libbpf: add support for bpf_call

- recognize relocation emitted by llvm
- since all regular function will be kept in .text section and llvm
  takes care of pc-relative offsets in bpf_call instruction
  simply copy all of .text to relevant program section while adjusting
  bpf_call instructions in program section to point to newly copied
  body of instructions from .text
- do so for all programs in the elf file
- set all programs types to the one passed to bpf_prog_load()

Note for elf files with multiple programs that use different
functions in .text section we need to do 'linker' style logic.
This work is still TBD

Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

tools/include/uapi/linux/bpf.h

@@ -197,8 +197,14 @@ enum bpf_attach_type {
  */
 #define BPF_F_STRICT_ALIGNMENT	(1U << 0)
 
+/* when bpf_ldimm64->src_reg == BPF_PSEUDO_MAP_FD, bpf_ldimm64->imm == fd */
 #define BPF_PSEUDO_MAP_FD	1
 
+/* when bpf_call->src_reg == BPF_PSEUDO_CALL, bpf_call->imm == pc-relative
+ * offset to another bpf function
+ */
+#define BPF_PSEUDO_CALL		1
+
 /* flags for BPF_MAP_UPDATE_ELEM command */
 #define BPF_ANY		0 /* create new element or update existing */
 #define BPF_NOEXIST	1 /* create new element if it didn't exist */