Re: [PATCH 04/11] vfs: check unlinked ancestors before mount

[Miklos Szeredi]

On Thu, Sep 5, 2013 at 1:18 PM, Al Viro <viro@xxxxxxxxxxxxxxxxxx> wrote:
> On Thu, Sep 05, 2013 at 11:44:37AM +0200, Miklos Szeredi wrote:
>> +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).
>> + */
>> +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 a45ba4f..91b1c39 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);
>> +     }
>
> 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.

Thanks,
Miklos
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