Re: [PATCH 1/8] rust: debugfs: Bind DebugFS directory creation

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Wed, Apr 30, 2025 at 08:31:29AM -0700, Matthew Maurer wrote:
> On Wed, Apr 30, 2025 at 8:23 AM Greg Kroah-Hartman
> <gregkh@linuxfoundation.org> wrote:
> >
> > On Wed, Apr 30, 2025 at 08:10:44AM -0700, Matthew Maurer wrote:
> > > On Wed, Apr 30, 2025 at 5:06 AM Greg Kroah-Hartman
> > > <gregkh@linuxfoundation.org> wrote:
> > > >
> > > > On Tue, Apr 29, 2025 at 11:15:55PM +0000, Matthew Maurer wrote:
> > > > > The basic API relies on `dput` to prevent leaks. Use of `debugfs_remove`
> > > > > is delayed until the more full-featured API, because we need to avoid
> > > > > the user having an reference to a dir that is recursively removed.
> > > > >
> > > > > Signed-off-by: Matthew Maurer <mmaurer@google.com>
> > > >
> > > > First off, many thanks for doing this.  I like this in general, but I
> > > > have loads of specific questions/comments.  Don't take that as a
> > > > criticism of this feature, I really want these bindings to be in the
> > > > tree and work hopefully better/cleaner than the userspace ones do.
> > > >
> > > > First off, the main "rule" of debugfs is that you should NEVER care
> > > > about the return value of any debugfs function.  So much so that the C
> > > > side hides errors almost entirely where possible.  I'd like to see this
> > > > carried through to the Rust side as well, but I think you didn't do that
> > > > for various "traditional" reasons.
> > >
> > > Sure, I mostly had to do error checking because I was using an
> > > `ARef<Dir>` to represent a directory, which meant that the underlying
> > > directory needed to be a valid pointer. Given that you've said that
> > > the returned `dentry *` should never be used as an actual `dentry`,
> > > only as an abstract handle to a DebugFS object, that requirement goes
> > > away, and I can remove the error handling code and always successfully
> > > return a `Dir`, even in cases where an error has occurred.
> >
> > Great!
> >
> > Except when debugfs is not enabled, then what are you going to return?
> > The same structure, or an error?
> >
> > I'd vote for the same pointer to the structure, just to make it more
> > obvious that this is a totally opaque thing and that no caller should
> > ever know or care if debugfs is working or even present in the system.
> >
> > Note that some drivers will want to save a bit of space if debugfs is
> > not enabled in the build, so be prepared to make the binding work
> > somehow that way too.  Can you have an "empty" object that takes no
> > memory?  Or is this too overthinking things?
> 
> Based on what you've expressed, I think what makes sense is:
> 
> * Initial patch will always return the same `Dir`, just sometimes it
> will be a wrapper around a pointer that is an error code, and
> sometimes it will be a useful `dentry` pointer. This will match the
> current behavior of C code to my understanding.

Great.

> * Follow-up (probably still in this series) will check
> `CONFIG_DEBUG_FS`, and if it's off, will just make `Dir` into a ZST,
> and just discard the pointer. This would be an improvement upon the C
> interface, because drivers would get the shrinkage without needing to
> add conditionals on `CONFIG_DEBUG_FS` in their own driver.

You're going to have to check CONFIG_DEBUG_FS anyway for this series,
otherwise things aren't going to build properly, so this sounds like a
great way to handle that.

thanks,

greg k-h