Subject: Re: [1/5] powerpc/lib: Fix off-by-one in alternate feature patching

From: Michael Ellerman <[email-masked]>
Date: Tue, 17 Apr 2018 20:13:36 +1000 (AEST)

On Mon, 2018-04-16 at 14:39:01 UTC, Michael Ellerman wrote:
> When we patch an alternate feature section, we have to adjust any
> relative branches that branch out of the alternate section.
> 
> But currently we have a bug if we have a branch that points to past
> the last instruction of the alternate section, eg:
> 
>   FTR_SECTION_ELSE
>   1:     b       2f
>          or      6,6,6
>   2:
>   ALT_FTR_SECTION_END(...)
>          nop
> 
> This will result in a relative branch at 1 with a target that equals
> the end of the alternate section.
> 
> That branch does not need adjusting when it's moved to the non-else
> location. Currently we do adjust it, resulting in a branch that goes
> off into the link-time location of the else section, which is junk.
> 
> The fix is to not patch branches that have a target == end of the
> alternate section.
> 
> Fixes: d20fe50a7b3c ("KVM: PPC: Book3S HV: Branch inside feature section")
> Fixes: 9b1a735de64c ("powerpc: Add logic to patch alternative feature sections")
> Cc: [email-masked] # v2.6.27+
> Signed-off-by: Michael Ellerman <[email-masked]>

Applied to powerpc fixes.

https://git.kernel.org/powerpc/c/b8858581febb050688e276b956796b

cheers