linux/tools/testing/selftests/bpf
Toke Høiland-Jørgensen f506439ec3 selftests/bpf: Add a test for using a cpumap from an freplace-to-XDP program
This adds a simple test for inserting an XDP program into a cpumap that is
"owned" by an XDP program that was loaded as PROG_TYPE_EXT (as libxdp
does). Prior to the kernel fix this would fail because the map type
ownership would be set to PROG_TYPE_EXT instead of being resolved to
PROG_TYPE_XDP.

v5:
- Fix a few nits from Andrii, add his ACK
v4:
- Use skeletons for selftest
v3:
- Update comment to better explain the cause
- Add Yonghong's ACK

Acked-by: Yonghong Song <yhs@fb.com>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
Link: https://lore.kernel.org/r/20221214230254.790066-2-toke@redhat.com
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
2022-12-14 21:30:40 -08:00
..
benchs selftests/bpf: Add benchmark for local_storage RCU Tasks Trace usage 2022-07-07 16:35:21 +02:00
bpf_testmod selftests/bpf: Add bpf_testmod_fentry_* functions 2022-10-25 10:14:51 -07:00
gnu
map_tests bpf: Set and check spin lock value in sk_storage_map_test 2022-11-21 11:45:38 -08:00
prog_tests selftests/bpf: Add a test for using a cpumap from an freplace-to-XDP program 2022-12-14 21:30:40 -08:00
progs selftests/bpf: Add a test for using a cpumap from an freplace-to-XDP program 2022-12-14 21:30:40 -08:00
verifier selftests/bpf: test case for relaxed prunning of active_lock.id 2022-12-10 13:36:22 -08:00
.gitignore selftests/bpf: add sign-file to .gitignore 2022-09-23 18:14:44 -07:00
bench.c selftests/bpf: Add benchmark for local_storage RCU Tasks Trace usage 2022-07-07 16:35:21 +02:00
bench.h selftests/bpf: Add benchmark for local_storage RCU Tasks Trace usage 2022-07-07 16:35:21 +02:00
bpf_experimental.h selftests/bpf: Add __contains macro to bpf_experimental.h 2022-11-17 19:22:14 -08:00
bpf_legacy.h selftests/bpf: Add GCC compatible builtins to bpf_legacy.h 2022-12-02 16:22:28 -08:00
bpf_rand.h
bpf_sockopt_helpers.h
bpf_tcp_helpers.h selftests/bpf: Verify first of struct mptcp_sock 2022-05-20 15:36:48 -07:00
bpf_util.h selftests/bpf: cgroup_helpers.c: Fix strncpy() fortify warning 2022-11-04 15:46:40 -07:00
btf_helpers.c selftests/bpf: Test new enum kflag and enum64 API functions 2022-06-07 10:20:43 -07:00
btf_helpers.h
cap_helpers.c bpf: selftests: Add helpers to directly use the capget and capset syscall 2022-03-16 15:07:49 -07:00
cap_helpers.h bpf: selftests: Add helpers to directly use the capget and capset syscall 2022-03-16 15:07:49 -07:00
cgroup_getset_retval_hooks.h selftests/bpf: Make sure bpf_{g,s}et_retval is exposed everywhere 2022-08-23 16:08:22 -07:00
cgroup_helpers.c selftests/bpf: Add cgroup helper remove_cgroup() 2022-11-21 17:40:42 +01:00
cgroup_helpers.h selftests/bpf: Add cgroup helper remove_cgroup() 2022-11-21 17:40:42 +01:00
config selftests/bpf: Select CONFIG_FUNCTION_ERROR_INJECTION 2022-12-14 18:35:41 +01:00
config.aarch64 selftests/bpf: Add config.aarch64 2022-10-21 16:27:25 -07:00
config.s390x selftests/bpf: Remove entries from config.s390x already present in config 2022-10-21 16:27:25 -07:00
config.x86_64 selftests/bpf: Panic on hard/soft lockup 2022-10-25 23:42:03 -07:00
DENYLIST selftests/bpf: Alphabetize DENYLISTs 2022-10-11 22:55:28 -07:00
DENYLIST.aarch64 selftests/bpf: Add bench test to arm64 and s390x denylist 2022-12-01 00:07:30 +01:00
DENYLIST.s390x selftests/bpf: add xfrm_info tests 2022-12-05 22:54:23 -08:00
flow_dissector_load.c selftests/bpf: Use libbpf 1.0 API mode instead of RLIMIT_MEMLOCK 2022-04-10 20:17:16 -07:00
flow_dissector_load.h selftests/bpf: Use explicit bpf_prog_test_load() calls everywhere 2021-11-07 08:34:23 -08:00
get_cgroup_id_user.c selftests/bpf: Store BPF object files with .bpf.o extension 2022-09-02 15:55:37 +02:00
ima_setup.sh selftests/bpf: Add test for bpf_lsm_kernel_read_file() 2022-03-10 18:57:55 -08:00
Makefile selftests/bpf: add generic BPF program tester-loader 2022-12-07 17:01:21 -08:00
Makefile.docs
netcnt_common.h bpf: Increase supported cgroup storage value size 2021-07-27 15:59:29 -07:00
network_helpers.c selftests/bpf: Remove the "/sys" mount and umount dance in {open,close}_netns 2022-11-30 22:47:43 +01:00
network_helpers.h selftests/bpf: Add MPTCP test base 2022-05-20 15:33:23 -07:00
README.rst bpf/docs: Summarize CI system and deny lists 2022-10-19 11:42:01 -07:00
sdt-config.h selftests/bpf: Add basic USDT selftests 2022-04-05 13:16:08 -07:00
sdt.h selftests/bpf: Add basic USDT selftests 2022-04-05 13:16:08 -07:00
settings
task_local_storage_helpers.h selftests/bpf: Use sys_pidfd_open() helper when possible 2022-10-13 12:09:19 -07:00
test_bpftool_build.sh selftests/bpf: Better clean up for runqslower in test_bpftool_build.sh 2021-10-08 12:02:38 -07:00
test_bpftool_metadata.sh selftests/bpf: fix missing BPF object files 2022-10-20 19:05:50 -07:00
test_bpftool_synctypes.py bpftool: remove support of --legacy option for bpftool 2022-11-20 16:17:46 -08:00
test_bpftool.py bpftool: Only probe trace_vprintk feature in 'full' mode 2021-09-17 14:02:05 -07:00
test_bpftool.sh selftests/bpf: Add missing files required by test_bpftool.sh for installing 2021-08-24 14:01:10 -07:00
test_btf.h selftests/bpf: allow BTF specs and func infos in test_verifier tests 2022-06-20 17:40:51 -07:00
test_cgroup_storage.c selftests/bpf: Use libbpf 1.0 API mode instead of RLIMIT_MEMLOCK 2022-04-10 20:17:16 -07:00
test_cpp.cpp selftests/bpf: Make sure enum-less bpf_enable_stats() API works in C++ mode 2022-11-30 22:56:47 +01:00
test_dev_cgroup.c selftests/bpf: Store BPF object files with .bpf.o extension 2022-09-02 15:55:37 +02:00
test_doc_build.sh selftests/bpf: Exit with KSFT_SKIP if no Makefile found 2021-08-24 14:01:10 -07:00
test_flow_dissector.c
test_flow_dissector.sh selftests/bpf: fix missing BPF object files 2022-10-20 19:05:50 -07:00
test_ftrace.sh
test_iptunnel_common.h
test_kmod.sh selftests/bpf: Fix passing arguments via function in test_kmod.sh 2022-09-27 15:57:54 +02:00
test_lirc_mode2_user.c selftests/bpf: Store BPF object files with .bpf.o extension 2022-09-02 15:55:37 +02:00
test_lirc_mode2.sh selftests/bpf/test_lirc_mode2.sh: Exit with proper code 2022-03-21 14:48:06 +01:00
test_loader.c selftests/bpf: add generic BPF program tester-loader 2022-12-07 17:01:21 -08:00
test_lpm_map.c selftests/bpf: Fix map tests errno checks 2022-04-21 09:51:57 -07:00
test_lru_map.c selftests/bpf: Fix map tests errno checks 2022-04-21 09:51:57 -07:00
test_lwt_ip_encap.sh selftests/bpf: fix missing BPF object files 2022-10-20 19:05:50 -07:00
test_lwt_seg6local.sh selftests/bpf: fix missing BPF object files 2022-10-20 19:05:50 -07:00
test_maps.c selftests/bpf: Free the allocated resources after test case succeeds 2022-09-22 16:41:27 -07:00
test_maps.h selftests/bpf: Add test result messages for test_task_storage_map_stress_lookup 2022-09-19 11:17:38 -07:00
test_offload.py selftests/bpf: Bring test_offload.py back to life 2022-12-08 15:25:59 -08:00
test_progs.c Merge git://git.kernel.org/pub/scm/linux/kernel/git/netdev/net 2022-11-17 18:30:39 -08:00
test_progs.h selftests/bpf: add generic BPF program tester-loader 2022-12-07 17:01:21 -08:00
test_select_reuseport_common.h
test_skb_cgroup_id_user.c selftests/bpf: Use libbpf 1.0 API mode instead of RLIMIT_MEMLOCK 2022-04-10 20:17:16 -07:00
test_skb_cgroup_id.sh selftests/bpf: Store BPF object files with .bpf.o extension 2022-09-02 15:55:37 +02:00
test_sock_addr.c selftests/bpf: Store BPF object files with .bpf.o extension 2022-09-02 15:55:37 +02:00
test_sock_addr.sh
test_sock.c selftests/bpf: Fix attach tests retcode checks 2022-04-21 16:34:55 +02:00
test_sockmap.c selftests/bpf: Add ingress tests for txmsg with apply_bytes 2022-12-01 01:07:41 +01:00
test_sysctl.c selftests/bpf: Store BPF object files with .bpf.o extension 2022-09-02 15:55:37 +02:00
test_tag.c selftests/bpf: Use libbpf 1.0 API mode instead of RLIMIT_MEMLOCK 2022-04-10 20:17:16 -07:00
test_tc_edt.sh selftests/bpf: fix missing BPF object files 2022-10-20 19:05:50 -07:00
test_tc_tunnel.sh selftests/bpf: fix missing BPF object files 2022-10-20 19:05:50 -07:00
test_tcp_check_syncookie_user.c selftests/bpf: Use libbpf 1.0 API mode instead of RLIMIT_MEMLOCK 2022-04-10 20:17:16 -07:00
test_tcp_check_syncookie.sh selftests/bpf: Store BPF object files with .bpf.o extension 2022-09-02 15:55:37 +02:00
test_tcp_hdr_options.h
test_tcpbpf.h
test_tcpnotify_user.c selftests/bpf: Store BPF object files with .bpf.o extension 2022-09-02 15:55:37 +02:00
test_tcpnotify.h
test_tunnel.sh selftests/bpf: fix missing BPF object files 2022-10-20 19:05:50 -07:00
test_verifier_log.c selftests/bpf: Use libbpf 1.0 API mode instead of RLIMIT_MEMLOCK 2022-04-10 20:17:16 -07:00
test_verifier.c Merge git://git.kernel.org/pub/scm/linux/kernel/git/netdev/net 2022-11-17 18:30:39 -08:00
test_xdp_meta.sh selftests/bpf: fix missing BPF object files 2022-10-20 19:05:50 -07:00
test_xdp_redirect_multi.sh selftests/bpf: Store BPF object files with .bpf.o extension 2022-09-02 15:55:37 +02:00
test_xdp_redirect.sh selftests/bpf: Store BPF object files with .bpf.o extension 2022-09-02 15:55:37 +02:00
test_xdp_veth.sh selftests/bpf: Store BPF object files with .bpf.o extension 2022-09-02 15:55:37 +02:00
test_xdp_vlan_mode_generic.sh
test_xdp_vlan_mode_native.sh
test_xdp_vlan.sh selftests/bpf: fix missing BPF object files 2022-10-20 19:05:50 -07:00
test_xdping.sh selftests/bpf: Add drv mode testing for xdping 2022-06-03 14:53:34 -07:00
test_xsk.sh selftests/xsk: Add support for executing tests on physical device 2022-09-02 15:37:57 +02:00
testing_helpers.c selftests/bpf: Improve by-name subtest selection logic in prog_tests 2022-04-10 20:08:20 -07:00
testing_helpers.h selftests/bpf: Improve by-name subtest selection logic in prog_tests 2022-04-10 20:08:20 -07:00
trace_helpers.c selftests/bpf: Add load_kallsyms_refresh function 2022-10-25 10:14:51 -07:00
trace_helpers.h selftests/bpf: Add load_kallsyms_refresh function 2022-10-25 10:14:51 -07:00
urandom_read_aux.c selftests/bpf: Add urandom_read shared lib and USDTs 2022-04-05 13:16:08 -07:00
urandom_read_lib1.c selftests/bpf: Add urandom_read shared lib and USDTs 2022-04-05 13:16:08 -07:00
urandom_read_lib2.c selftests/bpf: Add urandom_read shared lib and USDTs 2022-04-05 13:16:08 -07:00
urandom_read.c selftests/bpf: Add urandom_read shared lib and USDTs 2022-04-05 13:16:08 -07:00
verify_sig_setup.sh selftests/bpf: Add test for bpf_verify_pkcs7_signature() kfunc 2022-09-21 17:33:42 -07:00
veristat.c selftests/bpf: fix veristat's singular file-or-prog filter 2022-11-11 14:06:20 -08:00
veristat.cfg selftests/bpf: add ability to filter programs in veristat 2022-09-21 19:41:37 -07:00
vmtest.sh selftests/bpf: Update vmtests.sh to support aarch64 2022-10-21 16:27:25 -07:00
with_addr.sh
with_tunnels.sh
xdp_redirect_multi.c selftests/bpf: Store BPF object files with .bpf.o extension 2022-09-02 15:55:37 +02:00
xdp_synproxy.c selftests/bpf: Fix xdp_synproxy compilation failure in 32-bit arch 2022-11-11 12:22:21 -08:00
xdping.c selftests/bpf: Store BPF object files with .bpf.o extension 2022-09-02 15:55:37 +02:00
xdping.h
xsk_prereqs.sh selftests, xsk: Rename AF_XDP testing app 2022-07-08 14:22:15 +02:00
xsk.c selftests/bpf: cgroup_helpers.c: Fix strncpy() fortify warning 2022-11-04 15:46:40 -07:00
xsk.h selftests/xsk: Introduce XDP prog load based on existing AF_XDP socket 2022-06-30 22:49:05 +02:00
xskxceiver.c selftests/bpf: Fix u32 variable compared with less than zero 2022-11-07 16:53:50 -08:00
xskxceiver.h selftests/xsk: Add support for zero copy testing 2022-09-02 15:38:08 +02:00

==================
BPF Selftest Notes
==================
General instructions on running selftests can be found in
`Documentation/bpf/bpf_devel_QA.rst`__.

__ /Documentation/bpf/bpf_devel_QA.rst#q-how-to-run-bpf-selftests

=============
BPF CI System
=============

BPF employs a continuous integration (CI) system to check patch submission in an
automated fashion. The system runs selftests for each patch in a series. Results
are propagated to patchwork, where failures are highlighted similar to
violations of other checks (such as additional warnings being emitted or a
``scripts/checkpatch.pl`` reported deficiency):

  https://patchwork.kernel.org/project/netdevbpf/list/?delegate=121173

The CI system executes tests on multiple architectures. It uses a kernel
configuration derived from both the generic and architecture specific config
file fragments below ``tools/testing/selftests/bpf/`` (e.g., ``config`` and
``config.x86_64``).

Denylisting Tests
=================

It is possible for some architectures to not have support for all BPF features.
In such a case tests in CI may fail. An example of such a shortcoming is BPF
trampoline support on IBM's s390x architecture. For cases like this, an in-tree
deny list file, located at ``tools/testing/selftests/bpf/DENYLIST.<arch>``, can
be used to prevent the test from running on such an architecture.

In addition to that, the generic ``tools/testing/selftests/bpf/DENYLIST`` is
honored on every architecture running tests.

These files are organized in three columns. The first column lists the test in
question. This can be the name of a test suite or of an individual test. The
remaining two columns provide additional meta data that helps identify and
classify the entry: column two is a copy and paste of the error being reported
when running the test in the setting in question. The third column, if
available, summarizes the underlying problem. A value of ``trampoline``, for
example, indicates that lack of trampoline support is causing the test to fail.
This last entry helps identify tests that can be re-enabled once such support is
added.

=========================
Running Selftests in a VM
=========================

It's now possible to run the selftests using ``tools/testing/selftests/bpf/vmtest.sh``.
The script tries to ensure that the tests are run with the same environment as they
would be run post-submit in the CI used by the Maintainers, with the exception
that deny lists are not automatically honored.

This script uses the in-tree kernel configuration and downloads a VM userspace
image from the system used by the CI. It builds the kernel (without overwriting
your existing Kconfig), recompiles the bpf selftests, runs them (by default
``tools/testing/selftests/bpf/test_progs``) and saves the resulting output (by
default in ``~/.bpf_selftests``).

Script dependencies:
- clang (preferably built from sources, https://github.com/llvm/llvm-project);
- pahole (preferably built from sources, https://git.kernel.org/pub/scm/devel/pahole/pahole.git/);
- qemu;
- docutils (for ``rst2man``);
- libcap-devel.

For more information about using the script, run:

.. code-block:: console

  $ tools/testing/selftests/bpf/vmtest.sh -h

In case of linker errors when running selftests, try using static linking:

.. code-block:: console

  $ LDLIBS=-static vmtest.sh

.. note:: Some distros may not support static linking.

.. note:: The script uses pahole and clang based on host environment setting.
          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.

profiler[23] test failures with clang/llvm <12.0.0
==================================================

With clang/llvm <12.0.0, the profiler[23] test may fail.
The symptom looks like

.. code-block:: c

  // r9 is a pointer to map_value
  // r7 is a scalar
  17:       bf 96 00 00 00 00 00 00 r6 = r9
  18:       0f 76 00 00 00 00 00 00 r6 += r7
  math between map_value pointer and register with unbounded min value is not allowed

  // the instructions below will not be seen in the verifier log
  19:       a5 07 01 00 01 01 00 00 if r7 < 257 goto +1
  20:       bf 96 00 00 00 00 00 00 r6 = r9
  // r6 is used here

The verifier will reject such code with above error.
At insn 18 the r7 is indeed unbounded. The later insn 19 checks the bounds and
the insn 20 undoes map_value addition. It is currently impossible for the
verifier to understand such speculative pointer arithmetic.
Hence `this patch`__ addresses it on the compiler side. It was committed on llvm 12.

__ https://reviews.llvm.org/D85570

The corresponding C code

.. code-block:: c

  for (int i = 0; i < MAX_CGROUPS_PATH_DEPTH; i++) {
          filepart_length = bpf_probe_read_str(payload, ...);
          if (filepart_length <= MAX_PATH) {
                  barrier_var(filepart_length); // workaround
                  payload += filepart_length;
          }
  }

bpf_iter test failures with clang/llvm 10.0.0
=============================================

With clang/llvm 10.0.0, the following two bpf_iter tests failed:
  * ``bpf_iter/ipv6_route``
  * ``bpf_iter/netlink``

The symptom for ``bpf_iter/ipv6_route`` looks like

.. code-block:: c

  2: (79) r8 = *(u64 *)(r1 +8)
  ...
  14: (bf) r2 = r8
  15: (0f) r2 += r1
  ; BPF_SEQ_PRINTF(seq, "%pi6 %02x ", &rt->fib6_dst.addr, rt->fib6_dst.plen);
  16: (7b) *(u64 *)(r8 +64) = r2
  only read is supported

The symptom for ``bpf_iter/netlink`` looks like

.. code-block:: c

  ; struct netlink_sock *nlk = ctx->sk;
  2: (79) r7 = *(u64 *)(r1 +8)
  ...
  15: (bf) r2 = r7
  16: (0f) r2 += r1
  ; BPF_SEQ_PRINTF(seq, "%pK %-3d ", s, s->sk_protocol);
  17: (7b) *(u64 *)(r7 +0) = r2
  only read is supported

This is due to a llvm BPF backend bug. `The fix`__
has been pushed to llvm 10.x release branch and will be
available in 10.0.1. The patch is available in llvm 11.0.0 trunk.

__  https://reviews.llvm.org/D78466

bpf_verif_scale/loop6.bpf.o test failure with Clang 12
======================================================

With Clang 12, the following bpf_verif_scale test failed:
  * ``bpf_verif_scale/loop6.bpf.o``

The verifier output looks like

.. code-block:: c

  R1 type=ctx expected=fp
  The sequence of 8193 jumps is too complex.

The reason is compiler generating the following code

.. code-block:: c

  ;       for (i = 0; (i < VIRTIO_MAX_SGS) && (i < num); i++) {
      14:       16 05 40 00 00 00 00 00 if w5 == 0 goto +64 <LBB0_6>
      15:       bc 51 00 00 00 00 00 00 w1 = w5
      16:       04 01 00 00 ff ff ff ff w1 += -1
      17:       67 05 00 00 20 00 00 00 r5 <<= 32
      18:       77 05 00 00 20 00 00 00 r5 >>= 32
      19:       a6 01 01 00 05 00 00 00 if w1 < 5 goto +1 <LBB0_4>
      20:       b7 05 00 00 06 00 00 00 r5 = 6
  00000000000000a8 <LBB0_4>:
      21:       b7 02 00 00 00 00 00 00 r2 = 0
      22:       b7 01 00 00 00 00 00 00 r1 = 0
  ;       for (i = 0; (i < VIRTIO_MAX_SGS) && (i < num); i++) {
      23:       7b 1a e0 ff 00 00 00 00 *(u64 *)(r10 - 32) = r1
      24:       7b 5a c0 ff 00 00 00 00 *(u64 *)(r10 - 64) = r5

Note that insn #15 has w1 = w5 and w1 is refined later but
r5(w5) is eventually saved on stack at insn #24 for later use.
This cause later verifier failure. The bug has been `fixed`__ in
Clang 13.

__  https://reviews.llvm.org/D97479

BPF CO-RE-based tests and Clang version
=======================================

A set of selftests use BPF target-specific built-ins, which might require
bleeding-edge Clang versions (Clang 12 nightly at this time).

Few sub-tests of core_reloc test suit (part of test_progs test runner) require
the following built-ins, listed with corresponding Clang diffs introducing
them to Clang/LLVM. These sub-tests are going to be skipped if Clang is too
old to support them, they shouldn't cause build failures or runtime test
failures:

- __builtin_btf_type_id() [0_, 1_, 2_];
- __builtin_preserve_type_info(), __builtin_preserve_enum_value() [3_, 4_].

.. _0: https://reviews.llvm.org/D74572
.. _1: https://reviews.llvm.org/D74668
.. _2: https://reviews.llvm.org/D85174
.. _3: https://reviews.llvm.org/D83878
.. _4: https://reviews.llvm.org/D83242

Floating-point tests and Clang version
======================================

Certain selftests, e.g. core_reloc, require support for the floating-point
types, which was introduced in `Clang 13`__. The older Clang versions will
either crash when compiling these tests, or generate an incorrect BTF.

__  https://reviews.llvm.org/D83289

Kernel function call test and Clang version
===========================================

Some selftests (e.g. kfunc_call and bpf_tcp_ca) require a LLVM support
to generate extern function in BTF.  It was introduced in `Clang 13`__.

Without it, the error from compiling bpf selftests looks like:

.. code-block:: console

  libbpf: failed to find BTF for extern 'tcp_slow_start' [25] section: -2

__ https://reviews.llvm.org/D93563

btf_tag test and Clang version
==============================

The btf_tag selftest requires LLVM support to recognize the btf_decl_tag and
btf_type_tag attributes. They are introduced in `Clang 14` [0_, 1_].
The subtests ``btf_type_tag_user_{mod1, mod2, vmlinux}`` also requires
pahole version ``1.23``.

Without them, the btf_tag selftest will be skipped and you will observe:

.. code-block:: console

  #<test_num> btf_tag:SKIP

.. _0: https://reviews.llvm.org/D111588
.. _1: https://reviews.llvm.org/D111199

Clang dependencies for static linking tests
===========================================

linked_vars, linked_maps, and linked_funcs tests depend on `Clang fix`__ to
generate valid BTF information for weak variables. Please make sure you use
Clang that contains the fix.

__ https://reviews.llvm.org/D100362

Clang relocation changes
========================

Clang 13 patch `clang reloc patch`_  made some changes on relocations such
that existing relocation types are broken into more types and
each new type corresponds to only one way to resolve relocation.
See `kernel llvm reloc`_ for more explanation and some examples.
Using clang 13 to compile old libbpf which has static linker support,
there will be a compilation failure::

  libbpf: ELF relo #0 in section #6 has unexpected type 2 in .../bpf_tcp_nogpl.bpf.o

Here, ``type 2`` refers to new relocation type ``R_BPF_64_ABS64``.
To fix this issue, user newer libbpf.

.. Links
.. _clang reloc patch: https://reviews.llvm.org/D102712
.. _kernel llvm reloc: /Documentation/bpf/llvm_reloc.rst

Clang dependencies for the u32 spill test (xdpwall)
===================================================
The xdpwall selftest requires a change in `Clang 14`__.

Without it, the xdpwall selftest will fail and the error message
from running test_progs will look like:

.. code-block:: console

  test_xdpwall:FAIL:Does LLVM have https://reviews.llvm.org/D109073? unexpected error: -4007

__ https://reviews.llvm.org/D109073