Re: [PATCH][RFC] ecryptfs_lookup_interpose(): lower_dentry->d_inode is not stable

[Amir Goldstein]

On Wed, Nov 13, 2019 at 2:52 PM Al Viro <viro@xxxxxxxxxxxxxxxxxx> wrote:
>
> On Wed, Nov 13, 2019 at 09:01:36AM +0200, Amir Goldstein wrote:
> > > -       if (d_really_is_negative(lower_dentry)) {
> > > +       /*
> > > +        * negative dentry can go positive under us here - its parent is not
> > > +        * locked.  That's OK and that could happen just as we return from
> > > +        * ecryptfs_lookup() anyway.  Just need to be careful and fetch
> > > +        * ->d_inode only once - it's not stable here.
> > > +        */
> > > +       lower_inode = READ_ONCE(lower_dentry->d_inode);
> > > +
> > > +       if (!lower_inode) {
> > >                 /* We want to add because we couldn't find in lower */
> > >                 d_add(dentry, NULL);
> > >                 return NULL;
> >
> > Sigh!
> >
> > Open coding a human readable macro to solve a subtle lookup race.
> > That doesn't sound like a scalable solution.
> > I have a feeling this is not the last patch we will be seeing along
> > those lines.
> >
> > Seeing that developers already confused about when they should use
> > d_really_is_negative() over d_is_negative() [1] and we probably
> > don't want to add d_really_really_is_negative(), how about
> > applying that READ_ONCE into d_really_is_negative() and
> > re-purpose it as a macro to be used when races with lookup are
> > a concern?
>
> Would you care to explain what that "fix" would've achieved here,
> considering the fact that barriers are no-ops on UP and this is
> *NOT* an SMP race?
>
> And it's very much present on UP - we have
>         fetch ->d_inode into local variable
>         do blocking allocation
>         check if ->d_inode is NULL now
>         if it is not, use the value in local variable and expect it to be non-NULL
>
> That's not a case of missing barriers.  At all.  And no redefinition of
> d_really_is_negative() is going to help - it can't retroactively affect
> the value explicitly fetched into a local variable some time prior to
> that.
>

Indeed. I missed that part of your commit message and didn't
realize the variable was being used later.
The language in the comment "can go positive under us" implied
SMP race so I misunderstood the reason for READ_ONCE().

Sorry for the noise.

Amir.