locks: Allow disabling mandatory locking at compile time

Mandatory locking appears to be almost unused and buggy and there
appears no real interest in doing anything with it.  Since effectively
no one uses the code and since the code is buggy let's allow it to be
disabled at compile time.  I would just suggest removing the code but
undoubtedly that will break some piece of userspace code somewhere.

For the distributions that don't care about this piece of code
this gives a nice starting point to make mandatory locking go away.

Cc: Benjamin Coddington <bcodding@redhat.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Jeff Layton <jeff.layton@primarydata.com>
Cc: J. Bruce Fields <bfields@fieldses.org>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Jeff Layton <jeff.layton@primarydata.com>

3 files changed, 22 insertions, 0 deletions

diff --git a/fs/Kconfig b/fs/Kconfig
index da3f32f..59322e6 100644
--- a/fs/Kconfig
+++ b/fs/Kconfig
@@ -67,6 +67,16 @@ config FILE_LOCKING
           for filesystems like NFS and for the flock() system
           call. Disabling this option saves about 11k.
 
+config MANDATORY_FILE_LOCKING
+	bool "Enable Mandatory file locking"
+	depends on FILE_LOCKING
+	default y
+	help
+	  This option enables files appropriately marked files on appropriely
+	  mounted filesystems to support mandatory locking.
+
+	  To the best of my knowledge this is dead code that no one cares about.
+
 source "fs/notify/Kconfig"
 
 source "fs/quota/Kconfig"
diff --git a/fs/locks.c b/fs/locks.c
index 0d2b326..86c9467 100644
--- a/fs/locks.c
+++ b/fs/locks.c
@@ -1191,6 +1191,7 @@ static int posix_lock_inode_wait(struct inode *inode, struct file_lock *fl)
 	return error;
 }
 
+#ifdef CONFIG_MANDATORY_FILE_LOCKING
 /**
  * locks_mandatory_locked - Check for an active lock
  * @file: the file to check
@@ -1289,6 +1290,7 @@ int locks_mandatory_area(int read_write, struct inode *inode,
 }
 
 EXPORT_SYMBOL(locks_mandatory_area);
+#endif /* CONFIG_MANDATORY_FILE_LOCKING */
 
 static void lease_clear_pending(struct file_lock *fl, int arg)
 {
diff --git a/fs/namespace.c b/fs/namespace.c
index 0570729..4219885 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -1584,6 +1584,14 @@ static inline bool may_mount(void)
 	return ns_capable(current->nsproxy->mnt_ns->user_ns, CAP_SYS_ADMIN);
 }
 
+static inline bool may_mandlock(void)
+{
+#ifndef	CONFIG_MANDATORY_FILE_LOCKING
+	return false;
+#endif
+	return true;
+}
+
 /*
  * Now umount can handle mount points as well as block devices.
  * This is important for filesystems which use unnamed block devices.
@@ -2677,6 +2685,8 @@ long do_mount(const char *dev_name, const char __user *dir_name,
 				   type_page, flags, data_page);
 	if (!retval && !may_mount())
 		retval = -EPERM;
+	if (!retval && (flags & MS_MANDLOCK) && !may_mandlock())
+		retval = -EPERM;
 	if (retval)
 		goto dput_out;