Hermetic toolchain uses `wrapper/clang` as linker instead of `wrapper/ld`

[unterumarmung]

The hermetic C++ toolchain is currently using wrapper/clang for linking instead of the dedicated wrapper/ld script. As a result, all linker-related configuration in the toolchain is effectively ignored.

The root cause seems to be that action_configs are created but never passed to cc_common.create_cc_toolchain_config_info here:

rules_ml_toolchain/third_party/rules_cc_toolchain/toolchain_config.bzl

Lines 106 to 143 in aa27dbc

	def _cc_toolchain_config_impl(ctx):
	action_configs = [action_config(
	action_name = action,
	enabled = True,
	tools = [
	tool(ctx.attr.tool_paths["ld"]),
	],
	implies = [
	],
	) for action in ACTION_NAME_GROUPS.all_cc_link_actions]
	return cc_common.create_cc_toolchain_config_info(
	ctx = ctx,
	host_system_name = "local",
	target_system_name = ctx.attr.target_system_name,
	target_cpu = ctx.attr.target_cpu,
	target_libc = ctx.attr.target_libc,
	artifact_name_patterns = _create_artifact_name_patterns(ctx),
	toolchain_identifier = "aarch64_linux_clang_id",
	compiler = "clang",
	abi_version = "unknown",
	abi_libc_version = "unknown",
	tool_paths = [
	tool_path(name = name, path = path)
	for name, path in ctx.attr.tool_paths.items()
	],
	features = [
	label[FeatureInfo]
	for label in ctx.attr.compiler_features
	] + [_label_to_tool_path_feature({
	"gcc": ctx.file.c_compiler,
	"cpp": ctx.file.cc_compiler,
	"ld": ctx.file.linker,
	"ar": ctx.file.archiver,
	"strip": ctx.file.strip_tool,
	"in": ctx.file.install_name,
	})],
	)

Because action_configs are omitted from the create_cc_toolchain_config_info call, Bazel ends up invoking wrapper/clang for linking instead of wrapper/ld. This is why the toolchain currently needs the feature:

"//third_party/rules_cc_toolchain/features:use_lld",

in order to get the expected linker behavior.

I tried wiring action_configs through to create_cc_toolchain_config_info locally, but then hit linker errors like undefined reference to _main in binaries where main is definitely defined. I did not track that regression down further.

This behavior also makes it harder to switch to a different linker (for example, mold) while keeping the rest of the toolchain the same.

If omitting action_configs in create_cc_toolchain_config_info was intentional, a short comment in the code would be very helpful. It would have saved me a significant amount of debugging time and will likely help others too.

[yuriivcs]

Hi @unterumarmung
Thank you for informing us about the problem.
Please create Pull Request with a fix of the issue.
Then we can review it and test.
Thank you