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-rw-r--r--fs/namespace.c54
-rw-r--r--include/linux/mount.h3
2 files changed, 57 insertions, 0 deletions
diff --git a/fs/namespace.c b/fs/namespace.c
index 94f026ec990a..066b393578c1 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -80,6 +80,60 @@ struct vfsmount *alloc_vfsmnt(const char *name)
return mnt;
}
+/*
+ * Most r/o checks on a fs are for operations that take
+ * discrete amounts of time, like a write() or unlink().
+ * We must keep track of when those operations start
+ * (for permission checks) and when they end, so that
+ * we can determine when writes are able to occur to
+ * a filesystem.
+ */
+/**
+ * mnt_want_write - get write access to a mount
+ * @mnt: the mount on which to take a write
+ *
+ * This tells the low-level filesystem that a write is
+ * about to be performed to it, and makes sure that
+ * writes are allowed before returning success. When
+ * the write operation is finished, mnt_drop_write()
+ * must be called. This is effectively a refcount.
+ */
+int mnt_want_write(struct vfsmount *mnt)
+{
+ if (__mnt_is_readonly(mnt))
+ return -EROFS;
+ return 0;
+}
+EXPORT_SYMBOL_GPL(mnt_want_write);
+
+/**
+ * mnt_drop_write - give up write access to a mount
+ * @mnt: the mount on which to give up write access
+ *
+ * Tells the low-level filesystem that we are done
+ * performing writes to it. Must be matched with
+ * mnt_want_write() call above.
+ */
+void mnt_drop_write(struct vfsmount *mnt)
+{
+}
+EXPORT_SYMBOL_GPL(mnt_drop_write);
+
+/*
+ * __mnt_is_readonly: check whether a mount is read-only
+ * @mnt: the mount to check for its write status
+ *
+ * This shouldn't be used directly ouside of the VFS.
+ * It does not guarantee that the filesystem will stay
+ * r/w, just that it is right *now*. This can not and
+ * should not be used in place of IS_RDONLY(inode).
+ */
+int __mnt_is_readonly(struct vfsmount *mnt)
+{
+ return (mnt->mnt_sb->s_flags & MS_RDONLY);
+}
+EXPORT_SYMBOL_GPL(__mnt_is_readonly);
+
int simple_set_mnt(struct vfsmount *mnt, struct super_block *sb)
{
mnt->mnt_sb = sb;
diff --git a/include/linux/mount.h b/include/linux/mount.h
index 5ee2df217cdf..2eecd2c8c760 100644
--- a/include/linux/mount.h
+++ b/include/linux/mount.h
@@ -71,9 +71,12 @@ static inline struct vfsmount *mntget(struct vfsmount *mnt)
return mnt;
}
+extern int mnt_want_write(struct vfsmount *mnt);
+extern void mnt_drop_write(struct vfsmount *mnt);
extern void mntput_no_expire(struct vfsmount *mnt);
extern void mnt_pin(struct vfsmount *mnt);
extern void mnt_unpin(struct vfsmount *mnt);
+extern int __mnt_is_readonly(struct vfsmount *mnt);
static inline void mntput(struct vfsmount *mnt)
{