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

>
> 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
> vfsmount in the autofs superblock info when it is mounted. Then we
> can easily update the vfsmount in autofs4_follow_link().
>
> Signed-off-by: Jan Blunck <jblunck@suse.de>
> Signed-off-by: Valerie Aurora <vaurora@redhat.com>
> Acked-by: Ian Kent <raven@themaw.net>
> Cc: autofs@linux.kernel.org
> Cc: Alexander Viro <viro@zeniv.linux.org.uk>
> ---
> fs/autofs4/autofs_i.h | 1 +
> fs/autofs4/init.c | 11 ++++++++++-
> fs/autofs4/root.c | 6 ++++++
> fs/namei.c | 7 ++-----
> 4 files changed, 19 insertions(+), 6 deletions(-)
>
> diff --git a/fs/autofs4/autofs_i.h b/fs/autofs4/autofs_i.h
> index 3d283ab..de3af64 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 vfsmount *mnt;
> 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..5e0dcd7 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->mnt = mnt;
> + return 0;
> }
>
> static struct file_system_type autofs_fs_type = {
> diff --git a/fs/autofs4/root.c b/fs/autofs4/root.c
> index e8e5e63..c41e01d 100644
> --- a/fs/autofs4/root.c
> +++ b/fs/autofs4/root.c
> @@ -219,6 +219,12 @@ static void *autofs4_follow_link(struct dentry *dentry, struct nameidata *nd)
> DPRINTK("dentry=%p %.*s oz_mode=%d nd->flags=%d",
> dentry, dentry->d_name.len, dentry->d_name.name, oz_mode,
> nd->flags);
> +
> + dput(nd->path.dentry);
> + mntput(nd->path.mnt);
> + nd->path.mnt = mntget(sbi->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 3b43c48..f731108 100644
> --- a/fs/namei.c
> +++ b/fs/namei.c
> @@ -538,11 +538,8 @@ __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);




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