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Date: Mon, 21 Jun 2010 15:06:23 +0200
From: Miklos Szeredi <miklos@...redi.hu>
To: Ian Kent <raven@...maw.net>
CC: viro@...iv.linux.org.uk, vaurora@...hat.com,
autofs@...ux.kernel.org, miklos@...redi.hu,
linux-kernel@...r.kernel.org, hch@...radead.org,
linux-fsdevel@...r.kernel.org, jblunck@...e.de
Subject: Re: [autofs] [PATCH 04/38] autofs4: Save autofs trigger's vfsmount
in super block info
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@...e.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.
>
>
> autofs4 - save autofs trigger mountpoint in super block info
>
> From: Ian Kent <raven@...maw.net>
>
> Adapted from the original patch from Jan Blunck <jblunck@...e.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@...maw.net>
> Cc: Jan Blunck <jblunck@...e.de>
> Cc: Valerie Aurora <vaurora@...hat.com>
> Cc: Alexander Viro <viro@...iv.linux.org.uk>
> Cc: autofs@...ux.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)
> {
>
>
>
--
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