[RFC PATCH 02/14] fscrypt: add fscrypt_new_context_from_parent

[Jeff Layton <jlayton@xxxxxxxxxx>]

Network filesystems usually don't know what inode number they'll have
until after the reply to a create RPC comes in. But we do need a
(blank) crypto context before that point. Break out the part of
fscrypt_inherit_context that creates a new context without setting it
into the new inode just yet.

Signed-off-by: Jeff Layton <jlayton@xxxxxxxxxx>
---
 fs/crypto/policy.c      | 42 ++++++++++++++++++++++++++++++-----------
 include/linux/fscrypt.h |  6 ++++++
 2 files changed, 37 insertions(+), 11 deletions(-)

diff --git a/fs/crypto/policy.c b/fs/crypto/policy.c
index 2d73fd39ad96..13e0a50157d5 100644
--- a/fs/crypto/policy.c
+++ b/fs/crypto/policy.c
@@ -621,19 +621,19 @@ int fscrypt_has_permitted_context(struct inode *parent, struct inode *child)
 EXPORT_SYMBOL(fscrypt_has_permitted_context);
 
 /**
- * fscrypt_inherit_context() - Sets a child context from its parent
+ * fscrypt_new_context_from_parent() - generate a new fscrypt context from its parent
  * @parent: Parent inode from which the context is inherited.
- * @child:  Child inode that inherits the context from @parent.
- * @fs_data:  private data given by FS.
- * @preload:  preload child i_crypt_info if true
+ * @ctx: destination for new encryption context
  *
- * Return: 0 on success, -errno on failure
+ * Generate a new encryption context for an inode from its parent, suitable
+ * for later installation into a child dentry's inode. Returns positive
+ * length of the new context on success, or -errno on failure.
+ *
+ * Note that the caller must ensure that it provides sufficient buffer space
+ * to write out the context (at least FSCRYPT_SET_CONTEXT_MAX_SIZE bytes).
  */
-int fscrypt_inherit_context(struct inode *parent, struct inode *child,
-						void *fs_data, bool preload)
+int fscrypt_new_context_from_parent(struct inode *parent, void *ctx)
 {
-	union fscrypt_context ctx;
-	int ctxsize;
 	struct fscrypt_info *ci;
 	int res;
 
@@ -645,10 +645,30 @@ int fscrypt_inherit_context(struct inode *parent, struct inode *child,
 	if (ci == NULL)
 		return -ENOKEY;
 
-	ctxsize = fscrypt_new_context_from_policy(&ctx, &ci->ci_policy);
+	return fscrypt_new_context_from_policy(ctx, &ci->ci_policy);
+}
+EXPORT_SYMBOL(fscrypt_new_context_from_parent);
+
+/**
+ * fscrypt_inherit_context() - Sets a child context from its parent
+ * @parent: Parent inode from which the context is inherited.
+ * @child:  Child inode that inherits the context from @parent.
+ * @fs_data:  private data given by FS.
+ * @preload:  preload child i_crypt_info if true
+ *
+ * Return: 0 on success, -errno on failure
+ */
+int fscrypt_inherit_context(struct inode *parent, struct inode *child,
+						void *fs_data, bool preload)
+{
+	union fscrypt_context ctx;
+	int res;
 
 	BUILD_BUG_ON(sizeof(ctx) != FSCRYPT_SET_CONTEXT_MAX_SIZE);
-	res = parent->i_sb->s_cop->set_context(child, &ctx, ctxsize, fs_data);
+	res = fscrypt_new_context_from_parent(parent, &ctx);
+	if (res < 0)
+		return res;
+	res = parent->i_sb->s_cop->set_context(child, &ctx, res, fs_data);
 	if (res)
 		return res;
 	return preload ? fscrypt_get_encryption_info(child): 0;
diff --git a/include/linux/fscrypt.h b/include/linux/fscrypt.h
index eaf16eb55788..c5fe6f334a1d 100644
--- a/include/linux/fscrypt.h
+++ b/include/linux/fscrypt.h
@@ -156,6 +156,7 @@ int fscrypt_ioctl_get_policy(struct file *filp, void __user *arg);
 int fscrypt_ioctl_get_policy_ex(struct file *filp, void __user *arg);
 int fscrypt_ioctl_get_nonce(struct file *filp, void __user *arg);
 int fscrypt_has_permitted_context(struct inode *parent, struct inode *child);
+int fscrypt_new_context_from_parent(struct inode *parent, void *);
 int fscrypt_inherit_context(struct inode *parent, struct inode *child,
 			    void *fs_data, bool preload);
 
@@ -340,6 +341,11 @@ static inline int fscrypt_has_permitted_context(struct inode *parent,
 	return 0;
 }
 
+static inline int fscrypt_new_context_from_parent(struct inode *parent, void *ctx);
+{
+	return -EOPNOTSUPP;
+}
+
 static inline int fscrypt_inherit_context(struct inode *parent,
 					  struct inode *child,
 					  void *fs_data, bool preload)
-- 
2.26.2