On Tue, Aug 06, 2013 at 04:30:01PM +0200, Miklos Szeredi wrote:
> From: Miklos Szeredi <[email protected]>
>
> We check submounts before doing d_drop() on a non-empty directory dentry in
> NFS (have_submounts()). But we do not exclude a racing mount. Nor do we
> prevent mounts to be added to the disconnected subtree using relative paths
> after the d_drop().
>
> This patch fixes these issues by checking for unlinked (unhashed, non-root)
> ancestors before proceeding with the mount. This is done after setting
> DCACHE_MOUNTED on the soon-to-be mountpoint and with the rename seqlock
> taken for write. This ensures that the only one of have_submounts() or
> has_unlinked_ancestor() can succeed.
>
> Signed-off-by: Miklos Szeredi <[email protected]>
> ---
> fs/dcache.c | 28 ++++++++++++++++++++++++++++
> fs/internal.h | 1 +
> fs/namespace.c | 9 +++++++++
> 3 files changed, 38 insertions(+)
>
> diff --git a/fs/dcache.c b/fs/dcache.c
> index ba429d9..eae7cc1 100644
> --- a/fs/dcache.c
> +++ b/fs/dcache.c
> @@ -1103,6 +1103,34 @@ rename_retry:
> }
> EXPORT_SYMBOL(have_submounts);
>
> +static bool __has_unlinked_ancestor(struct dentry *dentry)
> +{
> + struct dentry *this;
> +
> + for (this = dentry; !IS_ROOT(this); this = this->d_parent) {
> + if (d_unhashed(this))
Actually we need to take ->d_lock around this d_unhashed() since the __d_drop()
is done outside the read read_seqbegin/read_seqretry() region but with d_lock
held.
Thanks,
Miklos
> + return true;
> + }
> + return false;
> +}
> +
> +/*
> + * Called by mount code to check if the mountpoint is reachable (e.g. NFS can
> + * unhash a directory dentry and then the complete subtree can become
> + * unreachable).
> + */
> +bool has_unlinked_ancestor(struct dentry *dentry)
> +{
> + bool found;
> +
> + /* Need exclusion wrt. check_submounts_and_drop() */
> + write_seqlock(&rename_lock);
> + found = __has_unlinked_ancestor(dentry);
> + write_sequnlock(&rename_lock);
> +
> + return found;
> +}
> +
> /*
> * Search the dentry child list of the specified parent,
> * and move any unused dentries to the end of the unused
> diff --git a/fs/internal.h b/fs/internal.h
> index 7c5f01c..d232355 100644
> --- a/fs/internal.h
> +++ b/fs/internal.h
> @@ -126,6 +126,7 @@ extern int invalidate_inodes(struct super_block *, bool);
> * dcache.c
> */
> extern struct dentry *__d_alloc(struct super_block *, const struct qstr *);
> +extern bool has_unlinked_ancestor(struct dentry *dentry);
>
> /*
> * read_write.c
> diff --git a/fs/namespace.c b/fs/namespace.c
> index 7b1ca9b..bb92a9c 100644
> --- a/fs/namespace.c
> +++ b/fs/namespace.c
> @@ -634,6 +634,15 @@ static struct mountpoint *new_mountpoint(struct dentry
> *dentry)
> }
> dentry->d_flags |= DCACHE_MOUNTED;
> spin_unlock(&dentry->d_lock);
> +
> + if (has_unlinked_ancestor(dentry)) {
> + spin_lock(&dentry->d_lock);
> + dentry->d_flags &= ~DCACHE_MOUNTED;
> + spin_unlock(&dentry->d_lock);
> + kfree(mp);
> + return ERR_PTR(-ENOENT);
> + }
> +
> mp->m_dentry = dentry;
> mp->m_count = 1;
> list_add(&mp->m_hash, chain);
> --
> 1.8.1.4
>
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