[PATCH v4 02/21] fscrypt: add fscrypt_have_same_policy() to check inode compatibility

[Sweet Tea Dorminy <sweettea-kernel@dorminy.me>]

From: Omar Sandoval <osandov@osandov.com>

Btrfs will need to check whether inode policies are identical for
various purposes: if two inodes want to share an extent, they must have
the same policy, including key identifier; symlinks must not span the
encrypted/unencrypted border; and certain encryption policies will allow
btrfs to store one fscrypt_context for multiple objects. Therefore, add
a function which allows checking the encryption policies of two inodes
to ensure they are identical.

Signed-off-by: Omar Sandoval <osandov@osandov.com>
Signed-off-by: Sweet Tea Dorminy <sweettea-kernel@dorminy.me>
---
 fs/crypto/policy.c      | 26 ++++++++++++++++++++++++++
 include/linux/fscrypt.h |  7 +++++++
 2 files changed, 33 insertions(+)

diff --git a/fs/crypto/policy.c b/fs/crypto/policy.c
index 46757c3052ef..b7c4820a8001 100644
--- a/fs/crypto/policy.c
+++ b/fs/crypto/policy.c
@@ -415,6 +415,32 @@ static int fscrypt_get_policy(struct inode *inode, union fscrypt_policy *policy)
 	return fscrypt_policy_from_context(policy, &ctx, ret);
 }
 
+/**
+ * fscrypt_have_same_policy() - check whether two inodes have the same policy
+ * @inode1: the first inode
+ * @inode2: the second inode
+ *
+ * Return: %true if they are definitely equal, else %false
+ */
+int fscrypt_have_same_policy(struct inode *inode1, struct inode *inode2)
+{
+	union fscrypt_policy policy1, policy2;
+	int err;
+
+	if (!IS_ENCRYPTED(inode1) && !IS_ENCRYPTED(inode2))
+		return true;
+	else if (!IS_ENCRYPTED(inode1) || !IS_ENCRYPTED(inode2))
+		return false;
+	err = fscrypt_get_policy(inode1, &policy1);
+	if (err)
+		return false;
+	err = fscrypt_get_policy(inode2, &policy2);
+	if (err)
+		return false;
+	return fscrypt_policies_equal(&policy1, &policy2);
+}
+EXPORT_SYMBOL(fscrypt_have_same_policy);
+
 static int set_encryption_policy(struct inode *inode,
 				 const union fscrypt_policy *policy)
 {
diff --git a/include/linux/fscrypt.h b/include/linux/fscrypt.h
index 4cdff7c15544..9cc5a61c1200 100644
--- a/include/linux/fscrypt.h
+++ b/include/linux/fscrypt.h
@@ -313,6 +313,7 @@ static inline struct page *fscrypt_pagecache_page(struct page *bounce_page)
 void fscrypt_free_bounce_page(struct page *bounce_page);
 
 /* policy.c */
+int fscrypt_have_same_policy(struct inode *inode1, struct inode *inode2);
 int fscrypt_ioctl_set_policy(struct file *filp, const void __user *arg);
 int fscrypt_ioctl_get_policy(struct file *filp, void __user *arg);
 int fscrypt_ioctl_get_policy_ex(struct file *filp, void __user *arg);
@@ -490,6 +491,12 @@ static inline void fscrypt_free_bounce_page(struct page *bounce_page)
 }
 
 /* policy.c */
+static inline int fscrypt_have_same_policy(struct inode *inode1,
+					   struct inode *inode2)
+{
+	return 1;
+}
+
 static inline int fscrypt_ioctl_set_policy(struct file *filp,
 					   const void __user *arg)
 {
-- 
2.35.1