Spectre V1 diagnostic / mitigation

[Richard Earnshaw (lists)]

On 19/12/2018 17:17, Richard Biener wrote:
> On Wed, 19 Dec 2018, Florian Weimer wrote:
> 
>> * Peter Bergner:
>>
>>> On 12/19/18 7:59 AM, Florian Weimer wrote:
>>>> * Richard Biener:
>>>>
>>>>> Sure, if we'd ever deploy this in production placing this in the
>>>>> TCB for glibc targets might be beneifical.  But as said the
>>>>> current implementation was just an experiment intended to be
>>>>> maximum portable.  I suppose the dynamic loader takes care
>>>>> of initializing the TCB data?
>>>>
>>>> Yes, the dynamic linker will initialize it.  If you need 100% reliable
>>>> initialization with something that is not zero, it's going to be tricky
>>>> though.  Initial-exec TLS memory has this covered, but in the TCB, we
>>>> only have zeroed-out reservations today.
>>>
>>> We have non-zero initialized TCB entries on powerpc*-linux which are used
>>> for the GCC __builtin_cpu_is() and __builtin_cpu_supports() builtin
>>> functions.  Tulio would know the magic that was used to get them setup.
>>
>> Yes, there's a special symbol, __parse_hwcap_and_convert_at_platform, to
>> verify that the dynamic linker sets up the TCB as required.  This way,
>> binaries which need the feature will fail to run on older loaders.  This
>> is why I said it's a bit tricky to implement this.  It's even more
>> complicated if you want to backport this into released glibcs, where we
>> normally do not accept ABI changes (not even ABI additions).
> 
> It's easy to change the mitigation scheme to use a zero for the
> non-speculated path, you'd simply replace ands with zero by
> ors with -1.  For address parts that gets you some possible overflows
> you do not want though.

And you have to invert the value before using it as a mask.

R.

> 
> Richard.
>