Re: Unaligned writes on the kexec path

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 29/11/11 11:35, Jan Beulich wrote:
>>>> On 29.11.11 at 11:49, Andrew Cooper <andrew.cooper3@citrix.com> wrote:
>> On 29/11/11 05:51, Simon Horman wrote:
>>> Hi Keir, Hi Andrew,
>>>
>>> On Mon, Nov 28, 2011 at 08:42:16PM +0000, Keir Fraser wrote:
>>>> On 28/11/2011 14:24, "Andrew Cooper" <andrew.cooper3@citrix.com> wrote:
>>>>
>>>>> Hello,
>>>>>
>>>>> In c/s 7493bb48d89f, you change the internals of kexec_crash_save_info()
>>>> The patch is by Simon Horman, cc'ed, not me.
>> Right.  Sorry.  I should have remembered that basing "who wrote the
>> patch" on a simple hg log was not a safe bet.
> I don't think there's much room for improvement - all members of
> crash_xen_info_t are of "unsigned long" type, but ELF note handling
> will only ever guarantee 4-byte alignment.
>
> Jan
>
Depending on how flexible we want to be, we can either specify that the
name field should be 2n words long plus 1-4 bytes, which will cause it
to align to an odd number of 4 bytes, which will cause the desc field to
be aligned to 8 bytes when the type field in the note header is taken
into account.  Then, the desc field should be constrained to be (2n+1) +
1-4bytes which would cause it to have 8 byte alignment, and subsequently
8 byte align the next note.

Alternatively, we could artificially extend the name up to an odd 4 byte
alignment, and desc field up to 8 byte alignment with trailing \0's and
include this as part of their length fields.  All names should be
processed as Null terminating strings (which wont suffer from having
extra Nulls at the end) and I have yet to see processing of a note which
doesn't take the buffer and cast it to a structure pointer.  This also
wont suffer from from trailing data.

Then again, this does sound like quite a lot of work for not a lot, and
there is no guarantee that we wont break some of the more special code
which works with elf files in 'special' ways.

(What really should have happened was for ELF64 to specify 64bit
alignment of things like this, but we live and learn)

-- 
Andrew Cooper - Dom0 Kernel Engineer, Citrix XenServer
T: +44 (0)1223 225 900, http://www.citrix.com