Re: V5 [PATCH 2/2] x86: Add a LD_PRELOAD IFUNC resolver test for CPU_FEATURE_USABLE

[Florian Weimer <fweimer at redhat dot com>]

* H. J. Lu:

> On 10/24/18, Florian Weimer <fweimer@redhat.com> wrote:
>> * H. J. Lu:
>>
>>> I guess you knew that this issue was independent of my new functions.
>>> You will get the same error regardless of what the get_free body has.
>>
>> Yes, the check is certainly overly conservative.  I thought we want to
>> remove it.  Don't we trigger it in glibc in a few places?  If the check
>> is gone, then I think we will see incorrect results from the new
>> interface.
>>
>> I think we are very consistent right now when it comes to relocations in
>> IFUNC handlers.  I want to see this settled before adding something that
>> requires a relocation which is (among other things) targeted at IFUNC
>> resolvers.
>>
>
> <sys/platform/x86.h> isn't targeted for IFUNC.   My first use is to add
> x86_tsc_to_ns and x86_ns_to_tsc.   I am enclosing 2 patches here.

That's a generic interface which should rely on internal CPU flags.
What's worse, the cached flag isn't updated by the kernel TSC watchdog,
so applications will use a known-broken time source.

What's wrong with clock_gettime?  It handles all these details.

Thanks,
Florian