libc++ support for Clang + Linux

[htuch]

In the Envoy project, we would like to use a combination of our Bazel build and oss-fuzz (https://github.com/google/oss-fuzz), tracked at envoyproxy/envoy#508.

oss-fuzz requires that we are able to build with a Clang toolchain in a Linux Docker container, and link against libc++ rather than libstdc++, see google/oss-fuzz#1114.

I don't see direct support for this in cc_configure.bzl toolchain autoconfigure today, there's direct references to libstdc++ in

bazel/tools/cpp/unix_cc_configure.bzl

Line 49 in 12471a7

def _build_link_content():

for example. Ideally we would have some flag or detection of -stdlib=libc++ as a --linkopt that would correctly configure for linking against libc++.

We'd find it helpful to hear from Bazel team on the ideal way of structuring this and hopefully suggestions for how we can work around this in the interim to avoid blocking our full oss-fuzz integration. I'm guessing we could do a manually crafted CROSSTOOL file (possibly generated on-the-fly prior to Bazel invocation), or continue with Envoy's fork of cc_configure.bzl and build in the support there. The situation is complicated by the fact that oss-fuzz may invoke our build with arbitrary CFLAGS/CXXFLAGS/CC that we need to map into the Bazel world. Thanks!

@jmillikin-stripe @mattklein123 @kcc

[meteorcloudy]

Using a custom CROSSTOOL sounds like the right way to me, but //cc @mhlopko for more insight.

[htuch]

@jmillikin-stripe if we have #4031 in the next release, it seems this won't allow us to drop the cc_configure.bzl fork still, since we need to be able to override this for OSS-Fuzz as described above. Thoughts?

[jmillikin-stripe]

If we can use the C++ toolchain support to define a custom envoy_cc_binary rule, then we can use it to do all the flag editing we want without having to worry about the crosstool. It should also be easier to keep up-to-date as the Bazel API changes, since we'll be poking at far fewer internal surfaces.

[hlopko]

@htuch @jmillikin-stripe pls verify that 19c6428 is good enough, and let me know if not and I'll reopen. Thanks!

[jmillikin-stripe]

@mhlopko The functionality looks good to me. Can you document somewhere what BAZEL_LINKOPTS is, its expected format, and how it's different from the --linkopt flag?

[hlopko]

Let me come back to that next Monday, I'm planning a present for the world later this week.

[hlopko]

Still working on (fully documented :) present.

[mihaimaruseac]

Hi. Any update on the document? I'm thinking we'll need this for OSSFuzz-tensorflow integration

[htuch]

@mihaimaruseac you can see what we did for Envoy + Bazel + oss-fuzz at https://github.com/google/oss-fuzz/blob/master/projects/envoy/build.sh.

[mihaimaruseac]

I'm following that, but I thought it might be easier/more readable with BAZEL_LINKOPT (which is not present there).

I got it to work just now, so it should be ok, thanks