On Tue, Jun 24, 2025 at 12:53 PM Amir Goldstein <amir73il@xxxxxxxxx> wrote:
>
> On Tue, Jun 24, 2025 at 11:30 AM Jan Kara <jack@xxxxxxx> wrote:
> >
> > On Tue 24-06-25 10:29:13, Christian Brauner wrote:
> > > Various filesystems such as pidfs (and likely drm in the future) have a
> > > use-case to support opening files purely based on the handle without
> > > having to require a file descriptor to another object. That's especially
> > > the case for filesystems that don't do any lookup whatsoever and there's
> > > zero relationship between the objects. Such filesystems are also
> > > singletons that stay around for the lifetime of the system meaning that
> > > they can be uniquely identified and accessed purely based on the file
> > > handle type. Enable that so that userspace doesn't have to allocate an
> > > object needlessly especially if they can't do that for whatever reason.
> > >
> > > Signed-off-by: Christian Brauner <brauner@xxxxxxxxxx>
> > > ---
> > > fs/fhandle.c | 22 ++++++++++++++++++++--
> > > fs/pidfs.c | 5 ++++-
> > > 2 files changed, 24 insertions(+), 3 deletions(-)
> > >
> > > diff --git a/fs/fhandle.c b/fs/fhandle.c
> > > index ab4891925b52..54081e19f594 100644
> > > --- a/fs/fhandle.c
> > > +++ b/fs/fhandle.c
> > > @@ -173,7 +173,7 @@ SYSCALL_DEFINE5(name_to_handle_at, int, dfd, const char __user *, name,
> > > return err;
> > > }
> > >
> > > -static int get_path_anchor(int fd, struct path *root)
> > > +static int get_path_anchor(int fd, struct path *root, int handle_type)
> > > {
> > > if (fd >= 0) {
> > > CLASS(fd, f)(fd);
> > > @@ -193,6 +193,24 @@ static int get_path_anchor(int fd, struct path *root)
> > > return 0;
> > > }
> > >
> > > + /*
> > > + * Only autonomous handles can be decoded without a file
> > > + * descriptor.
> > > + */
> > > + if (!(handle_type & FILEID_IS_AUTONOMOUS))
> > > + return -EOPNOTSUPP;
> >
> > This somewhat ties to my comment to patch 5 that if someone passed invalid
> > fd < 0 before, we'd be returning -EBADF and now we'd be returning -EINVAL
> > or -EOPNOTSUPP based on FILEID_IS_AUTONOMOUS setting. I don't care that
> > much about it so feel free to ignore me but I think the following might be
> > more sensible error codes:
> >
> > if (!(handle_type & FILEID_IS_AUTONOMOUS)) {
> > if (fd == FD_INVALID)
> > return -EOPNOTSUPP;
> > return -EBADF;
> > }
> >
> > if (fd != FD_INVALID)
> > return -EBADF; (or -EINVAL no strong preference here)
>
> FWIW, I like -EBADF better.
> it makes the error more descriptive and keeps the flow simple:
>
> + /*
> + * Only autonomous handles can be decoded without a file
> + * descriptor and only when FD_INVALID is provided.
> + */
> + if (fd != FD_INVALID)
> + return -EBADF;
> +
> + if (!(handle_type & FILEID_IS_AUTONOMOUS))
> + return -EOPNOTSUPP;
>
Thinking about it some more, as I am trying to address your concerns
about crafting autonomous file handles by systemd, as you already
decided to define a range for kernel reserved values for fd, why not,
instead of requiring FD_INVALID for autonomous file handle, that we
actually define a kernel fd value that translates to "the root of pidfs":
+ /*
+ * Autonomous handles can be decoded with a special file
+ * descriptor value that describes the filesystem.
+ */
+ switch (fd) {
+ case FD_PIDFS_ROOT:
+ pidfs_get_root(root);
+ break;
+ default:
+ return -EBADF;
+ }
+
Then you can toss all my old ideas, including FILEID_IS_AUTONOMOUS,
and EXPORT_OP_AUTONOMOUS_HANDLES and you do not even need
to define FILEID_PIDFS anymore, just keep exporting FILEID_KERNFS
as before (you can also keep the existing systemd code) and when you want
to open file by handle you just go
open_by_handle_at(FD_PIDFS, &handle, 0)
and that's it.
In the end, my one and only concern with autonomous file handles is that
there should be a user opt-in to request them.
Sorry for taking the long road to get to this simpler design.
WDYT?
Thanks,
Amir.
