On Tue, Nov 15, 2022 at 06:10:59PM -0800, Eric Biggers wrote: > On Thu, Nov 10, 2022 at 03:12:21PM +0100, Niels de Vos wrote: > > While more filesystems are getting support for fscrypt, it is useful to > > be able to disable fscrypt for a selection of filesystems, while > > enabling it for others. > > > > The new USE_FS_ENCRYPTION define gets picked up in > > include/linux/fscrypt.h. This allows filesystems to choose to use the > > empty function definitions, or the functional ones when fscrypt is to be > > used with the filesystem. > > > > Using USE_FS_ENCRYPTION is a relatively clean approach, and requires > > minimal changes to the filesystems supporting fscrypt. This RFC is > > mostly for checking the acceptance of this solution, or if an other > > direction is preferred. > > > > --- > > > > Niels de Vos (4): > > fscrypt: introduce USE_FS_ENCRYPTION > > fs: make fscrypt support an ext4 config option > > fs: make fscrypt support a f2fs config option > > fs: make fscrypt support a UBIFS config option > > So as others have pointed out, it doesn't seem worth the complexity to do this. > > For a bit of historical context, before Linux v5.1, we did have per-filesystem > options for this: CONFIG_EXT4_ENCRYPTION, CONFIG_F2FS_FS_ENCRYPTION, and > CONFIG_UBIFS_FS_ENCRYPTION. If you enabled one of these, it selected > CONFIG_FS_ENCRYPTION to get the code in fs/crypto/. CONFIG_FS_ENCRYPTION was a > tristate, so the code in fs/crypto/ could be built as a loadable module if it > was only needed by filesystems that were loadable modules themselves. > > Having fs/crypto/ possibly be a loadable module was problematic, though, because > it made it impossible to call into fs/crypto/ from built-in code such as > fs/buffer.c, fs/ioctl.c, fs/libfs.c, fs/super.c, fs/iomap/direct-io.c, etc. So > that's why we made CONFIG_FS_ENCRYPTION into a bool. At the same time, we > decided to simplify the kconfig options by removing the per-filesystem options > so that it worked like CONFIG_QUOTA, CONFIG_FS_DAX, CONFIG_FS_POSIX_ACL, etc. > > I suppose we *could* have *just* changed CONFIG_FS_ENCRYPTION to a bool to solve > the first problem, and kept the per-filesystem options. I think that wouldn't > have made a lot of sense, though, for the reasons that Ted has already covered. Yes, it seems that there is a move to reduce the Kconfig options and (re)adding per-filesystem encryption support would be counterproductive. > A further point, beyond what Ted has already covered, is that > non-filesystem-specific code can't honor filesystem-specific options. So e.g. > if you had a filesystem with encryption disabled by kconfig, that then called > into fs/iomap/direct-io.c to process an I/O request, it could potentially still > call into fs/crypto/ to enable encryption on that I/O request, since > fs/iomap/direct-io.c would think that encryption support is enabled. > > Granted, that *should* never actually happen, because this would only make a > difference on encrypted files, and the filesystem shouldn't have allowed an > encrypted file to be opened if it doesn't have encryption support enabled. But > it does seem a bit odd, given that it would go against the goal of compiling out > all encryption code for a filesystem. Ah, yes, indeed! The boundaries between the options would be less clear, and potential changes to shared functions under fs/ could have incorrect assumptions about CONFIG_FS_ENCRYPTION. Even if this is not the case now, optimizations/enhancements in the future might be more complicated because of this. Thanks for the additional details! Have a good weekend, Niels
