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

[Jeff Layton]

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?
-- 
Jeff Layton <jlayton@xxxxxxxxxx>