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Message-ID: <CAJfpegtXQebPEQqw8ESmM-Bp0QgCefmRwoNeJNtN-_bV35b-pw@mail.gmail.com>
Date: Thu, 5 Sep 2013 14:03:41 +0200
From: Miklos Szeredi <miklos@...redi.hu>
To: Al Viro <viro@...iv.linux.org.uk>
Cc: Linux-Fsdevel <linux-fsdevel@...r.kernel.org>,
Kernel Mailing List <linux-kernel@...r.kernel.org>,
"mszeredi@...e.cz" <mszeredi@...e.cz>,
David Howells <dhowells@...hat.com>,
Steven Whitehouse <swhiteho@...hat.com>,
Trond Myklebust <Trond.Myklebust@...app.com>,
Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH 04/11] vfs: check unlinked ancestors before mount
On Thu, Sep 5, 2013 at 2:02 PM, Miklos Szeredi <miklos@...redi.hu> wrote:
> On Thu, Sep 05, 2013 at 01:32:10PM +0200, Miklos Szeredi wrote:
>> On Thu, Sep 5, 2013 at 1:18 PM, Al Viro <viro@...iv.linux.org.uk> wrote:
>
>> > Something's really odd with locking here. You are take d_lock, do one
>> > check, set flag, drop d_lock, grab rename_lock, do another check (taking
>> > and dropping d_lock in process), and, in case that check fails, grab
>> > d_lock again to clear the flag.
>> >
>> > At the very least it's a massive overkill. Just grab rename_lock, then
>> > d_lock, then do the damn check and set the flag only on success. Moreover,
>> > with rename_lock held, do you need d_lock on ancestors to mess with in
>> > has_unlinked_ancestor()?
>>
>> Yes, we need hard exclusion for the __d_drop() part. rename_lock can
>> provide one if we always take it for write in
>> check_submounts_and_drop(). But if we only take it for read then
>> that's not enough.
>>
>> And we do in fact also need DCACHE_MOUNTED set *before* checking
>> ancestors. Otherwise check_submounts_and_drop() could succeed and
>> has_unlinked_ancestor() return false, resulting in a dropped dentry
>> and a mount below it. Though this is mostly theoretical at this
>> point.
>
> Maybe something like this. Has less ugly locking. Untested.
And is missing a "goto out;"
>
> Thanks,
> Miklos
>
>
> ---
> fs/dcache.c | 49 +++++++++++++++++++++++++++++++++++++++++++++++++
> fs/internal.h | 1 +
> fs/namespace.c | 11 +++++------
> 3 files changed, 55 insertions(+), 6 deletions(-)
>
> --- a/fs/dcache.c
> +++ b/fs/dcache.c
> @@ -1159,6 +1159,55 @@ 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 check if the mountpoint is reachable (e.g. NFS can
> + * unhash a directory dentry and then the complete subtree can become
> + * unreachable).
> + */
> +int d_set_mounted(struct dentry *dentry)
> +{
> + int ret = 0;
> +
> + write_seqlock(&rename_lock);
> + spin_lock(&dentry->d_lock);
> + dentry->d_flags |= DCACHE_MOUNTED;
> + if (!IS_ROOT(dentry)) {
> + ret = -ENOENT;
> + if (d_unhashed(dentry)) {
> + dentry->d_flags &= ~DCACHE_MOUNTED;
> + goto out;
> + }
> + 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);
> + }
> + 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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