Re: [Xen-devel] [PATCH v2 3/5] xen/livepatch/ARM32: Don't load and crash on livepatches loaded with wrong alignment.

["Jan Beulich" <jbeulich@xxxxxxxx>]

>>> Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx> 07/31/17 6:04 PM >>>
>On Mon, Jul 31, 2017 at 07:55:34AM -0600, Jan Beulich wrote:
>> >>> Konrad Rzeszutek Wilk <konrad@xxxxxxxxxx> 07/26/17 9:50 PM >>>
>> >--- a/docs/misc/livepatch.markdown
>> >+++ b/docs/misc/livepatch.markdown
>> >@@ -279,6 +279,10 @@ It may also have some architecture-specific sections. 
>> >For example:
>> >* Exception tables.
>> >* Relocations for each of these sections.
>>  >
>> >+Note that on ARM 32 the sections SHOULD be four byte aligned. Otherwise
>> >+we risk hitting Data Abort exception as un-aligned manipulation of data is
>> >+prohibited on ARM 32.
>> 
>> This (and hence the rest of the patch) is not in line with the outcome of the
>> earlier discussion we had. Nothing is wrong with a section having smaller
>> alignment, as long as there are no 32-bit (or wider, but I don't think there
>> are any such) relocations against such a section. And even if there were, I
>> think it should rather be the code doing the relocations needing to cope, as
>> I don't think the ARM ELF ABI imposes any such restriction.
>
>The idea behind this patch is to give advance warnings. Akin to what
>2ff229643b739e2fd0cd0536ee9fca506cfa92f8
>"xen/livepatch: Don't crash on encountering STN_UNDEF relocations" did.
>
>The other patches in this series fix the alignment issues.
>
>The ARM ELF ABI 
>(http://infocenter.arm.com/help/topic/com.arm.doc.ihi0044f/IHI0044F_aaelf.pdf)
>
>says:
>
>4.3.5 Section Alignment
>There is no minimum alignment required for a section. However, sections 
>containing thumb code must be at least
>16-bit aligned and sections containing ARM code must be at least 32-bit 
>aligned.
>Platform standards may set a limit on the maximum alignment that they can 
>guarantee (normally the page size).

Note the "thumb code" and "ARM code" in here - iirc you're checking _all_
sections, not just ones containing code.

Jan





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