Enable overriding the tool used by link actions in cc_configure

[nlopezgi]

Description of the problem / feature request:

Enable overriding the tool used by link actions in cc_configure.

Feature requests: what underlying problem are you trying to solve with this feature?

We want to support ubsan builds with Bazel / RBE without the need for a custom toolchain configuration. Currently, the only thing that our toolchain configuration defines is the tool used for link actions (e.g., https://github.com/bazelbuild/bazel-toolchains/blob/master/configs/experimental/ubuntu16_04_clang/1.2/bazel_0.23.0/ubsan/cc_toolchain_config.bzl#L116).
Every other sanitizer (asan, msan, tsan) currently works out of the box both with the Bazel container and with Bazel + RBE, except for ubsan. We spend too much effort currently to manually upgrade our ubsan custom toolchain with new Bazel versions.

We think it should be plausible to have cc_configure read a BAZEL_LINKTOOL env variable or something of the sort and produce the corresponding toolchain config that only has the relevant action configs overridden.

[keith]

Related #7883

[hlopko]

Can you link issues/discussions about ubsan needed clang++? Sounds weird that it does..

[hlopko]

And to be sure, you don't care about specifying the linker (e.g. specifying FOO when passing -fuse-ld=FOO to CC), you only want to specify the tool that will be used in linking related action configs.

I think the former is requested more often..

[hlopko]

Alternative that comes to mind is to use clang++ for CC - setting clang++ to be the compiler driver for everything. Did you consider that? What were the problems?

[nlopezgi]

Hi Marcel, thanks for your response.
I have tried using clang++ for CC and that does not work for ubsan, it has to only be the tool used for link actions.
Why clang++?
I used clang++ following advice from:
google/oss-fuzz#383 and
https://clang.llvm.org/docs/UndefinedBehaviorSanitizer.html#usage which states clang++ should be used as linker

[hlopko]

Thanks Nick, give me a few days to see if we can get away with using clang, I hope it will be possible.

[nlopezgi]

sure thing! I would love to find a simpler way to get ubsan to work.

[hlopko]

Closing since it seems there are ways to make upsan work with current APIs.