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Commit eed81007 authored by Miklos Szeredi's avatar Miklos Szeredi Committed by Al Viro
Browse files

vfs: check unlinked ancestors before mount 



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 with rename
seqlock taken for write and with ->d_lock grabbed on each ancestor in turn,
including our dentry itself.  This ensures that the only one of
check_submounts_and_drop() or has_unlinked_ancestor() can succeed.

Signed-off-by: default avatarMiklos Szeredi <miklos@szeredi.hu>
Signed-off-by: default avatarAl Viro <viro@zeniv.linux.org.uk>
parent 848ac114

fs/dcache.c

+33 −0
Original line number Diff line number Diff line
@@ -1183,6 +1183,39 @@ int have_submounts(struct dentry *parent) 
}

EXPORT_SYMBOL(have_submounts);



/*

 * Called by mount code to set a mountpoint and check if the mountpoint is

 * reachable (e.g. NFS can unhash a directory dentry and then the complete

 * subtree can become unreachable).

 *

 * Only one of check_submounts_and_drop() and d_set_mounted() must succeed.  For

 * this reason take rename_lock and d_lock on dentry and ancestors.

 */

int d_set_mounted(struct dentry *dentry)

{

	struct dentry *p;

	int ret = -ENOENT;

	write_seqlock(&rename_lock);

	for (p = dentry->d_parent; !IS_ROOT(p); p = p->d_parent) {

		/* Need exclusion wrt. check_submounts_and_drop() */

		spin_lock(&p->d_lock);

		if (unlikely(d_unhashed(p))) {

			spin_unlock(&p->d_lock);

			goto out;

		}

		spin_unlock(&p->d_lock);

	}

	spin_lock(&dentry->d_lock);

	if (!d_unlinked(dentry)) {

		dentry->d_flags |= DCACHE_MOUNTED;

		ret = 0;

	}

 	spin_unlock(&dentry->d_lock);

out:

	write_sequnlock(&rename_lock);

	return ret;

}



/*

 * Search the dentry child list of the specified parent,

 * and move any unused dentries to the end of the unused

fs/internal.h

+1 −0
Original line number Diff line number Diff line
@@ -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 int d_set_mounted(struct dentry *dentry);



/*

 * read_write.c

fs/namespace.c

+5 −6
Original line number Diff line number Diff line
@@ -611,6 +611,7 @@ static struct mountpoint *new_mountpoint(struct dentry *dentry) 
{

	struct list_head *chain = mountpoint_hashtable + hash(NULL, dentry);

	struct mountpoint *mp;

	int ret;



	list_for_each_entry(mp, chain, m_hash) {

		if (mp->m_dentry == dentry) {
@@ -626,14 +627,12 @@ static struct mountpoint *new_mountpoint(struct dentry *dentry) 
	if (!mp)

		return ERR_PTR(-ENOMEM);



	spin_lock(&dentry->d_lock);

	if (d_unlinked(dentry)) {

		spin_unlock(&dentry->d_lock);

	ret = d_set_mounted(dentry);

	if (ret) {

		kfree(mp);

		return ERR_PTR(-ENOENT);

		return ERR_PTR(ret);

	}

	dentry->d_flags |= DCACHE_MOUNTED;

	spin_unlock(&dentry->d_lock);



	mp->m_dentry = dentry;

	mp->m_count = 1;

	list_add(&mp->m_hash, chain);