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Add a short note to make contributors aware of the existence of the script. The documentation does not intentionally document all the options of the script to avoid mentioning it in two places (it's available in the usage / help message of the script). Signed-off-by: KP Singh <kpsingh@kernel.org> Signed-off-by: Andrii Nakryiko <andrii@kernel.org> Link: https://lore.kernel.org/bpf/20210204194544.3383814-3-kpsingh@kernel.org
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==================
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BPF Selftest Notes
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==================
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General instructions on running selftests can be found in
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`Documentation/bpf/bpf_devel_QA.rst`__.
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__ /Documentation/bpf/bpf_devel_QA.rst#q-how-to-run-bpf-selftests
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=========================
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Running Selftests in a VM
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=========================
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It's now possible to run the selftests using ``tools/testing/selftests/bpf/vmtest.sh``.
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The script tries to ensure that the tests are run with the same environment as they
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would be run post-submit in the CI used by the Maintainers.
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This script downloads a suitable Kconfig and VM userspace image from the system used by
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the CI. It builds the kernel (without overwriting your existing Kconfig), recompiles the
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bpf selftests, runs them (by default ``tools/testing/selftests/bpf/test_progs``) and
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saves the resulting output (by default in ``~/.bpf_selftests``).
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For more information on about using the script, run:
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.. code-block:: console
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$ tools/testing/selftests/bpf/vmtest.sh -h
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.. note:: The script uses pahole and clang based on host environment setting.
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If you want to change pahole and llvm, you can change `PATH` environment
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variable in the beginning of script.
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.. note:: The script currently only supports x86_64.
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Additional information about selftest failures are
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documented here.
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profiler[23] test failures with clang/llvm <12.0.0
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==================================================
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With clang/llvm <12.0.0, the profiler[23] test may fail.
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The symptom looks like
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.. code-block:: c
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// r9 is a pointer to map_value
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// r7 is a scalar
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17: bf 96 00 00 00 00 00 00 r6 = r9
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18: 0f 76 00 00 00 00 00 00 r6 += r7
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math between map_value pointer and register with unbounded min value is not allowed
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// the instructions below will not be seen in the verifier log
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19: a5 07 01 00 01 01 00 00 if r7 < 257 goto +1
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20: bf 96 00 00 00 00 00 00 r6 = r9
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// r6 is used here
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The verifier will reject such code with above error.
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At insn 18 the r7 is indeed unbounded. The later insn 19 checks the bounds and
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the insn 20 undoes map_value addition. It is currently impossible for the
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verifier to understand such speculative pointer arithmetic.
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Hence `this patch`__ addresses it on the compiler side. It was committed on llvm 12.
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__ https://reviews.llvm.org/D85570
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The corresponding C code
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.. code-block:: c
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for (int i = 0; i < MAX_CGROUPS_PATH_DEPTH; i++) {
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filepart_length = bpf_probe_read_str(payload, ...);
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if (filepart_length <= MAX_PATH) {
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barrier_var(filepart_length); // workaround
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payload += filepart_length;
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}
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}
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bpf_iter test failures with clang/llvm 10.0.0
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=============================================
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With clang/llvm 10.0.0, the following two bpf_iter tests failed:
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* ``bpf_iter/ipv6_route``
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* ``bpf_iter/netlink``
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The symptom for ``bpf_iter/ipv6_route`` looks like
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.. code-block:: c
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2: (79) r8 = *(u64 *)(r1 +8)
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...
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14: (bf) r2 = r8
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15: (0f) r2 += r1
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; BPF_SEQ_PRINTF(seq, "%pi6 %02x ", &rt->fib6_dst.addr, rt->fib6_dst.plen);
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16: (7b) *(u64 *)(r8 +64) = r2
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only read is supported
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The symptom for ``bpf_iter/netlink`` looks like
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.. code-block:: c
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; struct netlink_sock *nlk = ctx->sk;
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2: (79) r7 = *(u64 *)(r1 +8)
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...
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15: (bf) r2 = r7
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16: (0f) r2 += r1
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; BPF_SEQ_PRINTF(seq, "%pK %-3d ", s, s->sk_protocol);
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17: (7b) *(u64 *)(r7 +0) = r2
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only read is supported
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This is due to a llvm BPF backend bug. `The fix`__
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has been pushed to llvm 10.x release branch and will be
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available in 10.0.1. The patch is available in llvm 11.0.0 trunk.
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__ https://reviews.llvm.org/D78466
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BPF CO-RE-based tests and Clang version
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=======================================
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A set of selftests use BPF target-specific built-ins, which might require
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bleeding-edge Clang versions (Clang 12 nightly at this time).
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Few sub-tests of core_reloc test suit (part of test_progs test runner) require
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the following built-ins, listed with corresponding Clang diffs introducing
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them to Clang/LLVM. These sub-tests are going to be skipped if Clang is too
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old to support them, they shouldn't cause build failures or runtime test
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failures:
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- __builtin_btf_type_id() [0_, 1_, 2_];
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- __builtin_preserve_type_info(), __builtin_preserve_enum_value() [3_, 4_].
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.. _0: https://reviews.llvm.org/D74572
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.. _1: https://reviews.llvm.org/D74668
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.. _2: https://reviews.llvm.org/D85174
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.. _3: https://reviews.llvm.org/D83878
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.. _4: https://reviews.llvm.org/D83242
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