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

[Al Viro <viro@zeniv.linux.org.uk>]

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.

There are other patches dealing with ->d_inode accesses, but they are
generally not along the same lines.  The problem is rarely the same...