libbpf: Load global data maps lazily on legacy kernels

Load global data maps lazily, if kernel is too old to support global
data. Make sure that programs are still correct by detecting if any of
the to-be-loaded programs have relocation against any of such maps.

This allows to solve the issue ([0]) with bpf_printk() and Clang
generating unnecessary and unreferenced .rodata.strX.Y sections, but it
also goes further along the CO-RE lines, allowing to have a BPF object
in which some code can work on very old kernels and relies only on BPF
maps explicitly, while other BPF programs might enjoy global variable
support. If such programs are correctly set to not load at runtime on
old kernels, bpf_object will load and function correctly now.

  [0] https://lore.kernel.org/bpf/CAK-59YFPU3qO+_pXWOH+c1LSA=8WA1yabJZfREjOEXNHAqgXNg@mail.gmail.com/

Fixes: aed65917 ("libbpf: Support multiple .rodata.* and .data.* BPF maps")
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Song Liu <songliubraving@fb.com>
Link: https://lore.kernel.org/bpf/20211123200105.387855-1-andrii@kernel.org

tools/lib/bpf/libbpf.c

@@ -5006,6 +5006,24 @@ bpf_object__create_maps(struct bpf_object *obj)
 	for (i = 0; i < obj->nr_maps; i++) {
 		map = &obj->maps[i];
 
+		/* To support old kernels, we skip creating global data maps
+		 * (.rodata, .data, .kconfig, etc); later on, during program
+		 * loading, if we detect that at least one of the to-be-loaded
+		 * programs is referencing any global data map, we'll error
+		 * out with program name and relocation index logged.
+		 * This approach allows to accommodate Clang emitting
+		 * unnecessary .rodata.str1.1 sections for string literals,
+		 * but also it allows to have CO-RE applications that use
+		 * global variables in some of BPF programs, but not others.
+		 * If those global variable-using programs are not loaded at
+		 * runtime due to bpf_program__set_autoload(prog, false),
+		 * bpf_object loading will succeed just fine even on old
+		 * kernels.
+		 */
+		if (bpf_map__is_internal(map) &&
+		    !kernel_supports(obj, FEAT_GLOBAL_DATA))
+			continue;
+
 		retried = false;
 retry:
 		if (map->pin_path) {
@@ -5605,6 +5623,14 @@ bpf_object__relocate_data(struct bpf_object *obj, struct bpf_program *prog)
 				insn[0].src_reg = BPF_PSEUDO_MAP_IDX_VALUE;
 				insn[0].imm = relo->map_idx;
 			} else {
+				const struct bpf_map *map = &obj->maps[relo->map_idx];
+
+				if (bpf_map__is_internal(map) &&
+				    !kernel_supports(obj, FEAT_GLOBAL_DATA)) {
+					pr_warn("prog '%s': relo #%d: kernel doesn't support global data\n",
+						prog->name, i);
+					return -ENOTSUP;
+				}
 				insn[0].src_reg = BPF_PSEUDO_MAP_VALUE;
 				insn[0].imm = obj->maps[relo->map_idx].fd;
 			}
@@ -6139,6 +6165,8 @@ bpf_object__relocate(struct bpf_object *obj, const char *targ_btf_path)
 		 */
 		if (prog_is_subprog(obj, prog))
 			continue;
+		if (!prog->load)
+			continue;
 
 		err = bpf_object__relocate_calls(obj, prog);
 		if (err) {
@@ -6152,6 +6180,8 @@ bpf_object__relocate(struct bpf_object *obj, const char *targ_btf_path)
 		prog = &obj->programs[i];
 		if (prog_is_subprog(obj, prog))
 			continue;
+		if (!prog->load)
+			continue;
 		err = bpf_object__relocate_data(obj, prog);
 		if (err) {
 			pr_warn("prog '%s': failed to relocate data references: %d\n",
@@ -6939,10 +6969,6 @@ static int bpf_object__sanitize_maps(struct bpf_object *obj)
 	bpf_object__for_each_map(m, obj) {
 		if (!bpf_map__is_internal(m))
 			continue;
-		if (!kernel_supports(obj, FEAT_GLOBAL_DATA)) {
-			pr_warn("kernel doesn't support global data\n");
-			return -ENOTSUP;
-		}
 		if (!kernel_supports(obj, FEAT_ARRAY_MMAP))
 			m->def.map_flags ^= BPF_F_MMAPABLE;
 	}