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Re: Spectre V1 diagnostic / mitigation

* 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).


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