Merge tag 'fscrypt-for-linus' of git://git.kernel.org/pub/scm/fs/fscrypt/fscrypt

Pull fscrypt updates from Eric Biggers:
 "Some small fixes and cleanups for fs/crypto/:

   - Fix ->getattr() for ext4, f2fs, and ubifs to report the correct
     st_size for encrypted symlinks

   - Use base64url instead of a custom Base64 variant

   - Document struct fscrypt_operations"

* tag 'fscrypt-for-linus' of git://git.kernel.org/pub/scm/fs/fscrypt/fscrypt:
  fscrypt: document struct fscrypt_operations
  fscrypt: align Base64 encoding with RFC 4648 base64url
  fscrypt: remove mention of symlink st_size quirk from documentation
  ubifs: report correct st_size for encrypted symlinks
  f2fs: report correct st_size for encrypted symlinks
  ext4: report correct st_size for encrypted symlinks
  fscrypt: add fscrypt_symlink_getattr() for computing st_size

1 files changed, 44 insertions, 0 deletions

diff --git a/fs/crypto/hooks.c b/fs/crypto/hooks.c
index a73b0376e6f3..af74599ae1cf 100644
--- a/fs/crypto/hooks.c
+++ b/fs/crypto/hooks.c
@@ -384,3 +384,47 @@ err_kfree:
 	return ERR_PTR(err);
 }
 EXPORT_SYMBOL_GPL(fscrypt_get_symlink);
+
+/**
+ * fscrypt_symlink_getattr() - set the correct st_size for encrypted symlinks
+ * @path: the path for the encrypted symlink being queried
+ * @stat: the struct being filled with the symlink's attributes
+ *
+ * Override st_size of encrypted symlinks to be the length of the decrypted
+ * symlink target (or the no-key encoded symlink target, if the key is
+ * unavailable) rather than the length of the encrypted symlink target.  This is
+ * necessary for st_size to match the symlink target that userspace actually
+ * sees.  POSIX requires this, and some userspace programs depend on it.
+ *
+ * This requires reading the symlink target from disk if needed, setting up the
+ * inode's encryption key if possible, and then decrypting or encoding the
+ * symlink target.  This makes lstat() more heavyweight than is normally the
+ * case.  However, decrypted symlink targets will be cached in ->i_link, so
+ * usually the symlink won't have to be read and decrypted again later if/when
+ * it is actually followed, readlink() is called, or lstat() is called again.
+ *
+ * Return: 0 on success, -errno on failure
+ */
+int fscrypt_symlink_getattr(const struct path *path, struct kstat *stat)
+{
+	struct dentry *dentry = path->dentry;
+	struct inode *inode = d_inode(dentry);
+	const char *link;
+	DEFINE_DELAYED_CALL(done);
+
+	/*
+	 * To get the symlink target that userspace will see (whether it's the
+	 * decrypted target or the no-key encoded target), we can just get it in
+	 * the same way the VFS does during path resolution and readlink().
+	 */
+	link = READ_ONCE(inode->i_link);
+	if (!link) {
+		link = inode->i_op->get_link(dentry, inode, &done);
+		if (IS_ERR(link))
+			return PTR_ERR(link);
+	}
+	stat->size = strlen(link);
+	do_delayed_call(&done);
+	return 0;
+}
+EXPORT_SYMBOL_GPL(fscrypt_symlink_getattr);