namei: add mapping aware lookup helper

Various filesystems rely on the lookup_one_len() helper to lookup a
single path component relative to a well-known starting point. Allow
such filesystems to support idmapped mounts by adding a version of this
helper to take the idmap into account when calling inode_permission().
This change is a required to let btrfs (and other filesystems) support
idmapped mounts.

Cc: Christoph Hellwig <hch@infradead.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: linux-fsdevel@vger.kernel.org
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/namei.c

@@ -2575,8 +2575,9 @@ int vfs_path_lookup(struct dentry *dentry, struct vfsmount *mnt,
 }
 EXPORT_SYMBOL(vfs_path_lookup);
 
-static int lookup_one_len_common(const char *name, struct dentry *base,
-				 int len, struct qstr *this)
+static int lookup_one_common(struct user_namespace *mnt_userns,
+			     const char *name, struct dentry *base, int len,
+			     struct qstr *this)
 {
 	this->name = name;
 	this->len = len;
@@ -2604,7 +2605,7 @@ static int lookup_one_len_common(const char *name, struct dentry *base,
 			return err;
 	}
 
-	return inode_permission(&init_user_ns, base->d_inode, MAY_EXEC);
+	return inode_permission(mnt_userns, base->d_inode, MAY_EXEC);
 }
 
 /**
@@ -2628,7 +2629,7 @@ struct dentry *try_lookup_one_len(const char *name, struct dentry *base, int len
 
 	WARN_ON_ONCE(!inode_is_locked(base->d_inode));
 
-	err = lookup_one_len_common(name, base, len, &this);
+	err = lookup_one_common(&init_user_ns, name, base, len, &this);
 	if (err)
 		return ERR_PTR(err);
 
@@ -2655,7 +2656,7 @@ struct dentry *lookup_one_len(const char *name, struct dentry *base, int len)
 
 	WARN_ON_ONCE(!inode_is_locked(base->d_inode));
 
-	err = lookup_one_len_common(name, base, len, &this);
+	err = lookup_one_common(&init_user_ns, name, base, len, &this);
 	if (err)
 		return ERR_PTR(err);
 
@@ -2664,6 +2665,36 @@ struct dentry *lookup_one_len(const char *name, struct dentry *base, int len)
 }
 EXPORT_SYMBOL(lookup_one_len);
 
+/**
+ * lookup_one - filesystem helper to lookup single pathname component
+ * @mnt_userns:	user namespace of the mount the lookup is performed from
+ * @name:	pathname component to lookup
+ * @base:	base directory to lookup from
+ * @len:	maximum length @len should be interpreted to
+ *
+ * Note that this routine is purely a helper for filesystem usage and should
+ * not be called by generic code.
+ *
+ * The caller must hold base->i_mutex.
+ */
+struct dentry *lookup_one(struct user_namespace *mnt_userns, const char *name,
+			  struct dentry *base, int len)
+{
+	struct dentry *dentry;
+	struct qstr this;
+	int err;
+
+	WARN_ON_ONCE(!inode_is_locked(base->d_inode));
+
+	err = lookup_one_common(mnt_userns, name, base, len, &this);
+	if (err)
+		return ERR_PTR(err);
+
+	dentry = lookup_dcache(&this, base, 0);
+	return dentry ? dentry : __lookup_slow(&this, base, 0);
+}
+EXPORT_SYMBOL(lookup_one);
+
 /**
  * lookup_one_len_unlocked - filesystem helper to lookup single pathname component
  * @name:	pathname component to lookup
@@ -2683,7 +2714,7 @@ struct dentry *lookup_one_len_unlocked(const char *name,
 	int err;
 	struct dentry *ret;
 
-	err = lookup_one_len_common(name, base, len, &this);
+	err = lookup_one_common(&init_user_ns, name, base, len, &this);
 	if (err)
 		return ERR_PTR(err);
 

include/linux/namei.h

@@ -68,6 +68,7 @@ extern struct dentry *try_lookup_one_len(const char *, struct dentry *, int);
 extern struct dentry *lookup_one_len(const char *, struct dentry *, int);
 extern struct dentry *lookup_one_len_unlocked(const char *, struct dentry *, int);
 extern struct dentry *lookup_positive_unlocked(const char *, struct dentry *, int);
+struct dentry *lookup_one(struct user_namespace *, const char *, struct dentry *, int);
 
 extern int follow_down_one(struct path *);
 extern int follow_down(struct path *);