[PATCH] fs: load FS modules before pinning the mountpoint

David Herrmann
2014-11-13T14:15:10Z

Several pseudo file-systems create default mountpoints in /sys/fs/ or
/sys/kernel/, including:
/sys/kernel/debug
/sys/fs/mqueue
/sys/fs/cgroup
/sys/fs/pstore

If the given pseudo file-system is available as a module, user-space can
rely on mount(2) to load the module on-demand (assuming MODULE_ALIAS_FS).
However, in case the default mount-point is created by the file-system
itself, mount(2) will fail as it cannot pin the mountpoint.

This patch makes mount(2) load any requested file-systems before pinning
the target mount-point. This way, a file-system module can create default
mount-points in the module instead of creating them statically in the
core kernel.

Signed-off-by: David Herrmann <dh.herrmann@gmail.com>
---
fs/namespace.c | 11 ++++++++++-
1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/fs/namespace.c b/fs/namespace.c
index 5b66b2b..b1dc9f8 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -2565,6 +2565,7 @@ char *copy_mount_string(const void __user *data)
long do_mount(const char *dev_name, const char __user *dir_name,
const char *type_page, unsigned long flags, void *data_page)
{
+ struct file_system_type *fstype = NULL;
struct path path;
int retval = 0;
int mnt_flags = 0;
@@ -2577,10 +2578,15 @@ long do_mount(const char *dev_name, const char __user *dir_name,
if (data_page)
((char *)data_page)[PAGE_SIZE - 1] = 0;

+ /* load fs-modules before pinning the mountpoint */
+ if (type_page && !(flags & (MS_REMOUNT|MS_BIND|MS_MOVE|MS_SHARED|
+ MS_PRIVATE|MS_SLAVE|MS_UNBINDABLE)))
+ fstype = get_fs_type(type_page);
+
/* ... and get the mountpoint */
retval = user_path(dir_name, &path);
if (retval)
- return retval;
+ goto putfs_out;

retval = security_sb_mount(dev_name, &path,
type_page, flags, data_page);
@@ -2635,6 +2641,9 @@ long do_mount(const char *dev_name, const char __user *dir_name,
dev_name, data_page);
dput_out:
path_put(&path);
+putfs_out:
+ if (fstype)
+ put_filesystem(fstype);
return retval;
}

--
2.1.3