selftests/bpf: Fix trace_virtqueue_add_sgs test issue with LLVM 17.

LLVM commit https://reviews.llvm.org/D143726 introduced hoistMinMax optimization
that transformed
(i < VIRTIO_MAX_SGS) && (i < out_sgs)
into
i < MIN(VIRTIO_MAX_SGS, out_sgs)

and caused the verifier to stop recognizing such loop as bounded.

Which resulted in the following test failure:

libbpf: prog 'trace_virtqueue_add_sgs': BPF program load failed: Bad address
libbpf: prog 'trace_virtqueue_add_sgs': -- BEGIN PROG LOAD LOG --
The sequence of 8193 jumps is too complex.
verification time 789206 usec
stack depth 56
processed 156446 insns (limit 1000000) max_states_per_insn 7 total_states 1746 peak_states 1701 mark_read 12
-- END PROG LOAD LOG --
libbpf: prog 'trace_virtqueue_add_sgs': failed to load: -14
libbpf: failed to load object 'loop6.bpf.o'

Workaround the verifier limitation for now with inline asm that
prevents this particular optimization.

Signed-off-by: Alexei Starovoitov <ast@kernel.org>

selftests/bpf: Fix compiler warning in BPF_KPROBE definition in loop6.c

Add missing return type to BPF_KPROBE definition. Without it, compiler
generates the following warning:

progs/loop6.c:68:12: warning: type specifier missing, defaults to 'int' [-Wimplicit-int]
BPF_KPROBE(trace_virtqueue_add_sgs, void *unused, struct scatterlist **sgs,
^
1 warning generated.

Fixes: 86a35af628e5 ("selftests/bpf: Add a verifier scale test with unknown bounded loop")
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20210309044322.3487636-1-andrii@kernel.org

selftests/bpf: Add a verifier scale test with unknown bounded loop

The original bcc pull request https://github.com/iovisor/bcc/pull/3270 exposed
a verifier failure with Clang 12/13 while Clang 4 works fine.

Further investigation exposed two issues:

Issue 1: LLVM may generate code which uses less refined value. The issue is
fixed in LLVM patch: https://reviews.llvm.org/D97479

Issue 2: Spills with initial value 0 are marked as precise which makes later
state pruning less effective. This is my rough initial analysis and
further investigation is needed to find how to improve verifier
pruning in such cases.

With the above LLVM patch, for the new loop6.c test, which has smaller loop
bound compared to original test, I got:

$ test_progs -s -n 10/16
...
stack depth 64
processed 390735 insns (limit 1000000) max_states_per_insn 87
total_states 8658 peak_states 964 mark_read 6
#10/16 loop6.o:OK

Use the original loop bound, i.e., commenting out "#define WORKAROUND", I got:

$ test_progs -s -n 10/16
...
BPF program is too large. Processed 1000001 insn
stack depth 64
processed 1000001 insns (limit 1000000) max_states_per_insn 91
total_states 23176 peak_states 5069 mark_read 6
...
#10/16 loop6.o:FAIL

The purpose of this patch is to provide a regression test for the above LLVM fix
and also provide a test case for further analyzing the verifier pruning issue.

Signed-off-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Cc: Zhenwei Pi <pizhenwei@bytedance.com>
Link: https://lore.kernel.org/bpf/20210226223810.236472-1-yhs@fb.com