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

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

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).



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