fscrypt: Add in-place encryption mode

ext4 and f2fs require a bounce page when encrypting pages. However, not
all filesystems will need that (eg. UBIFS). This is handled via a
flag on fscrypt_operations where a fs implementation can select in-place
encryption over using a bounce page (which is the default).
Signed-off-by: David Gstir <david@sigma-star.at>
Signed-off-by: Richard Weinberger <richard@nod.at>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

fs/crypto/crypto.c

@@ -217,8 +217,9 @@ static struct page *alloc_bounce_page(struct fscrypt_ctx *ctx, gfp_t gfp_flags)
  * @plaintext_page: The page to encrypt. Must be locked.
  * @gfp_flags:      The gfp flag for memory allocation
  *
- * Allocates a ciphertext page and encrypts plaintext_page into it using the ctx
- * encryption context.
+ * Encrypts plaintext_page using the ctx encryption context. If
+ * the filesystem supports it, encryption is performed in-place, otherwise a
+ * new ciphertext_page is allocated and returned.
  *
  * Called on the page write path.  The caller must call
  * fscrypt_restore_control_page() on the returned ciphertext page to
@@ -231,7 +232,7 @@ struct page *fscrypt_encrypt_page(struct inode *inode,
 				struct page *plaintext_page, gfp_t gfp_flags)
 {
 	struct fscrypt_ctx *ctx;
-	struct page *ciphertext_page = NULL;
+	struct page *ciphertext_page = plaintext_page;
 	int err;
 
 	BUG_ON(!PageLocked(plaintext_page));
@@ -240,10 +241,12 @@ struct page *fscrypt_encrypt_page(struct inode *inode,
 	if (IS_ERR(ctx))
 		return (struct page *)ctx;
 
+	if (!(inode->i_sb->s_cop->flags & FS_CFLG_INPLACE_ENCRYPTION)) {
 		/* The encryption operation will require a bounce page. */
 		ciphertext_page = alloc_bounce_page(ctx, gfp_flags);
 		if (IS_ERR(ciphertext_page))
 			goto errout;
+	}
 
 	ctx->w.control_page = plaintext_page;
 	err = do_page_crypto(inode, FS_ENCRYPT, plaintext_page->index,
@@ -253,9 +256,11 @@ struct page *fscrypt_encrypt_page(struct inode *inode,
 		ciphertext_page = ERR_PTR(err);
 		goto errout;
 	}
+	if (!(inode->i_sb->s_cop->flags & FS_CFLG_INPLACE_ENCRYPTION)) {
 		SetPagePrivate(ciphertext_page);
 		set_page_private(ciphertext_page, (unsigned long)ctx);
 		lock_page(ciphertext_page);
+	}
 	return ciphertext_page;
 
 errout:

include/linux/fscrypto.h

@@ -153,10 +153,16 @@ struct fscrypt_name {
 #define fname_name(p)		((p)->disk_name.name)
 #define fname_len(p)		((p)->disk_name.len)
 
+/*
+ * fscrypt superblock flags
+ */
+#define FS_CFLG_INPLACE_ENCRYPTION (1U << 1)
+
 /*
  * crypto opertions for filesystems
  */
 struct fscrypt_operations {
+	unsigned int flags;
 	int (*get_context)(struct inode *, void *, size_t);
 	int (*key_prefix)(struct inode *, u8 **);
 	int (*prepare_context)(struct inode *);