <djwong@kernel.org>]

On Tue, Feb 25, 2025 at 12:24:08PM +0100, Christian Brauner wrote:
> 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.

Agreed, I don't think it's hard to enlarge struct fsxattr in the
existing ioctl interface; either we figure out how to make the kernel
fill out the "missing" bytes with an internal getfsxattr call, or we
make it return some errno if we would be truncating real output due to
struct size limits and leave a note in the manpage that "EL3HLT means
use a bigger structure definition"

Then both interfaces can plod along for another 30 years. :)

--D