selftests/bpf: detect testing prog flags support

Various tests specify extra testing prog_flags when loading BPF
programs, like BPF_F_TEST_RND_HI32, and more recently also
BPF_F_TEST_REG_INVARIANTS. While BPF_F_TEST_RND_HI32 is old enough to
not cause much problem on older kernels, BPF_F_TEST_REG_INVARIANTS is
very fresh and unconditionally specifying it causes selftests to fail on
even slightly outdated kernels.

This breaks libbpf CI test against 4.9 and 5.15 kernels, it can break
some local development (done outside of VM), etc.

To prevent this, and guard against similar problems in the future, do
runtime detection of supported "testing flags", and only provide those
that host kernel recognizes.

Acked-by: Song Liu <[e-mail]>
Acked-by: Jiri Olsa <[e-mail]>
Signed-off-by: Andrii Nakryiko <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Alexei Starovoitov <[e-mail]>

tools/testing/selftests/bpf/prog_tests/bpf_verif_scale.c

···
 	}
 
 	bpf_program__set_type(prog, type);
-	bpf_program__set_flags(prog, BPF_F_TEST_RND_HI32 | BPF_F_TEST_REG_INVARIANTS);
+	bpf_program__set_flags(prog, testing_prog_flags());
 	bpf_program__set_log_level(prog, 4 | extra_prog_load_log_flags);
 
 	err = bpf_object__load(obj);

tools/testing/selftests/bpf/prog_tests/reg_bounds.c

···
 		.log_level = 2,
 		.log_buf = log_buf,
 		.log_size = log_sz,
-		.prog_flags = BPF_F_TEST_REG_INVARIANTS,
+		.prog_flags = testing_prog_flags(),
 	);
 
 	/* ; skip exit block below

tools/testing/selftests/bpf/test_loader.c

···
 	memset(spec, 0, sizeof(*spec));
 
 	spec->prog_name = bpf_program__name(prog);
-	spec->prog_flags = BPF_F_TEST_REG_INVARIANTS; /* by default be strict */
+	spec->prog_flags = testing_prog_flags();
 
 	btf = bpf_object__btf(obj);
 	if (!btf) {

tools/testing/selftests/bpf/test_sock_addr.c

···
 #include <bpf/libbpf.h>
 
 #include "cgroup_helpers.h"
+#include "testing_helpers.h"
 #include "bpf_util.h"
 
 #ifndef ENOTSUPP
···
 
 	bpf_program__set_type(prog, BPF_PROG_TYPE_CGROUP_SOCK_ADDR);
 	bpf_program__set_expected_attach_type(prog, test->expected_attach_type);
-	bpf_program__set_flags(prog, BPF_F_TEST_RND_HI32 | BPF_F_TEST_REG_INVARIANTS);
+	bpf_program__set_flags(prog, testing_prog_flags());
 
 	err = bpf_object__load(obj);
 	if (err) {

tools/testing/selftests/bpf/test_verifier.c

···
 	if (fixup_skips != skips)
 		return;
 
-	pflags = BPF_F_TEST_RND_HI32 | BPF_F_TEST_REG_INVARIANTS;
+	pflags = testing_prog_flags();
 	if (test->flags & F_LOAD_WITH_STRICT_ALIGNMENT)
 		pflags |= BPF_F_STRICT_ALIGNMENT;
 	if (test->flags & F_NEEDS_EFFICIENT_UNALIGNED_ACCESS)

tools/testing/selftests/bpf/testing_helpers.c

···
 
 int extra_prog_load_log_flags = 0;
 
+int testing_prog_flags(void)
+{
+	static int cached_flags = -1;
+	static int prog_flags[] = { BPF_F_TEST_RND_HI32, BPF_F_TEST_REG_INVARIANTS };
+	static struct bpf_insn insns[] = {
+		BPF_MOV64_IMM(BPF_REG_0, 0),
+		BPF_EXIT_INSN(),
+	};
+	int insn_cnt = ARRAY_SIZE(insns), i, fd, flags = 0;
+	LIBBPF_OPTS(bpf_prog_load_opts, opts);
+
+	if (cached_flags >= 0)
+		return cached_flags;
+
+	for (i = 0; i < ARRAY_SIZE(prog_flags); i++) {
+		opts.prog_flags = prog_flags[i];
+		fd = bpf_prog_load(BPF_PROG_TYPE_SOCKET_FILTER, "flag-test", "GPL",
+				   insns, insn_cnt, &opts);
+		if (fd >= 0) {
+			flags |= prog_flags[i];
+			close(fd);
+		}
+	}
+
+	cached_flags = flags;
+	return cached_flags;
+}
+
 int bpf_prog_test_load(const char *file, enum bpf_prog_type type,
 		       struct bpf_object **pobj, int *prog_fd)
 {
···
 	if (type != BPF_PROG_TYPE_UNSPEC && bpf_program__type(prog) != type)
 		bpf_program__set_type(prog, type);
 
-	flags = bpf_program__flags(prog) | BPF_F_TEST_RND_HI32 | BPF_F_TEST_REG_INVARIANTS;
+	flags = bpf_program__flags(prog) | testing_prog_flags();
 	bpf_program__set_flags(prog, flags);
 
 	err = bpf_object__load(obj);
···
 {
 	LIBBPF_OPTS(bpf_prog_load_opts, opts,
 		.kern_version = kern_version,
-		.prog_flags = BPF_F_TEST_RND_HI32 | BPF_F_TEST_REG_INVARIANTS,
+		.prog_flags = testing_prog_flags(),
 		.log_level = extra_prog_load_log_flags,
 		.log_buf = log_buf,
 		.log_size = log_buf_sz,

tools/testing/selftests/bpf/testing_helpers.h

···
  * e.g. verifier.c:convert_ctx_access() is done.
  */
 int get_xlated_program(int fd_prog, struct bpf_insn **buf, __u32 *cnt);
+int testing_prog_flags(void);
 
 #endif /* __TESTING_HELPERS_H */