[PATCH 1/6] Add a dentry op to handle automounting rather than abusing follow_link() [ver #2]

[David Howells]

Here's an updated patch that:

 (1) Fixes a bug in error handling (needs to use path_put_conditional not
     path_put).

 (2) Absorbs autofs4's decisions about whether to automount or not.  This
     means that colour-ls won't cause automounts unless -L is supplied or it
     doesn't get a DT_DIR flag from getdents().  It also means that autofs4
     can dispense with this logic should it choose to use d_automount().

 (3) Moves all the automounter logic out of __follow_mount() into
     follow_automount().

 (4) Stops pathwalk at the automount point and returns that point in the fs
     tree if it decides not to automount rather than reporting ELOOP (see its
     use of EXDEV for this).

David

---
From: David Howells <dhowells@xxxxxxxxxx>
Subject: [PATCH] Add a dentry op to handle automounting rather than abusing follow_link()

Add a dentry op (d_automount) to handle automounting directories rather than
abusing the follow_link() inode operation.  The operation is keyed off a new
inode flag (S_AUTOMOUNT).

This makes it easier to add an AT_ flag to suppress terminal segment automount
during pathwalk.  It should also remove the need for the kludge code in the
pathwalk algorithm to handle directories with follow_link() semantics.

I've only changed __follow_mount() to handle automount points, but it might be
necessary to change follow_mount() too.  The latter is only used from
follow_dotdot(), but any automounts on ".." should be pinned whilst we're using
a child of it.

I've also extracted the mount/don't-mount logic from autofs4 and included it
here.  It makes the mount go ahead anyway if someone calls open() or creat(),
tries to traverse the directory, tries to chdir/chroot/etc. into the directory,
or sticks a '/' on the end of the pathname.  If they do a stat(), however,
they'll only trigger the automount if they didn't also say O_NOFOLLOW.

Signed-off-by: David Howells <dhowells@xxxxxxxxxx>
---

 Documentation/filesystems/Locking |    2 +
 Documentation/filesystems/vfs.txt |   13 +++++
 fs/namei.c                        |   96 ++++++++++++++++++++++++++++++-------
 include/linux/dcache.h            |    5 ++
 include/linux/fs.h                |    2 +
 5 files changed, 100 insertions(+), 18 deletions(-)


diff --git a/Documentation/filesystems/Locking b/Documentation/filesystems/Locking
index 96d4293..ccbfa98 100644
--- a/Documentation/filesystems/Locking
+++ b/Documentation/filesystems/Locking
@@ -16,6 +16,7 @@ prototypes:
 	void (*d_release)(struct dentry *);
 	void (*d_iput)(struct dentry *, struct inode *);
 	char *(*d_dname)((struct dentry *dentry, char *buffer, int buflen);
+	struct vfsmount *(*d_automount)(struct path *path);
 
 locking rules:
 	none have BKL
@@ -27,6 +28,7 @@ d_delete:	yes		no		yes		no
 d_release:	no		no		no		yes
 d_iput:		no		no		no		yes
 d_dname:	no		no		no		no
+d_automount:	no		no		no		yes
 
 --------------------------- inode_operations --------------------------- 
 prototypes:
diff --git a/Documentation/filesystems/vfs.txt b/Documentation/filesystems/vfs.txt
index 94677e7..31a9e8f 100644
--- a/Documentation/filesystems/vfs.txt
+++ b/Documentation/filesystems/vfs.txt
@@ -851,6 +851,7 @@ struct dentry_operations {
 	void (*d_release)(struct dentry *);
 	void (*d_iput)(struct dentry *, struct inode *);
 	char *(*d_dname)(struct dentry *, char *, int);
+	struct vfsmount *(*d_automount)(struct path *);
 };
 
   d_revalidate: called when the VFS needs to revalidate a dentry. This
@@ -885,6 +886,18 @@ struct dentry_operations {
 	at the end of the buffer, and returns a pointer to the first char.
 	dynamic_dname() helper function is provided to take care of this.
 
+  d_automount: called when an automount dentry is to be traversed (optional).
+	This should create a new VFS mount record, mount it on the directory
+	and return the record to the caller.  The caller is supplied with a
+	path parameter giving the automount directory to describe the automount
+	target and the parent VFS mount record to provide inheritable mount
+	parameters.  NULL should be returned if someone else managed to make
+	the automount first.  If the automount failed, then an error code
+	should be returned.
+
+	This function is only used if S_AUTOMOUNT is set on the inode to which
+	the dentry refers.
+
 Example :
 
 static char *pipefs_dname(struct dentry *dent, char *buffer, int buflen)
diff --git a/fs/namei.c b/fs/namei.c
index 868d0cb..6509ca5 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -617,24 +617,82 @@ int follow_up(struct path *path)
 	return 1;
 }
 
+/*
+ * Perform an automount
+ * - return -EXDEV to tell __follow_mount() to stop and return the path we were
+ *   called with.
+ */
+static int follow_automount(struct path *path, unsigned flags, int res)
+{
+	struct vfsmount *mnt;
+
+	if (!path->dentry->d_op || !path->dentry->d_op->d_automount)
+		return -EREMOTE;
+
+	/* We want to mount if someone is trying to open/create a file of any
+	 * type under the mountpoint, wants to traverse through the mountpoint
+	 * or wants to open the mounted directory.
+	 *
+	 * We don't want to mount if someone's just doing a stat and they've
+	 * set AT_SYMLINK_NOFOLLOW - unless they're stat'ing a directory and
+	 * appended a '/' to the name.
+	 */
+	if (!(flags & LOOKUP_FOLLOW) &&
+	    !(flags & (LOOKUP_CONTINUE | LOOKUP_DIRECTORY |
+		       LOOKUP_OPEN | LOOKUP_CREATE)))
+		return -EXDEV;
+
+	current->total_link_count++;
+	if (current->total_link_count >= 40)
+		return -ELOOP;
+
+	mnt = path->dentry->d_op->d_automount(path);
+	if (IS_ERR(mnt))
+		return PTR_ERR(mnt);
+	if (!mnt) /* mount collision */
+		return 0;
+
+	if (mnt->mnt_sb == path->mnt->mnt_sb &&
+	    mnt->mnt_root == path->dentry) {
+		mntput(mnt);
+		return -ELOOP;
+	}
+
+	dput(path->dentry);
+	if (res)
+		mntput(path->mnt);
+	path->mnt = mnt;
+	path->dentry = dget(mnt->mnt_root);
+	return 0;
+}
+
 /* no need for dcache_lock, as serialization is taken care in
  * namespace.c
  */
-static int __follow_mount(struct path *path)
+static int __follow_mount(struct path *path, unsigned flags)
 {
-	int res = 0;
-	while (d_mountpoint(path->dentry)) {
-		struct vfsmount *mounted = lookup_mnt(path);
-		if (!mounted)
+	struct vfsmount *mounted;
+	int ret, res = 0;
+	for (;;) {
+		while (d_mountpoint(path->dentry)) {
+			mounted = lookup_mnt(path);
+			if (!mounted)
+				break;
+			dput(path->dentry);
+			if (res)
+				mntput(path->mnt);
+			path->mnt = mounted;
+			path->dentry = dget(mounted->mnt_root);
+			res = 1;
+		}
+		if (!d_automount_point(path->dentry))
 			break;
-		dput(path->dentry);
-		if (res)
-			mntput(path->mnt);
-		path->mnt = mounted;
-		path->dentry = dget(mounted->mnt_root);
+		ret = follow_automount(path, flags, res);
+		if (ret < 0)
+			return ret == -EXDEV ? 0 : ret;
 		res = 1;
 	}
-	return res;
+	return 0;
 }
 
 static void follow_mount(struct path *path)
@@ -702,6 +760,8 @@ static int do_lookup(struct nameidata *nd, struct qstr *name,
 	struct vfsmount *mnt = nd->path.mnt;
 	struct dentry *dentry, *parent;
 	struct inode *dir;
+	int ret;
+
 	/*
 	 * See if the low-level filesystem might want
 	 * to use its own hash..
@@ -720,8 +780,10 @@ static int do_lookup(struct nameidata *nd, struct qstr *name,
 done:
 	path->mnt = mnt;
 	path->dentry = dentry;
-	__follow_mount(path);
-	return 0;
+	ret = __follow_mount(path, nd->flags);
+	if (unlikely(ret < 0))
+		path_put_conditional(path, nd);
+	return ret;
 
 need_lookup:
 	parent = nd->path.dentry;
@@ -1721,11 +1783,9 @@ static struct file *do_last(struct nameidata *nd, struct path *path,
 	if (open_flag & O_EXCL)
 		goto exit_dput;
 
-	if (__follow_mount(path)) {
-		error = -ELOOP;
-		if (open_flag & O_NOFOLLOW)
-			goto exit_dput;
-	}
+	error = __follow_mount(path, nd->flags);
+	if (error < 0)
+		goto exit_dput;
 
 	error = -ENOENT;
 	if (!path->dentry->d_inode)
diff --git a/include/linux/dcache.h b/include/linux/dcache.h
index eebb617..5380bff 100644
--- a/include/linux/dcache.h
+++ b/include/linux/dcache.h
@@ -139,6 +139,7 @@ struct dentry_operations {
 	void (*d_release)(struct dentry *);
 	void (*d_iput)(struct dentry *, struct inode *);
 	char *(*d_dname)(struct dentry *, char *, int);
+	struct vfsmount *(*d_automount)(struct path *);
 };
 
 /* the dentry parameter passed to d_hash and d_compare is the parent
@@ -157,6 +158,7 @@ d_compare:	no		yes		yes      no
 d_delete:	no		yes		no       no
 d_release:	no		no		no       yes
 d_iput:		no		no		no       yes
+d_automount:	no		no		no	 yes
  */
 
 /* d_flags entries */
@@ -389,6 +391,9 @@ static inline int d_mountpoint(struct dentry *dentry)
 	return dentry->d_mounted;
 }
 
+#define d_automount_point(dentry) \
+	(dentry->d_inode && IS_AUTOMOUNT(dentry->d_inode))
+
 extern struct vfsmount *lookup_mnt(struct path *);
 extern struct dentry *lookup_create(struct nameidata *nd, int is_dir);
 
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 68ca1b0..a83fc81 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -235,6 +235,7 @@ struct inodes_stat_t {
 #define S_NOCMTIME	128	/* Do not update file c/mtime */
 #define S_SWAPFILE	256	/* Do not truncate: swapon got its bmaps */
 #define S_PRIVATE	512	/* Inode is fs-internal */
+#define S_AUTOMOUNT	1024	/* Automount/referral quasi-directory */
 
 /*
  * Note that nosuid etc flags are inode-specific: setting some file-system
@@ -269,6 +270,7 @@ struct inodes_stat_t {
 #define IS_NOCMTIME(inode)	((inode)->i_flags & S_NOCMTIME)
 #define IS_SWAPFILE(inode)	((inode)->i_flags & S_SWAPFILE)
 #define IS_PRIVATE(inode)	((inode)->i_flags & S_PRIVATE)
+#define IS_AUTOMOUNT(inode)	((inode)->i_flags & S_AUTOMOUNT)
 
 /* the read-only stuff doesn't really belong here, but any other place is
    probably as bad and I don't want to create yet another include file. */
--
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