Re: [PATCH v5 4/4] rust: samples: Add debugfs sample

["Benno Lossin" <lossin@kernel.org>]

On Thu May 22, 2025 at 7:40 PM CEST, Alice Ryhl wrote:
> On Thu, May 22, 2025 at 04:15:46PM +0200, Greg Kroah-Hartman wrote:
>> No, sorry, but debugfs files are "create and forget" type of things.
>> The caller has NO reference back to the file at all in the C version,
>> let's not add that functionality back to the rust side after I spent a
>> long time removing it from the C code :)
>> 
>> If you really want to delete a debugfs file that you have created in the
>> past, then look it up and delete it with the call that is present for
>> that.
>> 
>> The only thing I think that might be worth "keeping" in some form, as an
>> object reference as discussed, is a debugfs directory.
>
> That could work if we don't have any Rust value for files at all. The
> problem is that if we do have such values, then code like this:
>
> let my_file = dir.create_file("my_file_name");
> dir.delete_file("my_file_name");
> my_file.do_something();

I might have misunderstood something, but "deleting a debugfs file" is
not the same as freeing the representing object (is that a dentry?). So
you could still call `do_something`, it just wouldn't do anything.

Or did I misunderstand?

---
Cheers,
Benno

> would be a UAF on the last line. We have to design the Rust API to avoid
> such UAF, which is why I suggested the ghost objects; the delete_file()
> call leaves my_file in a valid but useless state. And as a ghost object,
> the .do_something() call becomes a no-op since the file is now missing
> from the filesystem.
>
> Alice