Merge branch 'libbpf: support custom .rodata.*/.data.* sections'

Andrii Nakryiko says:

====================

This patch set refactors internals of libbpf to enable support for multiple
custom .rodata.* and .data.* sections. Each such section is backed by its own
BPF_MAP_TYPE_ARRAY, memory-mappable just like .rodata/.data. This is not
extended to .bss because .bss is not a great name, it is generated by compiler
with name that reflects completely irrelevant historical implementation
details. Given that users have to annotate their variables with
SEC(".data.my_sec") explicitly, standardizing on .rodata. and .data. prefixes
makes more sense and keeps things simpler.

Additionally, this patch set makes it simpler to work with those special
internal maps by allowing to look them up by their full ELF section name.

Patch #1 is a preparatory patch that deprecates one libbpf API and moves
custom logic into libbpf.c, where it's used. This code is later refactored
with the rest of libbpf.c logic to support multiple data section maps.

See individual patches for all the details.

For new custom "dot maps", their full ELF section names are used as the names
that are sent into the kernel. Object name isn't prepended like for
.data/.rodata/.bss. The reason is that with longer custom names, there isn't
much space left for object name anyways. Also, if BTF is supported,
btf_value_type_id points to DATASEC BTF type, which contains full original ELF
name of the section, so tools like bpftool could use that to recover full
name. This patch set doesn't add this logic yet, this is left for follow up
patches.

One interesting possibility that is now open by these changes is that it's
possible to do:

    bpf_trace_printk("My fmt %s", sizeof("My fmt %s"), "blah");

and it will work as expected. I haven't updated libbpf-provided helpers in
bpf_helpers.h for snprintf, seq_printf, and printk, because using
`static const char ___fmt[] = fmt;` trick is still efficient and doesn't fill
out the buffer at runtime (no copying). But we might consider updating them in
the future, especially with the array check that Kumar proposed (see [0]).

  [0] https://lore.kernel.org/bpf/20211012041524.udytbr2xs5wid6x2@apollo.localdomain/

v1->v2:
  - don't prepend object name for new dot maps;
  - add __read_mostly example in selftests (Daniel).
====================

Signed-off-by: Alexei Starovoitov <ast@kernel.org>

1 files changed, 105 insertions, 53 deletions

diff --git a/tools/bpf/bpftool/gen.c b/tools/bpf/bpftool/gen.c
index b2ffc18eafc1..c446405ab73f 100644
--- a/tools/bpf/bpftool/gen.c
+++ b/tools/bpf/bpftool/gen.c
@@ -33,6 +33,11 @@ static void sanitize_identifier(char *name)
 			name[i] = '_';
 }
 
+static bool str_has_prefix(const char *str, const char *prefix)
+{
+	return strncmp(str, prefix, strlen(prefix)) == 0;
+}
+
 static bool str_has_suffix(const char *str, const char *suffix)
 {
 	size_t i, n1 = strlen(str), n2 = strlen(suffix);
@@ -67,23 +72,47 @@ static void get_header_guard(char *guard, const char *obj_name)
 		guard[i] = toupper(guard[i]);
 }
 
-static const char *get_map_ident(const struct bpf_map *map)
+static bool get_map_ident(const struct bpf_map *map, char *buf, size_t buf_sz)
 {
+	static const char *sfxs[] = { ".data", ".rodata", ".bss", ".kconfig" };
 	const char *name = bpf_map__name(map);
+	int i, n;
+
+	if (!bpf_map__is_internal(map)) {
+		snprintf(buf, buf_sz, "%s", name);
+		return true;
+	}
+
+	for  (i = 0, n = ARRAY_SIZE(sfxs); i < n; i++) {
+		const char *sfx = sfxs[i], *p;
 
-	if (!bpf_map__is_internal(map))
-		return name;
-
-	if (str_has_suffix(name, ".data"))
-		return "data";
-	else if (str_has_suffix(name, ".rodata"))
-		return "rodata";
-	else if (str_has_suffix(name, ".bss"))
-		return "bss";
-	else if (str_has_suffix(name, ".kconfig"))
-		return "kconfig";
-	else
-		return NULL;
+		p = strstr(name, sfx);
+		if (p) {
+			snprintf(buf, buf_sz, "%s", p + 1);
+			sanitize_identifier(buf);
+			return true;
+		}
+	}
+
+	return false;
+}
+
+static bool get_datasec_ident(const char *sec_name, char *buf, size_t buf_sz)
+{
+	static const char *pfxs[] = { ".data", ".rodata", ".bss", ".kconfig" };
+	int i, n;
+
+	for  (i = 0, n = ARRAY_SIZE(pfxs); i < n; i++) {
+		const char *pfx = pfxs[i];
+
+		if (str_has_prefix(sec_name, pfx)) {
+			snprintf(buf, buf_sz, "%s", sec_name + 1);
+			sanitize_identifier(buf);
+			return true;
+		}
+	}
+
+	return false;
 }
 
 static void codegen_btf_dump_printf(void *ctx, const char *fmt, va_list args)
@@ -100,24 +129,14 @@ static int codegen_datasec_def(struct bpf_object *obj,
 	const char *sec_name = btf__name_by_offset(btf, sec->name_off);
 	const struct btf_var_secinfo *sec_var = btf_var_secinfos(sec);
 	int i, err, off = 0, pad_cnt = 0, vlen = btf_vlen(sec);
-	const char *sec_ident;
-	char var_ident[256];
+	char var_ident[256], sec_ident[256];
 	bool strip_mods = false;
 
-	if (strcmp(sec_name, ".data") == 0) {
-		sec_ident = "data";
-		strip_mods = true;
-	} else if (strcmp(sec_name, ".bss") == 0) {
-		sec_ident = "bss";
-		strip_mods = true;
-	} else if (strcmp(sec_name, ".rodata") == 0) {
-		sec_ident = "rodata";
-		strip_mods = true;
-	} else if (strcmp(sec_name, ".kconfig") == 0) {
-		sec_ident = "kconfig";
-	} else {
+	if (!get_datasec_ident(sec_name, sec_ident, sizeof(sec_ident)))
 		return 0;
-	}
+
+	if (strcmp(sec_name, ".kconfig") != 0)
+		strip_mods = true;
 
 	printf("	struct %s__%s {\n", obj_name, sec_ident);
 	for (i = 0; i < vlen; i++, sec_var++) {
@@ -194,22 +213,61 @@ 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;
+	char sec_ident[256], map_ident[256];
 	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))
+		if (!get_map_ident(map, map_ident, sizeof(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);
+			if (!get_datasec_ident(name, sec_ident, sizeof(sec_ident)))
+				continue;
+
+			if (strcmp(sec_ident, map_ident) == 0) {
+				sec = t;
+				break;
+			}
+		}
+
+		/* In some cases (e.g., sections like .rodata.cst16 containing
+		 * compiler allocated string constants only) there will be
+		 * special internal maps with no corresponding DATASEC BTF
+		 * type. In such case, generate empty structs for each such
+		 * map. It will still be memory-mapped and its contents
+		 * accessible from user-space through BPF skeleton.
+		 */
+		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;
@@ -385,6 +443,7 @@ static void codegen_destroy(struct bpf_object *obj, const char *obj_name)
 {
 	struct bpf_program *prog;
 	struct bpf_map *map;
+	char ident[256];
 
 	codegen("\
 		\n\
@@ -405,10 +464,7 @@ static void codegen_destroy(struct bpf_object *obj, const char *obj_name)
 	}
 
 	bpf_object__for_each_map(map, obj) {
-		const char *ident;
-
-		ident = get_map_ident(map);
-		if (!ident)
+		if (!get_map_ident(map, ident, sizeof(ident)))
 			continue;
 		if (bpf_map__is_internal(map) &&
 		    (bpf_map__def(map)->map_flags & BPF_F_MMAPABLE))
@@ -432,6 +488,7 @@ static int gen_trace(struct bpf_object *obj, const char *obj_name, const char *h
 	struct bpf_object_load_attr load_attr = {};
 	DECLARE_LIBBPF_OPTS(gen_loader_opts, opts);
 	struct bpf_map *map;
+	char ident[256];
 	int err = 0;
 
 	err = bpf_object__gen_loader(obj, &opts);
@@ -477,12 +534,10 @@ static int gen_trace(struct bpf_object *obj, const char *obj_name, const char *h
 		",
 		obj_name, opts.data_sz);
 	bpf_object__for_each_map(map, obj) {
-		const char *ident;
 		const void *mmap_data = NULL;
 		size_t mmap_size = 0;
 
-		ident = get_map_ident(map);
-		if (!ident)
+		if (!get_map_ident(map, ident, sizeof(ident)))
 			continue;
 
 		if (!bpf_map__is_internal(map) ||
@@ -544,15 +599,15 @@ static int gen_trace(struct bpf_object *obj, const char *obj_name, const char *h
 				return err;				    \n\
 		", obj_name);
 	bpf_object__for_each_map(map, obj) {
-		const char *ident, *mmap_flags;
+		const char *mmap_flags;
 
-		ident = get_map_ident(map);
-		if (!ident)
+		if (!get_map_ident(map, ident, sizeof(ident)))
 			continue;
 
 		if (!bpf_map__is_internal(map) ||
 		    !(bpf_map__def(map)->map_flags & BPF_F_MMAPABLE))
 			continue;
+
 		if (bpf_map__def(map)->map_flags & BPF_F_RDONLY_PROG)
 			mmap_flags = "PROT_READ";
 		else
@@ -602,7 +657,8 @@ static int do_skeleton(int argc, char **argv)
 	DECLARE_LIBBPF_OPTS(bpf_object_open_opts, opts);
 	char obj_name[MAX_OBJ_NAME_LEN] = "", *obj_data;
 	struct bpf_object *obj = NULL;
-	const char *file, *ident;
+	const char *file;
+	char ident[256];
 	struct bpf_program *prog;
 	int fd, err = -1;
 	struct bpf_map *map;
@@ -673,8 +729,7 @@ static int do_skeleton(int argc, char **argv)
 	}
 
 	bpf_object__for_each_map(map, obj) {
-		ident = get_map_ident(map);
-		if (!ident) {
+		if (!get_map_ident(map, ident, sizeof(ident))) {
 			p_err("ignoring unrecognized internal map '%s'...",
 			      bpf_map__name(map));
 			continue;
@@ -727,8 +782,7 @@ static int do_skeleton(int argc, char **argv)
 	if (map_cnt) {
 		printf("\tstruct {\n");
 		bpf_object__for_each_map(map, obj) {
-			ident = get_map_ident(map);
-			if (!ident)
+			if (!get_map_ident(map, ident, sizeof(ident)))
 				continue;
 			if (use_loader)
 				printf("\t\tstruct bpf_map_desc %s;\n", ident);
@@ -897,9 +951,7 @@ static int do_skeleton(int argc, char **argv)
 		);
 		i = 0;
 		bpf_object__for_each_map(map, obj) {
-			ident = get_map_ident(map);
-
-			if (!ident)
+			if (!get_map_ident(map, ident, sizeof(ident)))
 				continue;
 
 			codegen("\