selftests/bpf: Support local rootfs image for vmtest

Support vmtest to use local rootfs image generated by [0] that is
consistent with BPF CI. Now we can specify the local rootfs image
through the `-l` parameter like as follows:

  vmtest.sh -l ./libbpf-vmtest-rootfs-2024.08.22-noble-amd64.tar.zst -- ./test_progs

Meanwhile, some descriptions have been flushed.

Link: https://github.com/libbpf/ci/blob/main/rootfs/mkrootfs_debian.sh [0]
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Pu Lehui <pulehui@huawei.com>
Link: https://lore.kernel.org/r/20240905081401.1894789-6-pulehui@huaweicloud.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

tools/testing/selftests/bpf/README.rst

@@ -85,8 +85,6 @@ In case of linker errors when running selftests, try using static linking:
           If you want to change pahole and llvm, you can change `PATH` environment
           variable in the beginning of script.
 
-.. note:: The script currently only supports x86_64 and s390x architectures.
-
 Additional information about selftest failures are
 documented here.
 

tools/testing/selftests/bpf/vmtest.sh

@@ -4,9 +4,11 @@
 set -u
 set -e
 
-# This script currently only works for x86_64 and s390x, as
-# it is based on the VM image used by the BPF CI, which is
-# available only for these architectures.
+# This script currently only works for the following platforms,
+# as it is based on the VM image used by the BPF CI, which is
+# available only for these architectures. We can also specify
+# the local rootfs image generated by the following script:
+# https://github.com/libbpf/ci/blob/main/rootfs/mkrootfs_debian.sh
 ARCH="$(uname -m)"
 case "${ARCH}" in
 s390x)
@@ -34,6 +36,7 @@ aarch64)
 esac
 DEFAULT_COMMAND="./test_progs"
 MOUNT_DIR="mnt"
+LOCAL_ROOTFS_IMAGE=""
 ROOTFS_IMAGE="root.img"
 OUTPUT_DIR="$HOME/.bpf_selftests"
 KCONFIG_REL_PATHS=("tools/testing/selftests/bpf/config"
@@ -69,6 +72,7 @@ or
 
 Options:
 
+	-l)             Specify the path to the local rootfs image.
 	-i)		Update the rootfs image with a newer version.
 	-d)		Update the output directory (default: ${OUTPUT_DIR})
 	-j)		Number of jobs for compilation, similar to -j in make
@@ -128,7 +132,11 @@ load_rootfs()
 		exit 1
 	fi
 
-	download_rootfs | zstd -d | sudo tar -C "$dir" -x
+	if [[ -n "${LOCAL_ROOTFS_IMAGE}" ]]; then
+		cat "${LOCAL_ROOTFS_IMAGE}" | zstd -d | sudo tar -C "$dir" -x
+	else
+		download_rootfs | zstd -d | sudo tar -C "$dir" -x
+	fi
 }
 
 recompile_kernel()
@@ -342,8 +350,11 @@ main()
 	local exit_command="poweroff -f"
 	local debug_shell="no"
 
-	while getopts ':hskid:j:' opt; do
+	while getopts ':hskl:id:j:' opt; do
 		case ${opt} in
+		l)
+			LOCAL_ROOTFS_IMAGE="$OPTARG"
+			;;
 		i)
 			update_image="yes"
 			;;