Re: [PATCH v4 4/4] ovl: Optimize override/revert creds

[Amir Goldstein]

On Thu, Nov 14, 2024 at 9:56 AM Amir Goldstein <amir73il@xxxxxxxxx> wrote:
>
> On Wed, Nov 13, 2024 at 8:30 PM Vinicius Costa Gomes
> <vinicius.gomes@xxxxxxxxx> wrote:
> >
> > Amir Goldstein <amir73il@xxxxxxxxx> writes:
> >
> > > On Thu, Nov 7, 2024 at 1:57 AM Vinicius Costa Gomes
> > > <vinicius.gomes@xxxxxxxxx> wrote:
> >
> > [...]
> >
> > >
> > > Vinicius,
> > >
> > > While testing fanotify with LTP tests (some are using overlayfs),
> > > kmemleak consistently reports the problems below.
> > >
> > > Can you see the bug, because I don't see it.
> > > Maybe it is a false positive...
> >
> > Hm, if the leak wasn't there before and we didn't touch anything related to
> > prepare_creds(), I think that points to the leak being real.
> >
> > But I see your point, still not seeing it.
> >
> > This code should be equivalent to the code we have now (just boot
> > tested):
> >
> > ----
> > diff --git a/fs/overlayfs/dir.c b/fs/overlayfs/dir.c
> > index 136a2c7fb9e5..7ebc2fd3097a 100644
> > --- a/fs/overlayfs/dir.c
> > +++ b/fs/overlayfs/dir.c
> > @@ -576,8 +576,7 @@ static int ovl_setup_cred_for_create(struct dentry *dentry, struct inode *inode,
> >          * We must be called with creator creds already, otherwise we risk
> >          * leaking creds.
> >          */
> > -       WARN_ON_ONCE(override_creds(override_cred) != ovl_creds(dentry->d_sb));
> > -       put_cred(override_cred);
> > +       WARN_ON_ONCE(override_creds_light(override_cred) != ovl_creds(dentry->d_sb));
> >
> >         return 0;
> >  }
> > ----
> >
> > Does it change anything? (I wouldn't think so, just to try something)
>
> No, but I think this does:
>
> --- a/fs/overlayfs/dir.c
> +++ b/fs/overlayfs/dir.c
> @@ -576,7 +576,8 @@ static int ovl_setup_cred_for_create(struct dentry
> *dentry, struct inode *inode,
>          * We must be called with creator creds already, otherwise we risk
>          * leaking creds.
>          */
> -       WARN_ON_ONCE(override_creds(override_cred) != ovl_creds(dentry->d_sb));
> +       old_cred = override_creds(override_cred);
> +       WARN_ON_ONCE(old_cred != ovl_creds(dentry->d_sb));
>         put_cred(override_cred);
>
>         return 0;
>
> Compiler optimized out override_creds(override_cred)? :-/

Nope, this voodoo did not help either.

>
> However, this is not enough.
>
> Dropping the ref of the new creds is going to drop the refcount to zero,
> so that is incorrect, we need to return the reference to the new creds
> explicitly to the callers. I will send a patch.

And neither did this.
The suspect memleak is still reported.

Any other ideas?

Thanks,
Amir.