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
10Additional information about selftest failures are
11documented here.
12
13profiler[23] test failures with clang/llvm <12.0.0
14==================================================
15
16With clang/llvm <12.0.0, the profiler[23] test may fail.
17The symptom looks like
18
19.. code-block:: c
20
21  // r9 is a pointer to map_value
22  // r7 is a scalar
23  17:       bf 96 00 00 00 00 00 00 r6 = r9
24  18:       0f 76 00 00 00 00 00 00 r6 += r7
25  math between map_value pointer and register with unbounded min value is not allowed
26
27  // the instructions below will not be seen in the verifier log
28  19:       a5 07 01 00 01 01 00 00 if r7 < 257 goto +1
29  20:       bf 96 00 00 00 00 00 00 r6 = r9
30  // r6 is used here
31
32The verifier will reject such code with above error.
33At insn 18 the r7 is indeed unbounded. The later insn 19 checks the bounds and
34the insn 20 undoes map_value addition. It is currently impossible for the
35verifier to understand such speculative pointer arithmetic.
36Hence `this patch`__ addresses it on the compiler side. It was committed on llvm 12.
37
38__ https://reviews.llvm.org/D85570
39
40The corresponding C code
41
42.. code-block:: c
43
44  for (int i = 0; i < MAX_CGROUPS_PATH_DEPTH; i++) {
45          filepart_length = bpf_probe_read_str(payload, ...);
46          if (filepart_length <= MAX_PATH) {
47                  barrier_var(filepart_length); // workaround
48                  payload += filepart_length;
49          }
50  }
51
52bpf_iter test failures with clang/llvm 10.0.0
53=============================================
54
55With clang/llvm 10.0.0, the following two bpf_iter tests failed:
56  * ``bpf_iter/ipv6_route``
57  * ``bpf_iter/netlink``
58
59The symptom for ``bpf_iter/ipv6_route`` looks like
60
61.. code-block:: c
62
63  2: (79) r8 = *(u64 *)(r1 +8)
64  ...
65  14: (bf) r2 = r8
66  15: (0f) r2 += r1
67  ; BPF_SEQ_PRINTF(seq, "%pi6 %02x ", &rt->fib6_dst.addr, rt->fib6_dst.plen);
68  16: (7b) *(u64 *)(r8 +64) = r2
69  only read is supported
70
71The symptom for ``bpf_iter/netlink`` looks like
72
73.. code-block:: c
74
75  ; struct netlink_sock *nlk = ctx->sk;
76  2: (79) r7 = *(u64 *)(r1 +8)
77  ...
78  15: (bf) r2 = r7
79  16: (0f) r2 += r1
80  ; BPF_SEQ_PRINTF(seq, "%pK %-3d ", s, s->sk_protocol);
81  17: (7b) *(u64 *)(r7 +0) = r2
82  only read is supported
83
84This is due to a llvm BPF backend bug. `The fix`__
85has been pushed to llvm 10.x release branch and will be
86available in 10.0.1. The patch is available in llvm 11.0.0 trunk.
87
88__  https://reviews.llvm.org/D78466
89
90BPF CO-RE-based tests and Clang version
91=======================================
92
93A set of selftests use BPF target-specific built-ins, which might require
94bleeding-edge Clang versions (Clang 12 nightly at this time).
95
96Few sub-tests of core_reloc test suit (part of test_progs test runner) require
97the following built-ins, listed with corresponding Clang diffs introducing
98them to Clang/LLVM. These sub-tests are going to be skipped if Clang is too
99old to support them, they shouldn't cause build failures or runtime test
100failures:
101
102- __builtin_btf_type_id() [0_, 1_, 2_];
103- __builtin_preserve_type_info(), __builtin_preserve_enum_value() [3_, 4_].
104
105.. _0: https://reviews.llvm.org/D74572
106.. _1: https://reviews.llvm.org/D74668
107.. _2: https://reviews.llvm.org/D85174
108.. _3: https://reviews.llvm.org/D83878
109.. _4: https://reviews.llvm.org/D83242
110