Add an AT_NO_AUTOMOUNT flag to suppress terminal automount

Add an AT_NO_AUTOMOUNT flag to suppress terminal automounting of automount
point directories.  This can be used by fstatat() users to permit the
gathering of attributes on an automount point and also prevent
mass-automounting of a directory of automount points by ls.

Signed-off-by: David Howells <dhowells@redhat.com>
Acked-by: Ian Kent <raven@themaw.net>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

4 files changed, 12 insertions, 1 deletions

diff --git a/fs/namei.c b/fs/namei.c
index 9d3033dc22e9..dc50bfb2f5d6 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -908,6 +908,12 @@ static int follow_automount(struct path *path, unsigned flags,
         if (!path->dentry->d_op || !path->dentry->d_op->d_automount)
                 return -EREMOTE;
 
+        /* We don't want to mount if someone supplied AT_NO_AUTOMOUNT
+         * and this is the terminal part of the path.
+         */
+        if ((flags & LOOKUP_NO_AUTOMOUNT) && !(flags & LOOKUP_CONTINUE))
+                return -EISDIR; /* we actually want to stop here */
+
         /* 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.
diff --git a/fs/stat.c b/fs/stat.c
index 12e90e213900..d5c61cf2b703 100644
--- a/fs/stat.c
+++ b/fs/stat.c
@@ -75,11 +75,13 @@ int vfs_fstatat(int dfd, const char __user *filename, struct kstat *stat,
         int error = -EINVAL;
         int lookup_flags = 0;
 
-        if ((flag & ~AT_SYMLINK_NOFOLLOW) != 0)
+        if ((flag & ~(AT_SYMLINK_NOFOLLOW | AT_NO_AUTOMOUNT)) != 0)
                 goto out;
 
         if (!(flag & AT_SYMLINK_NOFOLLOW))
                 lookup_flags |= LOOKUP_FOLLOW;
+        if (flag & AT_NO_AUTOMOUNT)
+                lookup_flags |= LOOKUP_NO_AUTOMOUNT;
 
         error = user_path_at(dfd, filename, lookup_flags, &path);
         if (error)
diff --git a/include/linux/fcntl.h b/include/linux/fcntl.h
index afc00af3229b..a562fa5fb4e3 100644
--- a/include/linux/fcntl.h
+++ b/include/linux/fcntl.h
@@ -45,6 +45,7 @@
 #define AT_REMOVEDIR            0x200   /* Remove directory instead of
                                            unlinking file.  */
 #define AT_SYMLINK_FOLLOW       0x400   /* Follow symbolic links.  */
+#define AT_NO_AUTOMOUNT         0x800   /* Suppress terminal automount traversal */
 
 #ifdef __KERNEL__
 
diff --git a/include/linux/namei.h b/include/linux/namei.h
index 8ef2c789c2a8..f276d4fa01fc 100644
--- a/include/linux/namei.h
+++ b/include/linux/namei.h
@@ -45,6 +45,7 @@ enum {LAST_NORM, LAST_ROOT, LAST_DOT, LAST_DOTDOT, LAST_BIND};
  *  - ending slashes ok even for nonexistent files
  *  - internal "there are more path components" flag
  *  - dentry cache is untrusted; force a real lookup
+ *  - suppress terminal automount
  */
 #define LOOKUP_FOLLOW           0x0001
 #define LOOKUP_DIRECTORY        0x0002
@@ -53,6 +54,7 @@ enum {LAST_NORM, LAST_ROOT, LAST_DOT, LAST_DOTDOT, LAST_BIND};
 #define LOOKUP_PARENT           0x0010
 #define LOOKUP_REVAL            0x0020
 #define LOOKUP_RCU              0x0040
+#define LOOKUP_NO_AUTOMOUNT     0x0080
 /*
  * Intent data
  */