Re: [PATCH v2] common/encrypt: allow the use of 'fscrypt:' as key prefix

[Eric Biggers <ebiggers@kernel.org>]

The code looks fine, but the explanation needs some tweaks:

On Mon, Apr 04, 2022 at 11:25:54AM +0100, Luís Henriques wrote:
> fscrypt keys have used the $FSTYP as prefix.  However this format is being
> deprecated -- newer kernels already allow the usage of the generic
> 'fscrypt:' prefix for ext4 and f2fs.  This patch allows the usage of this
> new prefix for testing filesystems that have never supported the old
> format, but keeping the $FSTYP prefix for filesystems that support it, so
> that old kernels can be tested.

This explanation is inconsistent with the code, which uses FSTYP for only ext4
and f2fs, and fscrypt for everything else including ubifs.

A better explanation would be something like "Only use $FSTYP on filesystems
that never supported the 'fscrypt' prefix, i.e. ext4 and f2fs."

> +# Keys are named $FSTYP:KEYDESC where KEYDESC is the 16-character key descriptor
> +# hex string.  Newer kernels (ext4 4.8 and later, f2fs 4.6 and later) also allow
> +# the common key prefix "fscrypt:" in addition to their filesystem-specific key
> +# prefix ("ext4:", "f2fs:").  It would be nice to use the common key prefix, but
> +# for now use the filesystem- specific prefix for these 2 filesystems to make it
> +# possible to test older kernels, and the "fscrypt" prefix for anything else.
> +_get_fs_keyprefix()

The first part of this comment sort of implies that FSTYP is the default and
"fscrypt" is the exception, but it should be the other way around.

How about:

# When fscrypt keys are added using the legacy mechanism (process-subscribed
# keyrings rather than filesystem keyrings), they are normally named
# "fscrypt:KEYDESC" where KEYDESC is the 16-character key descriptor hex string.
# However, ext4 and f2fs didn't add support for the "fscrypt" prefix until
# kernel v4.8 and v4.6, respectively.  Before that, they used "ext4" and "f2fs",
# respectively.  To allow testing ext4 and f2fs encryption on kernels older than
# this, we use these filesystem-specific prefixes for ext4 and f2fs.