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linux-stable/tools/testing/selftests/bpf/README.rst
Yonghong Song 223f903e9c bpf: Rename BTF_KIND_TAG to BTF_KIND_DECL_TAG 
Patch set [1] introduced BTF_KIND_TAG to allow tagging
declarations for struct/union, struct/union field, var, func
and func arguments and these tags will be encoded into
dwarf. They are also encoded to btf by llvm for the bpf target.

After BTF_KIND_TAG is introduced, we intended to use it
for kernel __user attributes. But kernel __user is actually
a type attribute. Upstream and internal discussion showed
it is not a good idea to mix declaration attribute and
type attribute. So we proposed to introduce btf_type_tag
as a type attribute and existing btf_tag renamed to
btf_decl_tag ([2]).

This patch renamed BTF_KIND_TAG to BTF_KIND_DECL_TAG and some
other declarations with *_tag to *_decl_tag to make it clear
the tag is for declaration. In the future, BTF_KIND_TYPE_TAG
might be introduced per [3].

 [1] https://lore.kernel.org/bpf/20210914223004.244411-1-yhs@fb.com/
 [2] https://reviews.llvm.org/D111588
 [3] https://reviews.llvm.org/D111199

Fixes: b5ea834dde ("bpf: Support for new btf kind BTF_KIND_TAG")
Fixes: 5b84bd1036 ("libbpf: Add support for BTF_KIND_TAG")
Fixes: 5c07f2fec0 ("bpftool: Add support for BTF_KIND_TAG")
Signed-off-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20211012164838.3345699-1-yhs@fb.com
2021-10-18 18:35:36 -07:00

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==================
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
=========================
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.
This script downloads a suitable Kconfig and 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 on about using the script, run:
.. code-block:: console
$ tools/testing/selftests/bpf/vmtest.sh -h
.. 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.
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.o test failure with Clang 12
==================================================
With Clang 12, the following bpf_verif_scale test failed:
* ``bpf_verif_scale/loop6.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 require LLVM support to recognize the btf_decl_tag attribute.
It was introduced in `Clang 14`__.
Without it, the btf_tag selftest will be skipped and you will observe:
.. code-block:: console
#<test_num> btf_tag:SKIP
__ https://reviews.llvm.org/D111588
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.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
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