Re: [RFC PATCH 0/1] vfs: pass S_IFDIR mode to vfs_prepare_mode()

[Jori Koolstra]

> Op 13-06-2026 03:44 CEST schreef NeilBrown <neilb@xxxxxxxxxxx>:
> 
>  
> On Fri, 12 Jun 2026, Jori Koolstra wrote:
> > There is a comment in vfs_prepare_mode() that says:
> > 
> >   Note that it's currently valid for @type to be 0 if a directory is
> >   created. Filesystems raise that flag individually and we need to check
> >   whether each filesystem can deal with receiving S_IFDIR from the vfs
> >   before we enforce a non-zero type.
> > 
> > This is a bit challenging since there are many filesystems. Claude Opus
> > 4.8 was used to generate the context for each mkdir implementation from
> > which it can be judged whether passing S_IFDIR is safe. The result was
> > then verified by hand by looking at how the mode argument is used in
> > each case. To check whether all mkdir implementations are convered,
> > 'rg "\.mkdir" ' was used and checked against the list of uses Claude
> > found.
> > 
> > The only mkdir implementation that I am not as sure of is
> > cifs_mkdir(). So this goes cc to the cifs people. But of course, I might
> > have made errors in other places (except probably not for the A list as
> > those are all just mode | S_IFDIR cases without other uses of mode
> > besides logging)
> 
> I agree that cifs_mkdir() is not obviously safe.
> I think coda_mkdir() and fuse_mkdir() at also not obviously safe.
> In all three cases the mode, which now has S_IFDIR included, is sent
> unchanged to something outside of Linux - either user-space or over at
> network connection.

You are (of course) right. I missed to mention coda. Also, I missed that
for FUSE inarg.mode = mode; is set, and this is passed, despite the mode
in create_new_entry(idmap, fm, &args, dir, entry, S_IFDIR); being hardwired
to S_IFDIR.

> 
> I think you patch which adds S_IFDIR to mode should mask it back out in
> those three functions.  Then we could be certain it is safe.
> The maintainers of those filesystems might then choose to remove the
> mask if they know it to be safe to do so

That seems like a good idea, agreed.

> 
> Thanks,
> NeilBrown