[RFC PATCH v2 21/37] tools lib bpf: relocation programs.

[Wang Nan <wangnan0@huawei.com>]

If an eBPF program access a map, LLVM generates a relocated load
instruction. To enable the usage of that map, relocation must be done
by replacing original instructions by map loading instructions.

Based on relocation description collected during 'opening' phase, this
patch replaces the instructions with map loading with correct map fd.

Signed-off-by: Wang Nan <wangnan0@huawei.com>
---
 tools/lib/bpf/libbpf.c | 48 ++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 48 insertions(+)

diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
index 6ff4cb6..5e25ea7 100644
--- a/tools/lib/bpf/libbpf.c
+++ b/tools/lib/bpf/libbpf.c
@@ -676,6 +676,52 @@ bpf_obj_create_maps(struct bpf_object *obj)
 	return 0;
 }
 
+static int
+bpf_program_relocate(struct bpf_object *obj, struct bpf_program *prog)
+{
+	int i;
+
+	if (!prog || !prog->reloc_desc)
+		return 0;
+
+	for (i = 0; i < prog->nr_reloc; i++) {
+		int insn_idx, map_idx;
+		struct bpf_insn *insns = prog->insns;
+
+		insn_idx = prog->reloc_desc[i].insn_idx;
+		map_idx = prog->reloc_desc[i].map_idx;
+
+		if (insn_idx >= (int)prog->insns_cnt) {
+			pr_warning("relocation out of range: '%s'\n",
+				   prog->section_name);
+			return -ERANGE;
+		}
+		insns[insn_idx].src_reg = BPF_PSEUDO_MAP_FD;
+		insns[insn_idx].imm = obj->maps_fds[map_idx];
+	}
+
+	return 0;
+}
+
+static int
+bpf_obj_relocate(struct bpf_object *obj)
+{
+	struct bpf_program *prog;
+	size_t i;
+	int err;
+
+	for (i = 0; i < obj->nr_programs; i++) {
+		prog = &obj->programs[i];
+
+		if ((err = bpf_program_relocate(obj, prog))) {
+			pr_warning("failed to relocate '%s'\n",
+				   prog->section_name);
+			return err;
+		}
+	}
+	return 0;
+}
+
 static int bpf_obj_collect_reloc(struct bpf_object *obj)
 {
 	int i, err;
@@ -783,6 +829,8 @@ int bpf_load_object(struct bpf_object *obj)
 
 	if (bpf_obj_create_maps(obj))
 		goto out;
+	if (bpf_obj_relocate(obj))
+		goto out;
 
 	return 0;
 out:
-- 
1.8.3.4