[PATCH bpf-next 1/3] bpftool: improve skeleton generation for objects without BTF

[Andrii Nakryiko <andrii@kernel.org>]

If BPF object file is using global variables, but is compiled without BTF or
ends up having only some of DATASEC types due to static linking, generated
skeleton won't compile, as some parts of skeleton would assume memory-mapped
struct definitions for each special data section.

Fix this by generating empty struct definition for such data sections. The
benefit of that is that they still will be memory-mapped by skeleton handling
code in libbpf, and user-space parts of user application would be able to
access data in them (though pointer casting), assuming they know the memory
layout they need.

Fixes: 985ead416df3 ("bpftool: Add skeleton codegen command")
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
---
 tools/bpf/bpftool/gen.c | 81 ++++++++++++++++++++++++++++++++---------
 1 file changed, 64 insertions(+), 17 deletions(-)

diff --git a/tools/bpf/bpftool/gen.c b/tools/bpf/bpftool/gen.c
index 31ade77f5ef8..6ec7c2e90a01 100644
--- a/tools/bpf/bpftool/gen.c
+++ b/tools/bpf/bpftool/gen.c
@@ -85,6 +85,18 @@ static const char *get_map_ident(const struct bpf_map *map)
 	else
 		return NULL;
 }
+static const char *datasec_ident(const char *sec_name)
+{
+	if (strcmp(sec_name, ".data") == 0)
+		return "data";
+	if (strcmp(sec_name, ".bss") == 0)
+		return "bss";
+	if (strcmp(sec_name, ".rodata") == 0)
+		return "rodata";
+	if (strcmp(sec_name, ".kconfig") == 0)
+		return "kconfig";
+	return NULL;
+}
 
 static void codegen_btf_dump_printf(void *ctx, const char *fmt, va_list args)
 {
@@ -104,18 +116,12 @@ static int codegen_datasec_def(struct bpf_object *obj,
 	char var_ident[256];
 	bool strip_mods = false;
 
-	if (strcmp(sec_name, ".data") == 0) {
-		sec_ident = "data";
-	} else if (strcmp(sec_name, ".bss") == 0) {
-		sec_ident = "bss";
-	} else if (strcmp(sec_name, ".rodata") == 0) {
-		sec_ident = "rodata";
-		strip_mods = true;
-	} else if (strcmp(sec_name, ".kconfig") == 0) {
-		sec_ident = "kconfig";
-	} else {
+	sec_ident = datasec_ident(sec_name);
+	if (!sec_ident)
 		return 0;
-	}
+
+	if (strcmp(sec_name, ".rodata") == 0)
+		strip_mods = true;
 
 	printf("	struct %s__%s {\n", obj_name, sec_ident);
 	for (i = 0; i < vlen; i++, sec_var++) {
@@ -188,22 +194,63 @@ static int codegen_datasecs(struct bpf_object *obj, const char *obj_name)
 	struct btf *btf = bpf_object__btf(obj);
 	int n = btf__get_nr_types(btf);
 	struct btf_dump *d;
+	struct bpf_map *map;
+	const struct btf_type *sec;
+	const char *sec_ident, *map_ident;
 	int i, err = 0;
 
 	d = btf_dump__new(btf, NULL, NULL, codegen_btf_dump_printf);
 	if (IS_ERR(d))
 		return PTR_ERR(d);
 
-	for (i = 1; i <= n; i++) {
-		const struct btf_type *t = btf__type_by_id(btf, i);
+	bpf_object__for_each_map(map, obj) {
+		/* only generate definitions for memory-mapped internal maps */
+		if (!bpf_map__is_internal(map))
+			continue;
+		if (!(bpf_map__def(map)->map_flags & BPF_F_MMAPABLE))
+			continue;
 
-		if (!btf_is_datasec(t))
+		map_ident = get_map_ident(map);
+		if (!map_ident)
 			continue;
 
-		err = codegen_datasec_def(obj, btf, d, t, obj_name);
-		if (err)
-			goto out;
+		sec = NULL;
+		for (i = 1; i <= n; i++) {
+			const struct btf_type *t = btf__type_by_id(btf, i);
+			const char *name;
+
+			if (!btf_is_datasec(t))
+				continue;
+
+			name = btf__str_by_offset(btf, t->name_off);
+			sec_ident = datasec_ident(name);
+			if (!sec_ident)
+				continue;
+
+			if (strcmp(sec_ident, map_ident) == 0) {
+				sec = t;
+				break;
+			}
+		}
+
+		/* In rare cases when BPF object file is using global
+		 * variables, but is compiled without BTF, we will have
+		 * special internal map, but no corresponding DATASEC BTF
+		 * type. In such case, generate empty structs for each such
+		 * map. It will still be memory-mapped as a convenience for
+		 * applications that know exact memory layout to expect.
+		 */
+		if (!sec) {
+			printf("	struct %s__%s {\n", obj_name, map_ident);
+			printf("	} *%s;\n", map_ident);
+		} else {
+			err = codegen_datasec_def(obj, btf, d, sec, obj_name);
+			if (err)
+				goto out;
+		}
 	}
+
+
 out:
 	btf_dump__free(d);
 	return err;
-- 
2.30.2