fscrypt: constify inode parameter to filename encryption functions

Constify the struct inode parameter to fscrypt_fname_disk_to_usr() and
the other filename encryption functions so that users don't have to pass
in a non-const inode when they are dealing with a const one, as in [1].

[1] https://lkml.kernel.org/linux-ext4/20191203051049.44573-6-drosen@google.com/

Cc: Daniel Rosenberg <drosen@google.com>
Link: https://lore.kernel.org/r/20191215213947.9521-1-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@google.com>

fs/crypto/fname.c

@@ -34,12 +34,12 @@ static inline bool fscrypt_is_dot_dotdot(const struct qstr *str)
  *
  * Return: 0 on success, -errno on failure
  */
-int fname_encrypt(struct inode *inode, const struct qstr *iname,
+int fname_encrypt(const struct inode *inode, const struct qstr *iname,
 		  u8 *out, unsigned int olen)
 {
 	struct skcipher_request *req = NULL;
 	DECLARE_CRYPTO_WAIT(wait);
-	struct fscrypt_info *ci = inode->i_crypt_info;
+	const struct fscrypt_info *ci = inode->i_crypt_info;
 	struct crypto_skcipher *tfm = ci->ci_ctfm;
 	union fscrypt_iv iv;
 	struct scatterlist sg;
@@ -85,14 +85,14 @@ int fname_encrypt(struct inode *inode, const struct qstr *iname,
  *
  * Return: 0 on success, -errno on failure
  */
-static int fname_decrypt(struct inode *inode,
-				const struct fscrypt_str *iname,
-				struct fscrypt_str *oname)
+static int fname_decrypt(const struct inode *inode,
+			 const struct fscrypt_str *iname,
+			 struct fscrypt_str *oname)
 {
 	struct skcipher_request *req = NULL;
 	DECLARE_CRYPTO_WAIT(wait);
 	struct scatterlist src_sg, dst_sg;
-	struct fscrypt_info *ci = inode->i_crypt_info;
+	const struct fscrypt_info *ci = inode->i_crypt_info;
 	struct crypto_skcipher *tfm = ci->ci_ctfm;
 	union fscrypt_iv iv;
 	int res;
@@ -247,10 +247,10 @@ EXPORT_SYMBOL(fscrypt_fname_free_buffer);
  *
  * Return: 0 on success, -errno on failure
  */
-int fscrypt_fname_disk_to_usr(struct inode *inode,
-			u32 hash, u32 minor_hash,
-			const struct fscrypt_str *iname,
-			struct fscrypt_str *oname)
+int fscrypt_fname_disk_to_usr(const struct inode *inode,
+			      u32 hash, u32 minor_hash,
+			      const struct fscrypt_str *iname,
+			      struct fscrypt_str *oname)
 {
 	const struct qstr qname = FSTR_TO_QSTR(iname);
 	struct fscrypt_digested_name digested_name;

fs/crypto/fscrypt_private.h

@@ -260,7 +260,7 @@ void fscrypt_generate_iv(union fscrypt_iv *iv, u64 lblk_num,
 			 const struct fscrypt_info *ci);
 
 /* fname.c */
-extern int fname_encrypt(struct inode *inode, const struct qstr *iname,
+extern int fname_encrypt(const struct inode *inode, const struct qstr *iname,
 			 u8 *out, unsigned int olen);
 extern bool fscrypt_fname_encrypted_size(const struct inode *inode,
 					 u32 orig_len, u32 max_len,

include/linux/fscrypt.h

@@ -153,8 +153,10 @@ static inline void fscrypt_free_filename(struct fscrypt_name *fname)
 extern int fscrypt_fname_alloc_buffer(const struct inode *, u32,
 				struct fscrypt_str *);
 extern void fscrypt_fname_free_buffer(struct fscrypt_str *);
-extern int fscrypt_fname_disk_to_usr(struct inode *, u32, u32,
-			const struct fscrypt_str *, struct fscrypt_str *);
+extern int fscrypt_fname_disk_to_usr(const struct inode *inode,
+				     u32 hash, u32 minor_hash,
+				     const struct fscrypt_str *iname,
+				     struct fscrypt_str *oname);
 
 #define FSCRYPT_FNAME_MAX_UNDIGESTED_SIZE	32
 
@@ -438,7 +440,7 @@ static inline void fscrypt_fname_free_buffer(struct fscrypt_str *crypto_str)
 	return;
 }
 
-static inline int fscrypt_fname_disk_to_usr(struct inode *inode,
+static inline int fscrypt_fname_disk_to_usr(const struct inode *inode,
 					    u32 hash, u32 minor_hash,
 					    const struct fscrypt_str *iname,
 					    struct fscrypt_str *oname)