Re: [PATCH] fs: support filename refcount without atomics

[Mateusz Guzik <mjguzik@gmail.com>]

On Fri, Mar 7, 2025 at 5:42 PM Al Viro <viro@zeniv.linux.org.uk> wrote:
>
> On Fri, Mar 07, 2025 at 05:11:55PM +0100, Mateusz Guzik wrote:
> > Atomics are only needed for a combination of io_uring and audit.
> >
> > Regular file access (even with audit) gets around fine without them.
> >
> > With this patch 'struct filename' starts with being refcounted using
> > regular ops.
> >
> > In order to avoid API explosion in the getname*() family, a dedicated
> > routine is added to switch the obj to use atomics.
> >
> > This leaves the room for merely issuing getname(), not issuing the
> > switch and still trying to manipulate the refcount from another thread.
> >
> > Catching such cases is facilitated by CONFIG_DEBUG_VFS-dependent
> > tracking of who created the given filename object and having refname()
> > and putname() detect if another thread is trying to modify them.
>
> Not a good way to handle that, IMO.
>
> Atomics do hurt there, but they are only plastering over the real
> problem - names formed in one thread, inserted into audit context
> there and operation involving them happening in a different thread.
>
> Refcounting avoids an instant memory corruption, but the real PITA
> is in audit users of that stuff.
>
> IMO we should *NOT* grab an audit names slot at getname() time -
> that ought to be done explicitly at later points.
>
> The obstacle is that currently there still are several retry loop
> with getname() done in it; I've most of that dealt with, need to
> finish that series.
>
> And yes, refcount becomes non-atomic as the result.

Well yes, it was audit which caused the appearance of atomics in the
first place. I was looking for an easy way out.

If you have something which gets rid of the underlying problem and it
is going to land in the foreseeable future, I wont be defending this
approach.

-- 
Mateusz Guzik <mjguzik gmail.com>