[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [Xen-devel] Unaligned writes on the kexec path
On 29/11/11 11:35, Jan Beulich wrote: >>>> On 29.11.11 at 11:49, Andrew Cooper <andrew.cooper3@xxxxxxxxxx> 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@xxxxxxxxxx> 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 _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxxxxxxxx http://lists.xensource.com/xen-devel
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