[PATCH] r/o bind mounts: elevate write count for open()s

This is the first really tricky patch in the series.  It elevates the writer
count on a mount each time a non-special file is opened for write.

We used to do this in may_open(), but Miklos pointed out that __dentry_open()
is used as well to create filps.  This will cover even those cases, while a
call in may_open() would not have.

There is also an elevated count around the vfs_create() call in open_namei().
See the comments for more details, but we need this to fix a 'create, remount,
fail r/w open()' race.

Some filesystems forego the use of normal vfs calls to create
struct files.   Make sure that these users elevate the mnt
writer count because they will get __fput(), and we need
to make sure they're balanced.

Acked-by: Al Viro <viro@ZenIV.linux.org.uk>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Dave Hansen <haveblue@us.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

1 files changed, 14 insertions, 2 deletions

diff --git a/ipc/mqueue.c b/ipc/mqueue.c
index 34262c11f480..94fd3b08fb77 100644
--- a/ipc/mqueue.c
+++ b/ipc/mqueue.c
@@ -598,6 +598,7 @@ static struct file *do_create(struct dentry *dir, struct dentry *dentry,
                         int oflag, mode_t mode, struct mq_attr __user *u_attr)
 {
         struct mq_attr attr;
+        struct file *result;
         int ret;
 
         if (u_attr) {
@@ -612,13 +613,24 @@ static struct file *do_create(struct dentry *dir, struct dentry *dentry,
         }
 
         mode &= ~current->fs->umask;
+        ret = mnt_want_write(mqueue_mnt);
+        if (ret)
+                goto out;
         ret = vfs_create(dir->d_inode, dentry, mode, NULL);
         dentry->d_fsdata = NULL;
         if (ret)
-                goto out;
+                goto out_drop_write;
 
-        return dentry_open(dentry, mqueue_mnt, oflag);
+        result = dentry_open(dentry, mqueue_mnt, oflag);
+        /*
+         * dentry_open() took a persistent mnt_want_write(),
+         * so we can now drop this one.
+         */
+        mnt_drop_write(mqueue_mnt);
+        return result;
 
+out_drop_write:
+        mnt_drop_write(mqueue_mnt);
 out:
         dput(dentry);
         mntput(mqueue_mnt);