Re: [PATCH V2 3/7] fscrypt: remove filesystem specific build config option

[Chandan Rajendra <chandan@linux.vnet.ibm.com>]

On Wednesday, December 5, 2018 5:13:21 AM IST Eric Biggers wrote:
> Hi Chandan,
> 
> On Tue, Dec 04, 2018 at 03:26:46PM +0530, Chandan Rajendra wrote:
> > In order to have a common code base for fscrypt "post read" processing
> > for all filesystems which support encryption, this commit removes
> > filesystem specific build config option (e.g. CONFIG_EXT4_FS_ENCRYPTION)
> > and replaces it with a build option (i.e. CONFIG_FS_ENCRYPTION) whose
> > value affects all the filesystems making use of fscrypt.
> > 
> > Signed-off-by: Chandan Rajendra <chandan@linux.vnet.ibm.com>
> [...]
> > -config F2FS_FS_ENCRYPTION
> > -	bool "F2FS Encryption"
> > -	depends on F2FS_FS
> > -	depends on F2FS_FS_XATTR
> > -	select FS_ENCRYPTION
> > -	help
> > -	  Enable encryption of f2fs files and directories.  This
> > -	  feature is similar to ecryptfs, but it is more memory
> > -	  efficient since it avoids caching the encrypted and
> > -	  decrypted pages in the page cache.
> > -
> [...]
> > -config UBIFS_FS_ENCRYPTION
> > -	bool "UBIFS Encryption"
> > -	depends on UBIFS_FS && UBIFS_FS_XATTR && BLOCK
> > -	select FS_ENCRYPTION
> > -	default n
> > -	help
> > -	  Enable encryption of UBIFS files and directories. This
> > -	  feature is similar to ecryptfs, but it is more memory
> > -	  efficient since it avoids caching the encrypted and
> > -	  decrypted pages in the page cache.
> 
> Will it cause problems that now f2fs encryption can be "enabled" without
> F2FS_FS_XATTR, and ubifs encryption without UBIFS_FS_XATTR && BLOCK?
> 
> Otherwise I think this patch looks fine.  I'm a bit concerned about the bloat
> from making FS_ENCRYPTION non-modular, but given that it will make sharing I/O
> code much easier, it's probably worthwhile.
> 
> It would help to strip down the dependencies of FS_ENCRYPTION to just the stuff
> needed for just AES-256-XTS and AES-256-CTS.  I already sent out a patch a
> couple months ago (https://patchwork.kernel.org/patch/10589319/) to remove
> CONFIG_CTR which isn't used at all; I'll remind Ted to apply that.  But we could
> also drop CONFIG_SHA256, which is only needed for AES-128-CBC contents
> encryption.  If we do that, it should be a separate patch, though.

Hi Eric,

fscrypt_valid_enc_modes() allows FS_ENCRYPTION_MODE_AES_128_CBC to be used for
encryption of file's contents. This is consistent with what you had mentioned
above.

static inline bool fscrypt_valid_enc_modes(u32 contents_mode,
                                           u32 filenames_mode)
{
        if (contents_mode == FS_ENCRYPTION_MODE_AES_128_CBC &&
            filenames_mode == FS_ENCRYPTION_MODE_AES_128_CTS)
                return true;

        if (contents_mode == FS_ENCRYPTION_MODE_AES_256_XTS &&
            filenames_mode == FS_ENCRYPTION_MODE_AES_256_CTS)
                return true;

        return false;
}

Hence FS_ENCRYPTION does need to have AES-128-CBC and by extension SHA256 code
compiled in right? 

-- 
chandan