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Document which fixes are required to generate correct static linking selftests. Signed-off-by: Andrii Nakryiko <andrii@kernel.org> Signed-off-by: Alexei Starovoitov <ast@kernel.org> Acked-by: Yonghong Song <yhs@fb.com> Link: https://lore.kernel.org/bpf/20210423181348.1801389-19-andrii@kernel.org
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205 lines
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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_verif_scale/loop6.o test failure with Clang 12
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==================================================
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With Clang 12, the following bpf_verif_scale test failed:
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* ``bpf_verif_scale/loop6.o``
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The verifier output looks like
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.. code-block:: c
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R1 type=ctx expected=fp
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The sequence of 8193 jumps is too complex.
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The reason is compiler generating the following code
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.. code-block:: c
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; for (i = 0; (i < VIRTIO_MAX_SGS) && (i < num); i++) {
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14: 16 05 40 00 00 00 00 00 if w5 == 0 goto +64 <LBB0_6>
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15: bc 51 00 00 00 00 00 00 w1 = w5
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16: 04 01 00 00 ff ff ff ff w1 += -1
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17: 67 05 00 00 20 00 00 00 r5 <<= 32
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18: 77 05 00 00 20 00 00 00 r5 >>= 32
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19: a6 01 01 00 05 00 00 00 if w1 < 5 goto +1 <LBB0_4>
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20: b7 05 00 00 06 00 00 00 r5 = 6
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00000000000000a8 <LBB0_4>:
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21: b7 02 00 00 00 00 00 00 r2 = 0
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22: b7 01 00 00 00 00 00 00 r1 = 0
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; for (i = 0; (i < VIRTIO_MAX_SGS) && (i < num); i++) {
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23: 7b 1a e0 ff 00 00 00 00 *(u64 *)(r10 - 32) = r1
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24: 7b 5a c0 ff 00 00 00 00 *(u64 *)(r10 - 64) = r5
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Note that insn #15 has w1 = w5 and w1 is refined later but
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r5(w5) is eventually saved on stack at insn #24 for later use.
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This cause later verifier failure. The bug has been `fixed`__ in
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Clang 13.
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__ https://reviews.llvm.org/D97479
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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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Floating-point tests and Clang version
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======================================
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Certain selftests, e.g. core_reloc, require support for the floating-point
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types, which was introduced in `Clang 13`__. The older Clang versions will
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either crash when compiling these tests, or generate an incorrect BTF.
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__ https://reviews.llvm.org/D83289
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Kernel function call test and Clang version
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===========================================
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Some selftests (e.g. kfunc_call and bpf_tcp_ca) require a LLVM support
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to generate extern function in BTF. It was introduced in `Clang 13`__.
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Without it, the error from compiling bpf selftests looks like:
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.. code-block:: console
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libbpf: failed to find BTF for extern 'tcp_slow_start' [25] section: -2
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__ https://reviews.llvm.org/D93563
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Clang dependencies for static linking tests
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===========================================
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linked_vars, linked_maps, and linked_funcs tests depend on `Clang fix`__ to
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generate valid BTF information for weak variables. Please make sure you use
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Clang that contains the fix.
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__ https://reviews.llvm.org/D100362
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