[PATCH 10/25] fscrypt: new helper function - fscrypt_prepare_setattr()

[Eric Biggers]

From: Eric Biggers <ebiggers at google.com>

Introduce a helper function for filesystems to call when processing
->setattr() on a possibly-encrypted inode.  It handles enforcing that an
encrypted file can only be truncated if its encryption key is available.

Signed-off-by: Eric Biggers <ebiggers at google.com>
---
 include/linux/fscrypt_notsupp.h |  8 ++++++++
 include/linux/fscrypt_supp.h    | 25 +++++++++++++++++++++++++
 2 files changed, 33 insertions(+)

diff --git a/include/linux/fscrypt_notsupp.h b/include/linux/fscrypt_notsupp.h
index e9b437605f9c..b0dd2d1756b2 100644
--- a/include/linux/fscrypt_notsupp.h
+++ b/include/linux/fscrypt_notsupp.h
@@ -218,4 +218,12 @@ static inline int fscrypt_prepare_lookup(struct inode *dir,
 	return 0;
 }
 
+static inline int fscrypt_prepare_setattr(struct dentry *dentry,
+					  struct iattr *attr)
+{
+	if (IS_ENCRYPTED(d_inode(dentry)) && (attr->ia_valid & ATTR_SIZE))
+		return -EOPNOTSUPP;
+	return 0;
+}
+
 #endif	/* _LINUX_FSCRYPT_NOTSUPP_H */
diff --git a/include/linux/fscrypt_supp.h b/include/linux/fscrypt_supp.h
index 2b99bc01c59b..bdd01e02435b 100644
--- a/include/linux/fscrypt_supp.h
+++ b/include/linux/fscrypt_supp.h
@@ -270,4 +270,29 @@ static inline int fscrypt_prepare_lookup(struct inode *dir,
 	return 0;
 }
 
+/**
+ * fscrypt_prepare_setattr - prepare to change a possibly-encrypted inode's attributes
+ * @dentry: dentry through which the inode is being changed
+ * @attr: attributes to change
+ *
+ * Prepare for ->setattr() on a possibly-encrypted inode.  On an encrypted file,
+ * most attribute changes are allowed even without the encryption key.  However,
+ * without the encryption key we do have to forbid truncates.  This is needed
+ * because the size being truncated to may not be a multiple of the filesystem
+ * block size, and in that case we'd have to decrypt the final block, zero the
+ * portion past i_size, and re-encrypt it.  (We *could* allow truncating to a
+ * filesystem block boundary, but it's simpler to just forbid all truncates ---
+ * and we already forbid all other contents modifications without the key.)
+ *
+ * Return: 0 on success, -ENOKEY if the key is missing, or another -errno code
+ * if a problem occurred while setting up the encryption key.
+ */
+static inline int fscrypt_prepare_setattr(struct dentry *dentry,
+					  struct iattr *attr)
+{
+	if (IS_ENCRYPTED(d_inode(dentry)) && (attr->ia_valid & ATTR_SIZE))
+		return fscrypt_require_key(d_inode(dentry));
+	return 0;
+}
+
 #endif	/* _LINUX_FSCRYPT_SUPP_H */
-- 
2.14.1.821.g8fa685d3b7-goog