Re: [PATCH v6 07/12] vfs: add O_CREAT|O_DIRECTORY to open*(2)

[Jori Koolstra]

> Op 19-09-2026 02:01 CEST schreef NeilBrown <neilb@xxxxxxxxxxx>:
> 
> > 
> > nfsd_create_locked() used to do that before vfs_mkdir() could return a
> > dentry, but it doesn't any more.  The reason was because
> > d_splice_alias() on might return a different dentry. 
> > In this case we want the same dentry, but we need to do a lookup on it.
> > 
> > I'd rather fix this in kernfs, but maybe that is a longer-term goal.
> > 
> > The comment in kernfs_dop_revalidate() suggests the we should d_drop()
> > the negative dentry and d_alloc_parallel() a new one and ->lookup that.
> > I'm not certain that is needed if we keep the parent locked, but we
> > would need to be certain.
> > We at least need to d_drop() the dentry before ->lookup as ->lookup
> > cannot handle hashed dentries and a hashed-negative dentry is passed
> > to ->mkdir.
> > 
> > I wonder if we could just disable O_CREATE|O_DIRECTORY on kernfs ....
> > probably not.
> > 
> > Summary: I think that if vfs_mkdir() returns NULL (success) but the
> > dentry is negative, we need to d_drop() and call ->lookup with a big
> > comment about kernfs.  But we need to double-check that this will do the
> > right thing with ->d_time (I think it will).
> > We also need to think carefully about races with
> > kernfs_dop_revalidate(), which could happen concurrently with the
> > ->lookup.
> 
> I've thought a bit more about this ...  I think that doing a lookup after
> the vfs_mkdir() results in a negative is a bit ugly.  It assumes things
> about the fs that I would rather not assume.
> 
> I would rather have the current proposed code check for a negative
> dentry, and fail with -EIO or similar.
> 

I just noticed that there's precedent for this in overlayfs in super.c:

		/* Weird filesystem returning with hashed negative (kernfs)? */
		err = -EINVAL;
		if (d_really_is_negative(work))
			goto out_dput;

Shall we just do this for current kernel release, then we can add support
later if wanted.

(But let's do EOPNOTSUPP instead of EINVAL)

What do you think?

Thanks,
Jori.