Re: [PATCH] Revert "ext4 crypto: fix to check feature status before get policy"

[Eric Biggers]

Hi Douglas,

On Wed, Oct 30, 2019 at 10:06:25AM -0700, Douglas Anderson wrote:
> This reverts commit 0642ea2409f3 ("ext4 crypto: fix to check feature
> status before get policy").
> 
> The commit made a clear and documented ABI change that is not backward
> compatible.  There exists userspace code [1] that relied on the old
> behavior and is now broken.
> 
> While we could entertain the idea of updating the userspace code to
> handle the ABI change, it's my understanding that in general ABI
> changes that break userspace are frowned upon (to put it nicely).
> 
> NOTE: if we for some reason do decide to entertain the idea of
> allowing the ABI change and updating userspace, I'd appreciate any
> help on how we should make the change.  Specifically the old code
> relied on the different return values to differentiate between
> "KeyState::NO_KEY" and "KeyState::NOT_SUPPORTED".  I'm no expert on
> the ext4 encryption APIs (I just ended up here tracking down the
> regression [2]) so I'd need a bit of handholding from someone.
> 
> [1] https://chromium.googlesource.com/chromiumos/platform2/+/refs/heads/master/cryptohome/dircrypto_util.cc#73
> [2] https://crbug.com/1018265
> 
> Fixes: 0642ea2409f3 ("ext4 crypto: fix to check feature status before get policy")
> Signed-off-by: Douglas Anderson <dianders@xxxxxxxxxxxx>
> ---
> 
>  Documentation/filesystems/fscrypt.rst | 3 +--
>  fs/ext4/ioctl.c                       | 2 --
>  2 files changed, 1 insertion(+), 4 deletions(-)
> 
> diff --git a/Documentation/filesystems/fscrypt.rst b/Documentation/filesystems/fscrypt.rst
> index 8a0700af9596..4289c29d7c5a 100644
> --- a/Documentation/filesystems/fscrypt.rst
> +++ b/Documentation/filesystems/fscrypt.rst
> @@ -562,8 +562,7 @@ FS_IOC_GET_ENCRYPTION_POLICY_EX can fail with the following errors:
>    or this kernel is too old to support FS_IOC_GET_ENCRYPTION_POLICY_EX
>    (try FS_IOC_GET_ENCRYPTION_POLICY instead)
>  - ``EOPNOTSUPP``: the kernel was not configured with encryption
> -  support for this filesystem, or the filesystem superblock has not
> -  had encryption enabled on it
> +  support for this filesystem
>  - ``EOVERFLOW``: the file is encrypted and uses a recognized
>    encryption policy version, but the policy struct does not fit into
>    the provided buffer
> diff --git a/fs/ext4/ioctl.c b/fs/ext4/ioctl.c
> index 0b7f316fd30f..13d97fb797b4 100644
> --- a/fs/ext4/ioctl.c
> +++ b/fs/ext4/ioctl.c
> @@ -1181,8 +1181,6 @@ long ext4_ioctl(struct file *filp, unsigned int cmd, unsigned long arg)
>  #endif
>  	}
>  	case EXT4_IOC_GET_ENCRYPTION_POLICY:
> -		if (!ext4_has_feature_encrypt(sb))
> -			return -EOPNOTSUPP;
>  		return fscrypt_ioctl_get_policy(filp, (void __user *)arg);
>  

Thanks for reporting this.  Can you elaborate on exactly why returning
EOPNOTSUPP breaks things in the Chrome OS code?  Since encryption is indeed not
supported, why isn't "KeyState::NOT_SUPPORTED" correct?

Note that the state after this revert will be:

- FS_IOC_GET_ENCRYPTION_POLICY on ext4 => ENODATA
- FS_IOC_GET_ENCRYPTION_POLICY on f2fs => EOPNOTSUPP
- FS_IOC_GET_ENCRYPTION_POLICY_EX on ext4 => EOPNOTSUPP
- FS_IOC_GET_ENCRYPTION_POLICY_EX on f2fs => EOPNOTSUPP

So if this code change is made, the documentation would need to be updated to
explain that the error code from FS_IOC_GET_ENCRYPTION_POLICY is
filesystem-specific (which we'd really like to avoid...), and that
FS_IOC_GET_ENCRYPTION_POLICY_EX handles this case differently.  Or else the
other three would need to be changed to ENODATA -- which for
FS_IOC_GET_ENCRYPTION_POLICY on f2fs would be an ABI break in its own right,
though it's possible that no one would notice.

Is your proposal to keep the error filesystem-specific for now?

BTW, the crbug.com link is not publicly viewable, so should not be included in
the commit message.

- Eric