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Gold Linker Patch: Introduce the "retpoline" x86 mitigation technique for variant #2 of the speculative execution vulnerabilities disclosed today, specifically identified by CVE-2017-5715 and in some places called "spectre".

Sriraman Tallam via binutils binutils@sourceware.org
Wed Jan 10 01:10:00 GMT 2018 
+manojgupta

On Mon, Jan 8, 2018 at 1:23 PM, Sriraman Tallam <tmsriram@google.com> wrote:
> On Mon, Jan 8, 2018 at 1:14 PM, H.J. Lu <hjl.tools@gmail.com> wrote:
>> On Mon, Jan 8, 2018 at 1:09 PM, Sriraman Tallam <tmsriram@google.com> wrote:
>>> On Mon, Jan 8, 2018 at 12:22 PM, H.J. Lu <hjl.tools@gmail.com> wrote:
>>>> On Mon, Jan 8, 2018 at 12:17 PM, Sriraman Tallam via binutils
>>>> <binutils@sourceware.org> wrote:
>>>>> On Mon, Jan 8, 2018 at 11:01 AM, Florian Weimer <fweimer@redhat.com> wrote:
>>>>>> On 01/08/2018 07:51 PM, Rui Ueyama wrote:
>>>>>>>
>>>>>>> A drawback of using BIND_NOW is that an application that has a PLT entry
>>>>>>> that cannot be resolved but not used fails to start with that option.
>>>>>>
>>>>>>
>>>>>> That can be a good or bad thing, depending on your perspective.  With more
>>>>>> and more use of symbol versioning, the point is increasingly moot because
>>>>>> the set of symbol versions is not checked lazily.
>>>>>
>>>>> Ok, my attempt to summarize the discussions around this patch:
>>>>>
>>>>> a)  We don't need this patch.
>>>>>       * We could deploy fno-plt and now binding and remove PLTs
>>>>> altogether. We have to fix correctness issues related to these, like
>>>>> the one Rui pointed out.
>>>>
>>>> What correctness issue?
>>>
>>> If a lazy bound symbol cannot be resolved and is not resolved at
>>> run-time, now binding will expose the issue.
>>
>> Is this the missing definition issue?  I won't call it as "correctness".  I
>> consider it as "abuse".  It can even be a security issue when a program
>> crashes unexpectedly due to the missing definition.
>
> Agreed and I understood this was Florian's point about good and bad
> based on perspective.
>
>>
>>>>
>>>>>       * One other pain point is we do have internally is we use a
>>>>> configuration for tests where we build a number of shared objects and
>>>>> keep the main binary pretty thin.  We have explicitly disabled now
>>>>> binding for this due to performance reasons, huge increase in the
>>>>> number of dynamic relocations putting unacceptable overheads on our
>>>>> distributed build system.  We need to find a solution here.
>>>>
>>>> Have you measured performance impact of -fno-plt?
>>>
>>> I have conducted some experiments with fno-plt for binaries that
>>> mostly statically linked with some hot calls to libc.  fno-plt did
>>> gives us 0.5 %- 1% improvements here and we have plans to turn this on
>>> for performance sensitive binaries.  fno-plt seems to help in reducing
>>>  iTLB misses when used in conjunction with kernel huge pages.
>>
>> So removing PLT isn't that bad for performance.
>
> Yes, based on what I have seen so far.
>
>>
>>>>
>>>>>       * The compiler is eliminating indirect branches and calls
>>>>> anyway, might as well do it with fno-plt also.  With
>>>>> -mindirect-branch=think this might also be unnecessary but LLVM
>>>>> atleast does not support this yet.
>>>>
>>>> Shouldn't LLVM be fixed?
>>>
>>> Yes,  I can take a look at fixing LLVM fo this.
>>>
>>
>> I think you should experiment all options before changing PLT.
>>
>>
>> --
>> H.J.
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