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

[Bernd Schubert <bernd.schubert@fastmail.fm>]

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@templeofstupid.com>
> ---
>   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,
Bernd