bpf tools: Collect symbol table from SHT_SYMTAB section

This patch collects symbols section. This section is useful when linking
BPF maps.

What 'bpf_map_xxx()' functions actually require are map's file
descriptors (and the internal verifier converts fds into pointers to
'struct bpf_map'), which we don't know when compiling. Therefore, we
should make compiler generate a 'ldr_64 r1, <imm>' instruction, and
fill the 'imm' field with the actual file descriptor when loading in
libbpf.

BPF programs should be written in this way:

 struct bpf_map_def SEC("maps") my_map = {
    .type = BPF_MAP_TYPE_HASH,
    .key_size = sizeof(unsigned long),
    .value_size = sizeof(unsigned long),
    .max_entries = 1000000,
 };

 SEC("my_func=sys_write")
 int my_func(void *ctx)
 {
     ...
     bpf_map_update_elem(&my_map, &key, &value, BPF_ANY);
     ...
 }

Compiler should convert '&my_map' into a 'ldr_64, r1, <imm>'
instruction, where imm should be the address of 'my_map'. According to
the address, libbpf knows which map it actually referenced, and then
fills the imm field with the 'fd' of that map created by it.

However, since we never really 'link' the object file, the imm field is
only a record in relocation section. Therefore libbpf should do the
relocation:

 1. In relocation section (type == SHT_REL), positions of each such
    'ldr_64' instruction are recorded with a reference of an entry in
    symbol table (SHT_SYMTAB);

 2. From records in symbol table we can find the indics of map
    variables.

Libbpf first record SHT_SYMTAB and positions of each instruction which
required bu such operation. Then create file descriptor. Finally, after
map creation complete, replace the imm field.

This is the first patch of BPF map related stuff. It records SHT_SYMTAB
into object's efile field for further use.
Signed-off-by: Wang Nan <wangnan0@huawei.com>
Acked-by: Alexei Starovoitov <ast@plumgrid.com>
Cc: Brendan Gregg <brendan.d.gregg@gmail.com>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Cc: David Ahern <dsahern@gmail.com>
Cc: He Kuang <hekuang@huawei.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Kaixu Xia <xiakaixu@huawei.com>
Cc: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Zefan Li <lizefan@huawei.com>
Cc: pi3orama@163.com
Link: http://lkml.kernel.org/r/1435716878-189507-12-git-send-email-wangnan0@huawei.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/lib/bpf/libbpf.c

@@ -94,6 +94,7 @@ struct bpf_object {
 		size_t obj_buf_sz;
 		Elf *elf;
 		GElf_Ehdr ehdr;
+		Elf_Data *symbols;
 	} efile;
 	char path[];
 };
@@ -135,6 +136,7 @@ static void bpf_object__elf_finish(struct bpf_object *obj)
 		elf_end(obj->efile.elf);
 		obj->efile.elf = NULL;
 	}
+	obj->efile.symbols = NULL;
 	zclose(obj->efile.fd);
 	obj->efile.obj_buf = NULL;
 	obj->efile.obj_buf_sz = 0;
@@ -333,6 +335,14 @@ static int bpf_object__elf_collect(struct bpf_object *obj)
 		else if (strcmp(name, "maps") == 0)
 			err = bpf_object__init_maps(obj, data->d_buf,
 						    data->d_size);
+		else if (sh.sh_type == SHT_SYMTAB) {
+			if (obj->efile.symbols) {
+				pr_warning("bpf: multiple SYMTAB in %s\n",
+					   obj->path);
+				err = -EEXIST;
+			} else
+				obj->efile.symbols = data;
+		}
 		if (err)
 			goto out;
 	}