Allow for an arbitrary location of $KBUILD_OUTPUT in tar-artifacts.sh

There are two distinct KBUILD_OUTPUTs across libbpf/ci:

- $KBUILD_OUTPUT used by actions/cache (in prepare-incremental-build)
- $REPO_ROOT/kbuild-output used by ./run-vmtest

So far this difference wasn't important, because $KBUILD_OUTPUT
always pointed to $(pwd)/kbuild-output. And $(pwd) happened to be a
root of the linux source tree, also equal to github.workspace

However we don't want prepare-incremental-build action to put any
requirements on $KBUILD_OUTPUT path to allow for a convenient action
API.

With this, the difference becomes important, because only selected
files are put from $KBUILD_OUTPUT into kernel-build output artifacts,
while entire $KBUILD_OUTPUT has to be cached by
prepare-incremental-build.

.github/scripts/tar-artifact.sh

@@ -2,6 +2,15 @@
 
 set -eux
 
+# Assumptions:
+#   - $(pwd) is the root of kernel repo we're tarring
+#   - zstd is installed by default in the runner images
+
+if [ ! -d "${KBUILD_OUTPUT:-}" ]; then
+  echo "KBUILD_OUTPUT must be a directory"
+  exit 1
+fi
+
 arch="${1}"
 toolchain="${2}"
 archive_make_helpers="${3:-0}"
@@ -39,6 +48,28 @@ find selftests/ -name "*.o" -a ! -name "*.bpf.o" -print0 | \
 # DWARF to symbolize stacktraces).
 "${arch}"-linux-gnu-strip --strip-debug "${KBUILD_OUTPUT}"/vmlinux
 
+image_name=$(make ARCH="$(platform_to_kernel_arch "${arch}")" -s image_name)
+kbuild_output_file_list=(
+  ".config"
+  "${image_name}"
+  "include/config/auto.conf"
+  "include/generated/autoconf.h"
+  "vmlinux"
+)
+
+# While we are preparing the tarball, move $KBUILD_OUTPUT to a tmp
+# location just in case it's inside the repo root
+tmp=$(mktemp -d)
+mv "${KBUILD_OUTPUT}" "${tmp}"
+stashed_kbuild_output=${tmp}/$(basename "${KBUILD_OUTPUT}")
+local_kbuild_output=$(realpath kbuild-output)
+mkdir -p "${local_kbuild_output}"
+
+for file in "${kbuild_output_file_list[@]}"; do
+  mkdir -p "$(dirname "${local_kbuild_output}/${file}")"
+  cp -a "${stashed_kbuild_output}/${file}" "${local_kbuild_output}/${file}"
+done
+
 additional_file_list=()
 if [ $archive_make_helpers -ne 0 ]; then
   # Package up a bunch of additional infrastructure to support running
@@ -52,26 +83,17 @@ if [ $archive_make_helpers -ne 0 ]; then
   )
 fi
 
-image_name=$(make ARCH="$(platform_to_kernel_arch "${arch}")" -s image_name)
-
-kbuild_output_file_list=(
-  ".config"
-  "${image_name}"
-  "include/config/auto.conf"
-  "include/generated/autoconf.h"
-  "vmlinux"
-)
-
-# Assumptions:
-#   - $(pwd) is the root of kernel repo we're tarring
-#   - zstd is installed by default in the runner images
-tar -cf -                              \
-    -C "$KBUILD_OUTPUT"                \
-       "${kbuild_output_file_list[@]}" \
-    -C "$(pwd)"                        \
-       "${additional_file_list[@]}"    \
-       --exclude '*.cmd'               \
-       --exclude '*.d'                 \
-       --exclude '*.output'            \
-       selftests/bpf/                  \
+tar -cf - \
+    "${local_kbuild_output}/" \
+    "${additional_file_list[@]}" \
+    --exclude '*.cmd'                  \
+    --exclude '*.d'                    \
+    --exclude '*.h'                    \
+    --exclude '*.output'               \
+    selftests/bpf/                     \
   | zstd -T0 -19 -o "vmlinux-${arch}-${toolchain}.tar.zst"
+
+# Cleanup and restore the original KBUILD_OUTPUT
+# We have to put KBUILD_OUTPUT back to its original location for actions/cache
+rm -rf "${local_kbuild_output}"
+mv "${stashed_kbuild_output}" "${KBUILD_OUTPUT}"

.github/workflows/kernel-test.yml

@@ -41,7 +41,6 @@ jobs:
       KERNEL: ${{ inputs.kernel }}
       REPO_ROOT: ${{ github.workspace }}
       REPO_PATH: ""
-      KBUILD_OUTPUT: ${{ github.workspace }}/kbuild-output
       # https://github.com/actions/runner/issues/1483#issuecomment-1031671517
       # booleans are weird in GH.
       CONTINUE_ON_ERROR: ${{ inputs.continue_on_error }}
@@ -67,4 +66,6 @@ jobs:
           kernel-root: ${{ env.REPO_ROOT }}
           max-cpu: 8
           kernel-test: ${{ inputs.test }}
-          kbuild-output: ${{ env.KBUILD_OUTPUT }}
+          # Here we must use kbuild-output local to the repo, because
+          # it was extracted from the artifacts.
+          kbuild-output: ${{ env.REPO_ROOT }}/kbuild-output