[PATCH 15/17] vfs: add vfs_get_link() helper

[Miklos Szeredi]

This helper is for filesystems that want to read the symlink and are better
off with the get_link() interface (returning a char *) rather than the
readlink() interface (copy into a userspace buffer).

Also call the LSM hook for readlink (not get_link) since this is for
symlink reading not following.

Signed-off-by: Miklos Szeredi <mszeredi@xxxxxxxxxx>
---
 fs/namei.c         | 47 ++++++++++++++++++++++++++++++++++++++++++-----
 include/linux/fs.h |  2 ++
 2 files changed, 44 insertions(+), 5 deletions(-)

diff --git a/fs/namei.c b/fs/namei.c
index 76d1f061de3c..23c3fb8cec0a 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -4659,6 +4659,47 @@ out:
 	return len;
 }
 
+static const char *do_get_link(struct dentry *dentry, struct inode *inode,
+			       struct delayed_call *done)
+{
+	const char *link = inode->i_op->get_link(dentry, inode, done);
+
+	/* "jumping" is unacceptable, warn and return error */
+	if (!IS_ERR(link) && WARN_ON_ONCE(!link))
+		link = ERR_PTR(-EIO);
+
+	return link;
+}
+
+/**
+ * vfs_get_link - get symbolic link
+ * @dentry: dentry
+ * @inode: inode on which to get symbolic link
+ * @done: caller needs to free returned data with this
+ *
+ * Calls security hook and i_op->get_link() on the supplied inode.
+ *
+ * It does not touch atime.  That's up to the caller if necessary.
+ *
+ * Don't call this from RCU lookup mode (yet)
+ */
+const char *vfs_get_link(struct dentry *dentry, struct inode *inode,
+			 struct delayed_call *done)
+{
+	const char *res = ERR_PTR(-EINVAL);
+
+	if (inode->i_op->get_link) {
+		int error = security_inode_readlink(dentry);
+
+		res = ERR_PTR(error);
+		if (!error)
+			res = do_get_link(dentry, inode, done);
+	}
+
+	return res;
+}
+EXPORT_SYMBOL(vfs_get_link);
+
 /**
  * vfs_readlink - read symlink body
  * @dentry: read symlink from this dentry
@@ -4682,13 +4723,9 @@ int vfs_readlink(struct dentry *dentry, char __user *buffer, int buflen)
 	int res;
 
 	if (!link) {
-		link = inode->i_op->get_link(dentry, inode, &done);
+		link = do_get_link(dentry, inode, &done);
 		if (IS_ERR(link))
 			return PTR_ERR(link);
-
-		/* "jumping" is unacceptable, warn and return error */
-		if (WARN_ON_ONCE(!link))
-			return -EIO;
 	}
 	res = readlink_copy(buffer, buflen, link);
 	do_delayed_call(&done);
diff --git a/include/linux/fs.h b/include/linux/fs.h
index f71e70e3017b..58f42e979b12 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -2916,6 +2916,8 @@ extern int vfs_fstat(unsigned int, struct kstat *);
 extern int vfs_fstatat(int , const char __user *, struct kstat *, int);
 
 extern int vfs_readlink(struct dentry *, char __user *, int);
+extern const char *vfs_get_link(struct dentry *, struct inode *,
+				struct delayed_call *);
 
 extern int __generic_block_fiemap(struct inode *inode,
 				  struct fiemap_extent_info *fieinfo,
-- 
2.5.5