Re: Spectre V1 diagnostic / mitigation

[Florian Weimer <fweimer@redhat.com>]

* 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).

Thanks,
Florian