selftests: net: declare section names for bpf_offload

Non-ancient ip (iproute2-5.15.0, libbpf 0.7.0) refuses to load
the sample with maps because we don't generate BTF:

libbpf: BTF is required, but is missing or corrupted.
ERROR: opening BPF object file failed

Enable BTF by adding -g to clang flags. With that done
neither of the programs load:

libbpf: prog 'func': error relocating .BTF.ext function info: -22
libbpf: prog 'func': failed to relocate calls: -22
libbpf: failed to load object 'ksft-net-drv/net/sample_ret0.bpf.o'

Andrii explains that this is because we don't specify
section names for the code. Add the section names, too.

Acked-by: Alexei Starovoitov <[e-mail]>
Acked-by: Stanislav Fomichev <[e-mail]>
Acked-by: Martin KaFai Lau <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

tools/testing/selftests/net/Makefile

···
 	$(OUTPUT)/sample_map_ret0.bpf.o $(OUTPUT)/sample_ret0.bpf.o
 
 $(BPF_PROG_OBJS): $(OUTPUT)/%.o : %.c $(BPFOBJ) | $(MAKE_DIRS)
-	$(CLANG) -O2 --target=bpf -c $< $(CCINCLUDE) $(CLANG_SYS_INCLUDES) -o $@
+	$(CLANG) -O2 -g --target=bpf $(CCINCLUDE) $(CLANG_SYS_INCLUDES) \
+	-c $< -o $@
 
 $(BPFOBJ): $(wildcard $(BPFDIR)/*.[ch] $(BPFDIR)/Makefile)		       \
 	   $(APIDIR)/linux/bpf.h					       \

tools/testing/selftests/net/bpf_offload.py

···
 def ethtool(dev, opt, args, fail=True):
     return cmd("ethtool %s %s %s" % (opt, dev["ifname"], args), fail=fail)
 
-def bpf_obj(name, sec=".text", path=bpf_test_dir,):
+def bpf_obj(name, sec="xdp", path=bpf_test_dir,):
     return "obj %s sec %s" % (os.path.join(path, name), sec)
 
 def bpf_pinned(name):

tools/testing/selftests/net/sample_map_ret0.bpf.c

···
 } array SEC(".maps");
 
 /* Sample program which should always load for testing control paths. */
-SEC(".text") int func()
+SEC("xdp") int func()
 {
 	__u64 key64 = 0;
 	__u32 key = 0;

tools/testing/selftests/net/sample_ret0.bpf.c

···
 /* SPDX-License-Identifier: (GPL-2.0 OR BSD-2-Clause) */
 
+#define SEC(name) __attribute__((section(name), used))
+
 /* Sample program which should always load for testing control paths. */
+SEC("xdp")
 int func()
 {
 	return 0;