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".
Mon Jan 8 20:22:00 GMT 2018
On Mon, Jan 8, 2018 at 12:17 PM, Sriraman Tallam via binutils
> On Mon, Jan 8, 2018 at 11:01 AM, Florian Weimer <email@example.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?
> * 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?
> * 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?
> * We still have to find a solution to avoid PLTs for shared
> objects, needs re-building and fixing performance issues.
> * We could use static linking but that is not an immediate solution.
> b) We have this patch in the linker:
> * If we want to continue to use lazy binding or just keep PLTs as
> it is and take the penalty for the project, this is easy.
> * My testing with retpoline for large programs shows this is
> pretty straightforward to deploy, I did not run into any correctness
> issues that requires large-scale fixing of builds.
> * All other problems from a) exist. Shared libraries still need
> to be fixed, compiler needs to be fixed to avoid indirect calls, etc.
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