Re: [RFC PATCH 2/2] fuse: ensure that submounts lookup their root

[Krister Johansen]

On Tue, Jul 25, 2023 at 01:16:08AM +0200, Bernd Schubert wrote:
> On 7/11/23 03:37, Krister Johansen wrote:
> > Prior to this commit, the submount code assumed that the inode for the
> > root filesystem could not be evicted.  When eviction occurs the server
> > may forget the inode.  This author has observed a submount get an EBADF
> > from a virtiofsd server that resulted from the sole dentry / inode
> > pair getting evicted from a mount namespace and superblock where they
> > were originally referenced.  The dentry shrinker triggered a forget
> > after killing the dentry with the last reference.
> > 
> > As a result, a container that was also using this submount failed to
> > access its filesystem because it had borrowed the reference instead of
> > taking its own when setting up its superblock for the submount.
> > 
> > Fix by ensuring that submount superblock configuration looks up the
> > nodeid for the submount as well.
> > 
> > Signed-off-by: Krister Johansen <kjlx@xxxxxxxxxxxxxxxxxx>
> > ---
> >   fs/fuse/dir.c    | 10 +++++-----
> >   fs/fuse/fuse_i.h |  6 ++++++
> >   fs/fuse/inode.c  | 32 ++++++++++++++++++++++++++++----
> >   3 files changed, 39 insertions(+), 9 deletions(-)
> > 
> > diff --git a/fs/fuse/dir.c b/fs/fuse/dir.c
> > index bdf5526a0733..fe6b3fd4a49c 100644
> > --- a/fs/fuse/dir.c
> > +++ b/fs/fuse/dir.c
> > @@ -193,11 +193,11 @@ static void fuse_lookup_init(struct fuse_conn *fc, struct fuse_args *args,
> >   	args->out_args[0].value = outarg;
> >   }
> > -static int fuse_dentry_revalidate_lookup(struct fuse_mount *fm,
> > -					 struct dentry *entry,
> > -					 struct inode *inode,
> > -					 struct fuse_entry_out *outarg,
> > -					 bool *lookedup)
> > +int fuse_dentry_revalidate_lookup(struct fuse_mount *fm,
> > +				  struct dentry *entry,
> > +				  struct inode *inode,
> > +				  struct fuse_entry_out *outarg,
> > +				  bool *lookedup)
> >   {
> >   	struct dentry *parent;
> >   	struct fuse_forget_link *forget;
> > diff --git a/fs/fuse/fuse_i.h b/fs/fuse/fuse_i.h
> > index 9b7fc7d3c7f1..77b123eddb6d 100644
> > --- a/fs/fuse/fuse_i.h
> > +++ b/fs/fuse/fuse_i.h
> > @@ -1309,6 +1309,12 @@ void fuse_dax_dontcache(struct inode *inode, unsigned int flags);
> >   bool fuse_dax_check_alignment(struct fuse_conn *fc, unsigned int map_alignment);
> >   void fuse_dax_cancel_work(struct fuse_conn *fc);
> > +/* dir.c */
> > +int fuse_dentry_revalidate_lookup(struct fuse_mount *fm, struct dentry *entry,
> > +				  struct inode *inode,
> > +				  struct fuse_entry_out *outarg,
> > +				  bool *lookedup);
> > +
> >   /* ioctl.c */
> >   long fuse_file_ioctl(struct file *file, unsigned int cmd, unsigned long arg);
> >   long fuse_file_compat_ioctl(struct file *file, unsigned int cmd,
> > diff --git a/fs/fuse/inode.c b/fs/fuse/inode.c
> > index f19d748890f0..1032e4b05d9c 100644
> > --- a/fs/fuse/inode.c
> > +++ b/fs/fuse/inode.c
> > @@ -1441,6 +1441,10 @@ static int fuse_fill_super_submount(struct super_block *sb,
> >   	struct super_block *parent_sb = parent_fi->inode.i_sb;
> >   	struct fuse_attr root_attr;
> >   	struct inode *root;
> > +	struct inode *parent;
> > +	struct dentry *pdent;
> > +	bool lookedup = false;
> > +	int ret;
> >   	fuse_sb_defaults(sb);
> >   	fm->sb = sb;
> > @@ -1456,14 +1460,34 @@ static int fuse_fill_super_submount(struct super_block *sb,
> >   	if (parent_sb->s_subtype && !sb->s_subtype)
> >   		return -ENOMEM;
> > +	/*
> > +	 * It is necessary to lookup the parent_if->nodeid in case the dentry
> > +	 * that triggered the automount of the submount is later evicted.
> > +	 * If this dentry is evicted without the lookup count getting increased
> > +	 * on the submount root, then the server can subsequently forget this
> > +	 * nodeid which leads to errors when trying to access the root of the
> > +	 * submount.
> > +	 */
> > +	parent = &parent_fi->inode;
> > +	pdent = d_find_alias(parent);
> > +	if (pdent) {
> > +		struct fuse_entry_out outarg;
> > +
> > +		ret = fuse_dentry_revalidate_lookup(fm, pdent, parent, &outarg,
> > +						    &lookedup);
> > +		dput(pdent);
> > +		if (ret < 0)
> > +			return ret;
> > +	}
> > +
> >   	fuse_fill_attr_from_inode(&root_attr, parent_fi);
> >   	root = fuse_iget(sb, parent_fi->nodeid, 0, &root_attr, 0, 0);
> >   	/*
> > -	 * This inode is just a duplicate, so it is not looked up and
> > -	 * its nlookup should not be incremented.  fuse_iget() does
> > -	 * that, though, so undo it here.
> > +	 * fuse_iget() sets nlookup to 1 at creation time.  If this nodeid was
> > +	 * not successfully looked up then decrement the count.
> >   	 */
> > -	get_fuse_inode(root)->nlookup--;
> > +	if (!lookedup)
> > +		get_fuse_inode(root)->nlookup--;
> 
> How does a submount work with a parent mismatch? I wonder if this function
> should return an error if lookup of the parent failed.

Thanks for the feedback.  This was definitely one of the things that I
didn't like about this patch.  The original code doesn't care if the
parent nodeid is valid, which is why I kept the old behavior as a
fallback if lookup failed.  I managed to console myself that it wasn't
any worse, but would love to remove that if it's okay to fail the mount
at this point.

The other thing that was annoying me was my refactoring of the
fuse_dentry_revalidate() function.  I'm annoyed by having the lookedup
boolean pointer as an argument to fuse_dentry_revalidate_lookup, but
couldn't quite work out how to handle the weird nlookup manipulations in
fuse_dentry_revalidate and fuse_fill_super_submount without it.

-K