diff --git a/build/toolchain/musl-host-gcc/sysroot/tarball.bzl b/build/toolchain/musl-host-gcc/sysroot/tarball.bzl
new file mode 100644
index 0000000..4f12049
--- /dev/null
+++ b/build/toolchain/musl-host-gcc/sysroot/tarball.bzl
@@ -0,0 +1,76 @@
+#  Copyright 2020 The Monogon Project Authors.
+#
+#  SPDX-License-Identifier: Apache-2.0
+#
+#  Licensed under the Apache License, Version 2.0 (the "License");
+#  you may not use this file except in compliance with the License.
+#  You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+#  Unless required by applicable law or agreed to in writing, software
+#  distributed under the License is distributed on an "AS IS" BASIS,
+#  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+#  See the License for the specific language governing permissions and
+#  limitations under the License.
+
+load(
+    "//build/utils:detect_root.bzl",
+    "detect_root",
+)
+
+"""
+Build a sysroot-style tarball containing musl/linux headers and libraries.
+
+This can then be used to build a C toolchain that builds for Smalltown.
+"""
+
+def _musl_gcc_tarball(ctx):
+    tarball_name = ctx.attr.name + ".tar.xz"
+    tarball = ctx.actions.declare_file(tarball_name)
+
+    musl_headers = ctx.file.musl_headers
+    musl_headers_path = musl_headers.path
+    linux_headers = ctx.file.linux_headers
+    linux_headers_path = linux_headers.path
+
+    musl_root = detect_root(ctx.attr.musl)
+    musl_files = ctx.files.musl
+
+    # This builds a tarball containing musl, musl headers and linux headers.
+    # This is done by some carefully crafted tar command line arguments that rewrite
+    # paths to ensure that everything lands in lib/ and include/ in the tarball.
+
+    # TODO(q3k): write nice, small static Go utility for this.
+
+    arguments = [tarball.path]
+    command = "tar -chJf $1"
+
+    arguments += [musl_headers_path]
+    command += " --transform 's|^'$2'|include|' $2"
+
+    arguments += [linux_headers_path]
+    command += " --transform 's|^'$3'|include|' $3"
+
+    arguments += [musl_root]
+    command += " --transform 's|^'$4'|lib|' $4"
+
+    ctx.actions.run_shell(
+        inputs = [musl_headers, linux_headers] + ctx.files.musl,
+        outputs = [tarball],
+        progress_message = "Building toolchain tarball",
+        mnemonic = "BuildToolchainTarball",
+        arguments = arguments,
+        use_default_shell_env = True,
+        command = command,
+    )
+    return [DefaultInfo(files=depset([tarball]))]
+
+musl_gcc_tarball = rule(
+    implementation = _musl_gcc_tarball,
+    attrs = {
+        "musl": attr.label(mandatory = True),
+        "musl_headers": attr.label(mandatory = True, allow_single_file = True),
+        "linux_headers": attr.label(mandatory = True, allow_single_file = True),
+    },
+)