1================== 2BPF Selftest Notes 3================== 4General instructions on running selftests can be found in 5`Documentation/bpf/bpf_devel_QA.rst`__. 6 7__ /Documentation/bpf/bpf_devel_QA.rst#q-how-to-run-bpf-selftests 8 9========================= 10Running Selftests in a VM 11========================= 12 13It's now possible to run the selftests using ``tools/testing/selftests/bpf/vmtest.sh``. 14The script tries to ensure that the tests are run with the same environment as they 15would be run post-submit in the CI used by the Maintainers. 16 17This script downloads a suitable Kconfig and VM userspace image from the system used by 18the CI. It builds the kernel (without overwriting your existing Kconfig), recompiles the 19bpf selftests, runs them (by default ``tools/testing/selftests/bpf/test_progs``) and 20saves the resulting output (by default in ``~/.bpf_selftests``). 21 22For more information on about using the script, run: 23 24.. code-block:: console 25 26 $ tools/testing/selftests/bpf/vmtest.sh -h 27 28.. note:: The script uses pahole and clang based on host environment setting. 29 If you want to change pahole and llvm, you can change `PATH` environment 30 variable in the beginning of script. 31 32.. note:: The script currently only supports x86_64. 33 34Additional information about selftest failures are 35documented here. 36 37profiler[23] test failures with clang/llvm <12.0.0 38================================================== 39 40With clang/llvm <12.0.0, the profiler[23] test may fail. 41The symptom looks like 42 43.. code-block:: c 44 45 // r9 is a pointer to map_value 46 // r7 is a scalar 47 17: bf 96 00 00 00 00 00 00 r6 = r9 48 18: 0f 76 00 00 00 00 00 00 r6 += r7 49 math between map_value pointer and register with unbounded min value is not allowed 50 51 // the instructions below will not be seen in the verifier log 52 19: a5 07 01 00 01 01 00 00 if r7 < 257 goto +1 53 20: bf 96 00 00 00 00 00 00 r6 = r9 54 // r6 is used here 55 56The verifier will reject such code with above error. 57At insn 18 the r7 is indeed unbounded. The later insn 19 checks the bounds and 58the insn 20 undoes map_value addition. It is currently impossible for the 59verifier to understand such speculative pointer arithmetic. 60Hence `this patch`__ addresses it on the compiler side. It was committed on llvm 12. 61 62__ https://reviews.llvm.org/D85570 63 64The corresponding C code 65 66.. code-block:: c 67 68 for (int i = 0; i < MAX_CGROUPS_PATH_DEPTH; i++) { 69 filepart_length = bpf_probe_read_str(payload, ...); 70 if (filepart_length <= MAX_PATH) { 71 barrier_var(filepart_length); // workaround 72 payload += filepart_length; 73 } 74 } 75 76bpf_iter test failures with clang/llvm 10.0.0 77============================================= 78 79With clang/llvm 10.0.0, the following two bpf_iter tests failed: 80 * ``bpf_iter/ipv6_route`` 81 * ``bpf_iter/netlink`` 82 83The symptom for ``bpf_iter/ipv6_route`` looks like 84 85.. code-block:: c 86 87 2: (79) r8 = *(u64 *)(r1 +8) 88 ... 89 14: (bf) r2 = r8 90 15: (0f) r2 += r1 91 ; BPF_SEQ_PRINTF(seq, "%pi6 %02x ", &rt->fib6_dst.addr, rt->fib6_dst.plen); 92 16: (7b) *(u64 *)(r8 +64) = r2 93 only read is supported 94 95The symptom for ``bpf_iter/netlink`` looks like 96 97.. code-block:: c 98 99 ; struct netlink_sock *nlk = ctx->sk; 100 2: (79) r7 = *(u64 *)(r1 +8) 101 ... 102 15: (bf) r2 = r7 103 16: (0f) r2 += r1 104 ; BPF_SEQ_PRINTF(seq, "%pK %-3d ", s, s->sk_protocol); 105 17: (7b) *(u64 *)(r7 +0) = r2 106 only read is supported 107 108This is due to a llvm BPF backend bug. `The fix`__ 109has been pushed to llvm 10.x release branch and will be 110available in 10.0.1. The patch is available in llvm 11.0.0 trunk. 111 112__ https://reviews.llvm.org/D78466 113 114bpf_verif_scale/loop6.o test failure with Clang 12 115================================================== 116 117With Clang 12, the following bpf_verif_scale test failed: 118 * ``bpf_verif_scale/loop6.o`` 119 120The verifier output looks like 121 122.. code-block:: c 123 124 R1 type=ctx expected=fp 125 The sequence of 8193 jumps is too complex. 126 127The reason is compiler generating the following code 128 129.. code-block:: c 130 131 ; for (i = 0; (i < VIRTIO_MAX_SGS) && (i < num); i++) { 132 14: 16 05 40 00 00 00 00 00 if w5 == 0 goto +64 <LBB0_6> 133 15: bc 51 00 00 00 00 00 00 w1 = w5 134 16: 04 01 00 00 ff ff ff ff w1 += -1 135 17: 67 05 00 00 20 00 00 00 r5 <<= 32 136 18: 77 05 00 00 20 00 00 00 r5 >>= 32 137 19: a6 01 01 00 05 00 00 00 if w1 < 5 goto +1 <LBB0_4> 138 20: b7 05 00 00 06 00 00 00 r5 = 6 139 00000000000000a8 <LBB0_4>: 140 21: b7 02 00 00 00 00 00 00 r2 = 0 141 22: b7 01 00 00 00 00 00 00 r1 = 0 142 ; for (i = 0; (i < VIRTIO_MAX_SGS) && (i < num); i++) { 143 23: 7b 1a e0 ff 00 00 00 00 *(u64 *)(r10 - 32) = r1 144 24: 7b 5a c0 ff 00 00 00 00 *(u64 *)(r10 - 64) = r5 145 146Note that insn #15 has w1 = w5 and w1 is refined later but 147r5(w5) is eventually saved on stack at insn #24 for later use. 148This cause later verifier failure. The bug has been `fixed`__ in 149Clang 13. 150 151__ https://reviews.llvm.org/D97479 152 153BPF CO-RE-based tests and Clang version 154======================================= 155 156A set of selftests use BPF target-specific built-ins, which might require 157bleeding-edge Clang versions (Clang 12 nightly at this time). 158 159Few sub-tests of core_reloc test suit (part of test_progs test runner) require 160the following built-ins, listed with corresponding Clang diffs introducing 161them to Clang/LLVM. These sub-tests are going to be skipped if Clang is too 162old to support them, they shouldn't cause build failures or runtime test 163failures: 164 165- __builtin_btf_type_id() [0_, 1_, 2_]; 166- __builtin_preserve_type_info(), __builtin_preserve_enum_value() [3_, 4_]. 167 168.. _0: https://reviews.llvm.org/D74572 169.. _1: https://reviews.llvm.org/D74668 170.. _2: https://reviews.llvm.org/D85174 171.. _3: https://reviews.llvm.org/D83878 172.. _4: https://reviews.llvm.org/D83242 173 174Floating-point tests and Clang version 175====================================== 176 177Certain selftests, e.g. core_reloc, require support for the floating-point 178types, which was introduced in `Clang 13`__. The older Clang versions will 179either crash when compiling these tests, or generate an incorrect BTF. 180 181__ https://reviews.llvm.org/D83289 182 183Kernel function call test and Clang version 184=========================================== 185 186Some selftests (e.g. kfunc_call and bpf_tcp_ca) require a LLVM support 187to generate extern function in BTF. It was introduced in `Clang 13`__. 188 189Without it, the error from compiling bpf selftests looks like: 190 191.. code-block:: console 192 193 libbpf: failed to find BTF for extern 'tcp_slow_start' [25] section: -2 194 195__ https://reviews.llvm.org/D93563 196 197Clang dependencies for static linking tests 198=========================================== 199 200linked_vars, linked_maps, and linked_funcs tests depend on `Clang fix`__ to 201generate valid BTF information for weak variables. Please make sure you use 202Clang that contains the fix. 203 204__ https://reviews.llvm.org/D100362 205 206Clang relocation changes 207======================== 208 209Clang 13 patch `clang reloc patch`_ made some changes on relocations such 210that existing relocation types are broken into more types and 211each new type corresponds to only one way to resolve relocation. 212See `kernel llvm reloc`_ for more explanation and some examples. 213Using clang 13 to compile old libbpf which has static linker support, 214there will be a compilation failure:: 215 216 libbpf: ELF relo #0 in section #6 has unexpected type 2 in .../bpf_tcp_nogpl.o 217 218Here, ``type 2`` refers to new relocation type ``R_BPF_64_ABS64``. 219To fix this issue, user newer libbpf. 220 221.. Links 222.. _clang reloc patch: https://reviews.llvm.org/D102712 223.. _kernel llvm reloc: /Documentation/bpf/llvm_reloc.rst 224