BPF Selftest Notes

General instructions on running selftests can be found in `Documentation/bpf/bpf_devel_QA.rst`_.

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

// 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

https://reviews.llvm.org/D85570

addresses it on the compiler side. It was committed on llvm 12.

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

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

; 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

https://reviews.llvm.org/D78466

has been pushed to llvm 10.x release branch and will be available in 10.0.1. The fix is available in llvm 11.0.0 trunk.

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