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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