Re: [PATCH v5 1/4] rust: debugfs: Bind DebugFS directory creation

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

On Wed, May 14, 2025 at 09:33:05AM +0200, Benno Lossin wrote:
> On Tue May 6, 2025 at 1:51 AM CEST, Matthew Maurer wrote:
> > +impl<'a> Dir<'a> {
> > +    /// Create a new directory in DebugFS. If `parent` is [`None`], it will be created at the root.
> > +    #[cfg(CONFIG_DEBUG_FS)]
> > +    fn create<'b>(name: &CStr, parent: Option<&'a Dir<'b>>) -> Self {
> > +        let parent_ptr = match parent {
> > +            Some(parent) => parent.0.as_ptr(),
> > +            None => core::ptr::null_mut(),
> > +        };
> > +        // SAFETY:
> > +        // * `name` argument points to a NUL-terminated string that lives across the call, by
> > +        //   invariants of `&CStr`.
> > +        // * If `parent` is `None`, `parent` accepts null pointers to mean create at root.
> > +        // * If `parent` is `Some`, `parent` accepts live dentry debugfs pointers.
> > +        //   so we can call `Self::from_ptr`.
> > +        let dir = unsafe { bindings::debugfs_create_dir(name.as_char_ptr(), parent_ptr) };
> > +
> > +        // SAFETY: `debugfs_create_dir` either returns an error code or a legal `dentry` pointer,
> > +        Self(unsafe { Entry::from_ptr(dir) })
> > +    }
> > +
> > +    #[cfg(not(CONFIG_DEBUG_FS))]
> > +    fn create<'b>(_name: &CStr, _parent: Option<&'a Dir<'b>>) -> Self {
> > +        Self(Entry::new())
> > +    }
> > +
> > +    /// Create a DebugFS subdirectory.
> 
> I'm not familiar with debugfs, if I run `Dir::create(c"foo", None)`
> twice, will both of the returned values refer to the same or different
> directories? 

You can not create a directory, or file, in the same location with the
same name.  The call will fail, so don't do that :)

> What if I give a parent?

Same thing, it will fail.

> If the answer in both cases is that they will refer to the same
> directory, then I'd change the docs to mention that.

Nope, that does not happen.

> So instead of
> "Creates" we could say "Finds or creates" or something better.

Find does not happen.

> If they refer to different files, then I am confused how that would look
> like in user-land :)

Agreed, which is why that does not happen :)

thanks,

greg k-h