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SubjectRe: Bug report: unaligned access with ext4 encryption
On Sun, Dec 30, 2018 at 06:29:06PM +0200, Aaro Koskinen wrote:
> Hi,
>
> When using ext4 encryption on SPARC, there's plenty of dmesg noise about
> unaligned access:
>
> [ 167.269526] Kernel unaligned access at TPC[5497a0] find_and_lock_process_key+0x80/0x120
> [ 167.270152] Kernel unaligned access at TPC[5497a0] find_and_lock_process_key+0x80/0x120
> [ 181.087451] log_unaligned: 5 callbacks suppressed
> [ 181.087511] Kernel unaligned access at TPC[5497a0] find_and_lock_process_key+0x80/0x120
> [ 181.092435] Kernel unaligned access at TPC[5497a0] find_and_lock_process_key+0x80/0x120
> [ 181.095816] Kernel unaligned access at TPC[5497a0] find_and_lock_process_key+0x80/0x120
>
> And also seen on an ARM machine:
>
> $ cat /proc/cpu/alignment
> User: 0
> System: 1028193 (find_and_lock_process_key+0x84/0x10c)
> Skipped: 0
> Half: 0
> Word: 1028193
> DWord: 0
> Multi: 0
> User faults: 0 (ignored)
>
> Looks like user_key_payload layout is not optimal when data address
> is used for fscrypt_key... I tried the below change and got rid of the
> messages. Not sure what the proper fix should be?
>
> A.
>
> diff --git a/include/keys/user-type.h b/include/keys/user-type.h
> index e098cbe27db5..6495ffcfe510 100644
> --- a/include/keys/user-type.h
> +++ b/include/keys/user-type.h
> @@ -31,7 +31,7 @@
> struct user_key_payload {
> struct rcu_head rcu; /* RCU destructor */
> unsigned short datalen; /* length of this data */
> - char data[0]; /* actual data */
> + char data[0] __aligned(4); /* actual data */
> };
>
> extern struct key_type key_type_user;
>

Hi Aaro, thanks for the bug report! I think you're on the right track; it makes
much more sense to have the keyrings subsystem store the payload with better
alignment, than to work around the 2-byte alignment in fscrypt.

But how about '__aligned(__alignof__(u64))' instead? 4 bytes may not be enough.

David, what do you think?

- Eric

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