ecryptfs: use vfs_get_link()

Here again we are copying form one buffer to another, while jumping through
hoops to make kernel memory look like userspace memory.

For no good reason, since vfs_get_link() provides exactly what is needed.

As a bonus, now the security hook for readlink is also called on the
underlying inode.

Note: this can be called from link-following context.  But this is okay:

 - not in RCU mode

 - commit e54ad7f1ee26 ("proc: prevent stacking filesystems on top")

 - ecryptfs is *reading* the underlying symlink not following it, so the
   right security hook is being called

Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Cc: Tyler Hicks <tyhicks@canonical.com>

1 files changed, 12 insertions, 17 deletions

diff --git a/fs/ecryptfs/inode.c b/fs/ecryptfs/inode.c
index cf390dceddd2..4ea9d69152dc 100644
--- a/fs/ecryptfs/inode.c
+++ b/fs/ecryptfs/inode.c
@@ -631,28 +631,23 @@ out_lock:
 
 static char *ecryptfs_readlink_lower(struct dentry *dentry, size_t *bufsiz)
 {
+        DEFINE_DELAYED_CALL(done);
         struct dentry *lower_dentry = ecryptfs_dentry_to_lower(dentry);
-        char *lower_buf;
+        const char *link;
         char *buf;
-        mm_segment_t old_fs;
         int rc;
 
-        lower_buf = kmalloc(PATH_MAX, GFP_KERNEL);
-        if (!lower_buf)
-                return ERR_PTR(-ENOMEM);
-        old_fs = get_fs();
-        set_fs(get_ds());
-        rc = d_inode(lower_dentry)->i_op->readlink(lower_dentry,
-                                                   (char __user *)lower_buf,
-                                                   PATH_MAX);
-        set_fs(old_fs);
-        if (rc < 0)
-                goto out;
+        link = vfs_get_link(lower_dentry, &done);
+        if (IS_ERR(link))
+                return ERR_CAST(link);
+
         rc = ecryptfs_decode_and_decrypt_filename(&buf, bufsiz, dentry->d_sb,
-                                                  lower_buf, rc);
-out:
-        kfree(lower_buf);
-        return rc ? ERR_PTR(rc) : buf;
+                                                  link, strlen(link));
+        do_delayed_call(&done);
+        if (rc)
+                return ERR_PTR(rc);
+
+        return buf;
 }
 
 static const char *ecryptfs_get_link(struct dentry *dentry,