Re: [PATCH 2/3] fs: namei: Allow follow_down() to uncover auto mounts

[Ian Kent]

On 18/11/22 05:01, Jeff Layton wrote:
> On Thu, 2022-11-17 at 20:11 +0100, Richard Weinberger wrote:
> > This function is only used by NFSD to cross mount points.
> > If a mount point is of type auto mount, follow_down() will
> > not uncover it. Add LOOKUP_AUTOMOUNT to the lookup flags
> > to have ->d_automount() called when NFSD walks down the
> > mount tree.
> >
> > Signed-off-by: Richard Weinberger <richard@xxxxxx>
> > ---
> >   fs/namei.c | 2 +-
> >   1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/fs/namei.c b/fs/namei.c
> > index 578c2110df02..000c4b84e6be 100644
> > --- a/fs/namei.c
> > +++ b/fs/namei.c
> > @@ -1462,7 +1462,7 @@ int follow_down(struct path *path)
> >   {
> >   	struct vfsmount *mnt = path->mnt;
> >   	bool jumped;
> > -	int ret = traverse_mounts(path, &jumped, NULL, 0);
> > +	int ret = traverse_mounts(path, &jumped, NULL, LOOKUP_AUTOMOUNT);
> >   
> >   	if (path->mnt != mnt)
> >   		mntput(mnt);
>
> What happens when CROSSMOUNT isn't enabled and someone tries to stroll
> into an automount point? I'm guessing the automount happens but the
> export is denied? It seems like LOOKUP_AUTOMOUNT ought to be conditional
> on the parent export having CROSSMOUNT set.
>
> There's also another caller of follow_down too, the UNIX98 pty code.
> This may be harmless for it, but it'd be best not to perturb that if we
> can help it.
>
> Maybe follow_down can grow a lookupflags argument?es, I think that's needed too.


Changing the core VFS unconditionally ricks breaking things.


For example this:

        if (!(lookup_flags & (LOOKUP_PARENT | LOOKUP_DIRECTORY |
                           LOOKUP_OPEN | LOOKUP_CREATE | 
LOOKUP_AUTOMOUNT)) &&
            dentry->d_inode)

will never be true now so that, at the least, the handling of this case

will change for automount(8). I don't remember now the reasons behind

doing this but I do remember there was a special case that needed to

be handled by it.


Ian