libbpf: Add support for new llvm bpf relocations

LLVM patch https://reviews.llvm.org/D102712
narrowed the scope of existing R_BPF_64_64
and R_BPF_64_32 relocations, and added three
new relocations, R_BPF_64_ABS64, R_BPF_64_ABS32
and R_BPF_64_NODYLD32. The main motivation is
to make relocations linker friendly.

This change, unfortunately, breaks libbpf build,
and we will see errors like below:
  libbpf: ELF relo #0 in section #6 has unexpected type 2 in
     /home/yhs/work/bpf-next/tools/testing/selftests/bpf/bpf_tcp_nogpl.o
  Error: failed to link
     '/home/yhs/work/bpf-next/tools/testing/selftests/bpf/bpf_tcp_nogpl.o':
     Unknown error -22 (-22)
The new relocation R_BPF_64_ABS64 is generated
and libbpf linker sanity check doesn't understand it.
Relocation section '.rel.struct_ops' at offset 0x1410 contains 1 entries:
    Offset             Info             Type               Symbol's Value  Symbol's Name
0000000000000018  0000000700000002 R_BPF_64_ABS64         0000000000000000 nogpltcp_init

Look at the selftests/bpf/bpf_tcp_nogpl.c,
  void BPF_STRUCT_OPS(nogpltcp_init, struct sock *sk)
  {
  }

  SEC(".struct_ops")
  struct tcp_congestion_ops bpf_nogpltcp = {
          .init           = (void *)nogpltcp_init,
          .name           = "bpf_nogpltcp",
  };
The new llvm relocation scheme categorizes 'nogpltcp_init' reference
as R_BPF_64_ABS64 instead of R_BPF_64_64 which is used to specify
ld_imm64 relocation in the new scheme.

Let us fix the linker sanity checking by including
R_BPF_64_ABS64 and R_BPF_64_ABS32. There is no need to
check R_BPF_64_NODYLD32 which is used for .BTF and .BTF.ext.

Signed-off-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Link: https://lore.kernel.org/bpf/20210522162341.3687617-1-yhs@fb.com

tools/lib/bpf/libbpf_internal.h

@@ -28,6 +28,12 @@
 #ifndef R_BPF_64_64
 #define R_BPF_64_64 1
 #endif
+#ifndef R_BPF_64_ABS64
+#define R_BPF_64_ABS64 2
+#endif
+#ifndef R_BPF_64_ABS32
+#define R_BPF_64_ABS32 3
+#endif
 #ifndef R_BPF_64_32
 #define R_BPF_64_32 10
 #endif

tools/lib/bpf/linker.c

@@ -892,7 +892,8 @@ static int linker_sanity_check_elf_relos(struct src_obj *obj, struct src_sec *se
 		size_t sym_idx = ELF64_R_SYM(relo->r_info);
 		size_t sym_type = ELF64_R_TYPE(relo->r_info);
 
-		if (sym_type != R_BPF_64_64 && sym_type != R_BPF_64_32) {
+		if (sym_type != R_BPF_64_64 && sym_type != R_BPF_64_32 &&
+		    sym_type != R_BPF_64_ABS64 && sym_type != R_BPF_64_ABS32) {
 			pr_warn("ELF relo #%d in section #%zu has unexpected type %zu in %s\n",
 				i, sec->sec_idx, sym_type, obj->filename);
 			return -EINVAL;