Re: [autofs] [PATCH 04/38] autofs4: Save autofs trigger's vfsmount in super block info

[Ian Kent <raven@themaw.net>]

On 21/06/2010, at 9:06 PM, Miklos Szeredi <miklos@szeredi.hu> wrote:

> On Mon, 21 Jun 2010, Ian Kent wrote:
>> On Wed, 2010-06-16 at 12:04 +0800, Ian Kent wrote:
>>> On Tue, 2010-06-15 at 11:39 -0700, Valerie Aurora wrote:
>>>> From: Jan Blunck <jblunck@suse.de>
>>>> 
>>>> XXX - This is broken and included just to make union mounts work.  See
>>>> discussion at:
>>>> 
>>>> http://kerneltrap.org/mailarchive/linux-fsdevel/2010/1/15/6708053/thread
>>> 
>>> Instead of saving the vfsmount we could save a pointer to the dentry of
>>> the mount point in the autofs super block info struct. I think that's
>>> the bit I don't have so it would be sufficient for a lookup_mnt() for
>>> the needed vfsmount in ->follow_mount().
>>> 
>>> Objections?
>>> Suggestions?
>> 
>> No comments so far.
>> 
>> Before I dive into testing if this actually does what I need, can I get
>> an "in principal" ack or nack for the patch so union mounts can move on
>> please?
>> 
>> Note that this patch hasn't even been compile tested so the point is to
>> decide whether it is worth going ahead with it.
> 
> mnt_mountpoint is NULL at the point you try to save it, so this is not
> going to work.

Hahaha, OK, I'd better look more closely then!

> 
>> 
>> 
>> autofs4 - save autofs trigger mountpoint in super block info
>> 
>> From: Ian Kent <raven@themaw.net>
>> 
>> Adapted from the original patch from Jan Blunck <jblunck@suse.de>.
>> 
>> Original commit message:
>> 
>> This is a bugfix/replacement for commit
>> 051d381259eb57d6074d02a6ba6e90e744f1a29f:
>> 
>>    During a path walk if an autofs trigger is mounted on a dentry,
>>    when the follow_link method is called, the nameidata struct
>>    contains the vfsmount and mountpoint dentry of the parent mount
>>    while the dentry that is passed in is the root of the autofs
>>    trigger mount.  I believe it is impossible to get the vfsmount of
>>    the trigger mount, within the follow_link method, when only the
>>    parent vfsmount and the root dentry of the trigger mount are
>>    known.
>> 
>> The solution in this commit was to replace the path embedded in the
>> parent's nameidata with the path of the link itself in
>> __do_follow_link().  This is a relatively harmless misuse of the
>> field, but union mounts ran into a bug during follow_link() caused by
>> the nameidata containing the wrong path (we count on it being what it
>> is all other places - the path of the parent).
>> 
>> A cleaner and easier to understand solution is to save the necessary
>> mountpoint dentry in the autofs superblock info when it is mounted.
>> Then we can cwlookup the needed vfsmount in autofs4_follow_link().
>> 
>> Signed-off-by: Ian Kent <raven@themaw.net>
>> Cc: Jan Blunck <jblunck@suse.de>
>> Cc: Valerie Aurora <vaurora@redhat.com>
>> Cc: Alexander Viro <viro@zeniv.linux.org.uk>
>> Cc: autofs@linux.kernel.org
>> ---
>> 
>> fs/autofs4/autofs_i.h |    1 +
>> fs/autofs4/init.c     |   11 ++++++++++-
>> fs/autofs4/root.c     |   13 +++++++++++++
>> fs/namei.c            |    7 ++-----
>> fs/namespace.c        |    1 +
>> 5 files changed, 27 insertions(+), 6 deletions(-)
>> 
>> 
>> diff --git a/fs/autofs4/autofs_i.h b/fs/autofs4/autofs_i.h
>> index 3d283ab..9fc6d69 100644
>> --- a/fs/autofs4/autofs_i.h
>> +++ b/fs/autofs4/autofs_i.h
>> @@ -133,6 +133,7 @@ struct autofs_sb_info {
>>  	int reghost_enabled;
>>  	int needs_reghost;
>>  	struct super_block *sb;
>> +	struct dentry *mountpoint;
>>  	struct mutex wq_mutex;
>>  	spinlock_t fs_lock;
>>  	struct autofs_wait_queue *queues; /* Wait queue pointer */
>> diff --git a/fs/autofs4/init.c b/fs/autofs4/init.c
>> index 9722e4b..f305b7d 100644
>> --- a/fs/autofs4/init.c
>> +++ b/fs/autofs4/init.c
>> @@ -17,7 +17,16 @@
>> static int autofs_get_sb(struct file_system_type *fs_type,
>>  	int flags, const char *dev_name, void *data, struct vfsmount *mnt)
>> {
>> -	return get_sb_nodev(fs_type, flags, data, autofs4_fill_super, mnt);
>> +	struct autofs_sb_info *sbi;
>> +	int ret;
>> +
>> +	ret = get_sb_nodev(fs_type, flags, data, autofs4_fill_super, mnt);
>> +	if (ret)
>> +		return ret;
>> +
>> +	sbi = autofs4_sbi(mnt->mnt_sb);
>> +	sbi->mountpoint = mnt->mnt_mountpoint;
>> +	return 0;
>> }
>> 
>> static struct file_system_type autofs_fs_type = {
>> diff --git a/fs/autofs4/root.c b/fs/autofs4/root.c
>> index db4117e..be89fd2 100644
>> --- a/fs/autofs4/root.c
>> +++ b/fs/autofs4/root.c
>> @@ -215,11 +215,24 @@ static void *autofs4_follow_link(struct dentry *dentry, struct nameidata *nd)
>>  	struct autofs_info *ino = autofs4_dentry_ino(dentry);
>>  	int oz_mode = autofs4_oz_mode(sbi);
>>  	unsigned int lookup_type;
>> +	struct vfsmount *mnt;
>>  	int status;
>> 
>>  	DPRINTK("dentry=%p %.*s oz_mode=%d nd->flags=%d",
>>  		dentry, dentry->d_name.len, dentry->d_name.name, oz_mode,
>>  		nd->flags);
>> +
>> +	mnt = lookup_mount(nd->path.mnt, sbi->mountpoint);
>> +	if (!mnt) {
>> +		status = -ENOENT;
>> +		goto out_error;
>> +	}
>> +
>> +	dput(nd->path.dentry);
>> +	mntput(nd->path.mnt);
>> +	nd->path.mnt = mnt;
>> +	nd->path.dentry = dget(dentry);
>> +
>>  	/*
>>  	 * For an expire of a covered direct or offset mount we need
>>  	 * to break out of follow_down() at the autofs mount trigger
>> diff --git a/fs/namei.c b/fs/namei.c
>> index 868d0cb..69a78ee 100644
>> --- a/fs/namei.c
>> +++ b/fs/namei.c
>> @@ -539,11 +539,8 @@ __do_follow_link(struct path *path, struct nameidata *nd, void **p)
>>  	touch_atime(path->mnt, dentry);
>>  	nd_set_link(nd, NULL);
>> 
>> -	if (path->mnt != nd->path.mnt) {
>> -		path_to_nameidata(path, nd);
>> -		dget(dentry);
>> -	}
>> -	mntget(path->mnt);
>> +	if (path->mnt == nd->path.mnt)
>> +		mntget(nd->path.mnt);
>>  	nd->last_type = LAST_BIND;
>>  	*p = dentry->d_inode->i_op->follow_link(dentry, nd);
>>  	error = PTR_ERR(*p);
>> diff --git a/fs/namespace.c b/fs/namespace.c
>> index 88058de..19b7fc9 100644
>> --- a/fs/namespace.c
>> +++ b/fs/namespace.c
>> @@ -445,6 +445,7 @@ struct vfsmount *lookup_mnt(struct path *path)
>>  	spin_unlock(&vfsmount_lock);
>>  	return child_mnt;
>> }
>> +EXPORT_SYMBOL_GPL(lookup_mnt);
>> 
>> static inline int check_mnt(struct vfsmount *mnt)
>> {
>> 
>> 
>>