Re: PING: [PATCH] i386: Add TARGET_INDIRECT_BRANCH_REGISTER

[Steve Beattie <sbeattie@ubuntu.com>]

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Hi Jeff,

On Thu, Feb 22, 2018 at 10:10:13AM -0700, Jeff Law wrote:
> A few notes.
> 
> 1. It's not even clear at this time that retpolining user space binaries
> makes any sense at all.   SO before doing anything to make this easier
> I'd like to see a justification for why it's really needed.

Do you have a reference that gives evidence that retpolining user
space is not needed or not preferred for x86? Everything that I've
seen has suggested user space to user space attacks are possible,
if difficult. And it does not seem likely that microcode updates will
occur for all processor generations out there.

> 2. On the other hand, the existing thunk options do make it easier to
> test independent of hte kernel.  ie, I can turn on inline thunks by
> default and test things in user space (ie, do thunks generally work
> properly).

If thunk-extern is to be the only maintained option, and its deemed
sensible for user space in at least some situations, is there a
preferred location for the thunks to end up?

(I ask these questions because you can already find individual users
recompiling apps important to them with retpoline options, and there
is pressure (with associated deadlines) in some quarters to rebuild
vast tracts of user space with retpolines for x86.)

Thanks.

-- 
Steve Beattie
<sbeattie@ubuntu.com>
http://NxNW.org/~steve/

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