[cfe-dev] [llvm-dev] Is it really valid to discard externally instantiated functions from a TU when marked inline?

Petr Hosek via cfe-dev cfe-dev at lists.llvm.org
Fri Jul 20 12:35:52 PDT 2018

I might be completely wrong about this, but I don't think libFuzzer relies
on __always_inline, at least not intentionally. The way libFuzzer
statically links libc++ is more complicated than simply merging all object
files into a single archive. What we do is first compiling libc++ with
_LIBCPP_ABI_VERSION=Fuzzer so all symbols that would normally end up
in std::__1 (or __2 if you use ABI v2) namespace end up in std::__Fuzzer
namespace (this may not even be needed given what follows). We also also
disable the default visibility annotations by
setting _LIBCPP_DISABLE_VISIBILITY_ANNOTATIONS and compile libc++ with
-fvisibility=hidden set so all symbols should end up as hidden. We then
link all libFuzzer and libc++ object files into a single relocatable .o
file and we use objcopy --localize-hidden to make all symbols that were
marked as hidden (i.e. all libc++ symbols) local. Given that, there should
be no collisions between libFuzzer's internal libc++ copy and libc++
application is using (even when linking libc++ statically), so I don't
think removing __always_inline would make any difference for this use case.

On Fri, Jul 20, 2018 at 8:35 AM Louis Dionne via cfe-dev <
cfe-dev at lists.llvm.org> wrote:

> On Jul 19, 2018, at 19:51, Dean Michael Berris <dean.berris at gmail.com>
> wrote:
> On 20 Jul 2018, at 01:49, James Y Knight via cfe-dev <
> cfe-dev at lists.llvm.org> wrote:
> I haven't looked at what libfuzzer does, but as you've described it, I'd
> say this _doesn't_ seem a reasonable use-case. If you want to link libc++
> (or any other library!) hidden inside your library, you should be using a
> version-script to only expose symbols that you intend to expose.
> In XRay, we go out of our way to not use any standard-library defined
> symbols that need linkage. We have a tests which ensure that we are able to
> link the XRay runtime without the standard library dependency. This has
> been discussed before, I think, precisely in this context.
> The options for libFuzzer might be more limited since it’s a special
> use-case, and I’m sure unless the fuzzer runtime removes dependencies on
> standard library symbols that libc++ shouldn’t be prioritising this
> use-case. Another way of saying this is that the cost of ensuring that
> libFuzzer continues to work despite changes to libc++ should be borne by
> libFuzzer, not the other way around.
> Well, that’s also my impression, but I don’t know how we can actually
> change this given there are clients using it and the status quo usually
> wins when no consensus can be reached. What I suggest is that we instead
> start implementing my proposal at
> http://lists.llvm.org/pipermail/cfe-dev/2018-July/058457.html (I’m
> waiting for reviews on https://reviews.llvm.org/D49240 BTW), which will
> solve most concrete problems we have right now. In the not too distant
> future, we can then evaluate the feasibility of removing support for
> interoperability of TUs built with different versions and the benefits of
> doing so — as a separate effort (which I’d be glad to volunteer for).
> My primary concern at this point is about moving forward and solving the
> concrete problems we have with the use of __always_inline__, and I’m
> willing to compromise to get there, and to do it in stages if I have to.
> Cheers,
> Louis
> I don’t know how much work is required to use a work-around as James
> describes with symbol visibility or “privatising” the symbols libFuzzer
> uses from libc++. It could be less than trying to remove the dependency on
> libc++ instead.
> -- Dean
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