linux-ext4 - Re: [PATCH] fscrypto: move ioctl processing more fully into common code

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Date:   Sun, 27 Nov 2016 12:39:34 -0500
From:   Theodore Ts'o <tytso@....edu>
To:     Eric Biggers <ebiggers@...gle.com>
Cc:     linux-fsdevel@...r.kernel.org, linux-ext4@...r.kernel.org,
        linux-f2fs-devel@...ts.sourceforge.net, jaegeuk@...nel.org
Subject: Re: [PATCH] fscrypto: move ioctl processing more fully into common
 code

On Sat, Nov 26, 2016 at 08:20:48PM -0800, Eric Biggers wrote:
> 
> I guess I'm okay with that, since struct fscrypt_policy won't have any padding
> bytes because its members are all bytes.  Plus it's marked __packed, though I
> think that was a mistake given that the struct isn't stored on disk directly.
>

It wouldn't have mattered if wasn't marked __packed, since the first
four fields are __u8, and the master_key_descriptor is a 4 byte
aligned __u8 array of size 8.

The use of __packed in the fscrypt code came from Michael, and I
suspect it's more of a Microsoft thing, since his previous experience
was as the architect for Bitlocker.  It's actually pretty rare that we
use __packed in Linux kernel sources in general, and in ext4
specifically.

Personally, I tend to depend on __uNN declaration and various
assumptions that we make about "sane" packing rules which are assumed
by the kernel.  See how the on-disk ext4 superblock is defined; that's
not the only place where we make assumptions about sane structure
packing, and anyone who tried porting Linux to a 18-bit or 36-bit
architecture would have lots of other problems, even if a modern Linux
kernel could be made small enough to fit in the memory available to a
PDP-10 or a PDP-15. :-)

We probably could remove a few of them, but I haven't bothered, since
in general they aren't doing any harm.

Cheers,

					- Ted

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