Re: [PATCH] fscrypt: remove kernel-internal constants from UAPI header

[Eric Biggers <ebiggers@kernel.org>]

On Fri, Oct 23, 2020 at 05:51:31PM -0700, Eric Biggers wrote:
> From: Eric Biggers <ebiggers@google.com>
> 
> There isn't really any valid reason to use __FSCRYPT_MODE_MAX or
> FSCRYPT_POLICY_FLAGS_VALID in a userspace program.  These constants are
> only meant to be used by the kernel internally, and they are defined in
> the UAPI header next to the mode numbers and flags only so that kernel
> developers don't forget to update them when adding new modes or flags.
> 
> In https://lkml.kernel.org/r/20201005074133.1958633-2-satyat@google.com
> there was an example of someone wanting to use __FSCRYPT_MODE_MAX in a
> user program, and it was wrong because the program would have broken if
> __FSCRYPT_MODE_MAX were ever increased.  So having this definition
> available is harmful.  FSCRYPT_POLICY_FLAGS_VALID has the same problem.
> 
> So, remove these definitions from the UAPI header.  Replace
> FSCRYPT_POLICY_FLAGS_VALID with just listing the valid flags explicitly
> in the one kernel function that needs it.  Move __FSCRYPT_MODE_MAX to
> fscrypt_private.h, remove the double underscores (which were only
> present to discourage use by userspace), and add a BUILD_BUG_ON() and
> comments to (hopefully) ensure it is kept in sync.
> 
> Keep the old name FS_POLICY_FLAGS_VALID, since it's been around for
> longer and there's a greater chance that removing it would break source
> compatibility with some program.  Indeed, mtd-utils is using it in
> an #ifdef, and removing it would introduce compiler warnings (about
> FS_POLICY_FLAGS_PAD_* being redefined) into the mtd-utils build.
> However, reduce its value to 0x07 so that it only includes the flags
> with old names (the ones present before Linux 5.4), and try to make it
> clear that it's now "frozen" and no new flags should be added to it.
> 
> Fixes: 2336d0deb2d4 ("fscrypt: use FSCRYPT_ prefix for uapi constants")
> Cc: <stable@vger.kernel.org> # v5.4+
> Signed-off-by: Eric Biggers <ebiggers@google.com>

Applied to fscrypt.git#master for 5.11.

- Eric