Messages in this thread Patch in this message | | | Date | Thu, 5 Sep 2013 14:39:11 +0200 | From | Miklos Szeredi <> | Subject | Re: [PATCH 04/11] vfs: check unlinked ancestors before mount |
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This one actually has a slight chance of working.
Thanks, Miklos
--- Subject: vfs: check unlinked ancestors before mount From: Miklos Szeredi <mszeredi@suse.cz>
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 check_submounts_and_drop() or has_unlinked_ancestor() can succeed.
Signed-off-by: Miklos Szeredi <mszeredi@suse.cz> CC: David Howells <dhowells@redhat.com> CC: Steven Whitehouse <swhiteho@redhat.com> CC: Trond Myklebust <Trond.Myklebust@netapp.com> CC: Greg Kroah-Hartman <gregkh@linuxfoundation.org> --- fs/dcache.c | 52 ++++++++++++++++++++++++++++++++++++++++++++++++++++ fs/internal.h | 1 + fs/namespace.c | 11 +++++------ 3 files changed, 58 insertions(+), 6 deletions(-)
--- a/fs/dcache.c +++ b/fs/dcache.c @@ -1159,6 +1159,58 @@ int have_submounts(struct dentry *parent } 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) { + int is_unhashed; + + /* Need exclusion wrt. check_submounts_and_drop() */ + spin_lock(&this->d_lock); + is_unhashed = d_unhashed(this); + spin_unlock(&this->d_lock); + + if (is_unhashed) + return true; + } + return false; +} + +/* + * 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) +{ + int ret = -ENOENT; + + write_seqlock(&rename_lock); + spin_lock(&dentry->d_lock); + if (d_unlinked(dentry)) { + spin_unlock(&dentry->d_lock); + goto out; + } + + dentry->d_flags |= DCACHE_MOUNTED; + spin_unlock(&dentry->d_lock); + + if (__has_unlinked_ancestor(dentry->d_parent)) { + spin_lock(&dentry->d_lock); + dentry->d_flags &= ~DCACHE_MOUNTED; + spin_unlock(&dentry->d_lock); + goto out; + } + ret = 0; +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 --- a/fs/internal.h +++ b/fs/internal.h @@ -126,6 +126,7 @@ extern int invalidate_inodes(struct supe * dcache.c */ extern struct dentry *__d_alloc(struct super_block *, const struct qstr *); +extern int d_set_mounted(struct dentry *dentry); /* * read_write.c --- a/fs/namespace.c +++ b/fs/namespace.c @@ -611,6 +611,7 @@ static struct mountpoint *new_mountpoint { 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 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);
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