[RFC PATCH dwarves] btf: add support for split BTF loading and encoding

[Andrii Nakryiko <andrii@kernel.org>]

Add support for generating split BTF, in which there is a designated base
BTF, containing a base set of types, and a split BTF, which extends main BTF
with extra types, that can reference types and strings from the main BTF.

This is going to be used to generate compact BTFs for kernel modules, with
vmlinux BTF being a main BTF, which all kernel modules are based off of.

These changes rely on patch set [0] to be present in libbpf submodule.

  [0] https://patchwork.kernel.org/project/netdevbpf/list/?series=377859&state=*

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
---

This is posted before libbpf changes landed to show end-to-end how kernel
module BTFs are going to be integrated into the kernel. Once libbpf split BTF
support lands, I'll sync it into Github repo and will post a proper v1.

 btf_encoder.c | 15 ++++++++-------
 btf_loader.c  |  2 +-
 libbtf.c      | 43 +++++++++++++++++++++++++++----------------
 libbtf.h      |  4 +++-
 pahole.c      | 23 +++++++++++++++++++++++
 5 files changed, 62 insertions(+), 25 deletions(-)

diff --git a/btf_encoder.c b/btf_encoder.c
index 4c92908beab2..d67e29b9cbee 100644
--- a/btf_encoder.c
+++ b/btf_encoder.c
@@ -12,6 +12,7 @@
 #include "dwarves.h"
 #include "libbtf.h"
 #include "lib/bpf/include/uapi/linux/btf.h"
+#include "lib/bpf/src/libbpf.h"
 #include "hash.h"
 #include "elf_symtab.h"
 #include "btf_encoder.h"
@@ -343,7 +344,7 @@ int cu__encode_btf(struct cu *cu, int verbose, bool force,
 	}
 
 	if (!btfe) {
-		btfe = btf_elf__new(cu->filename, cu->elf);
+		btfe = btf_elf__new(cu->filename, cu->elf, base_btf);
 		if (!btfe)
 			return -1;
 
@@ -358,22 +359,22 @@ int cu__encode_btf(struct cu *cu, int verbose, bool force,
 			printf("File %s:\n", btfe->filename);
 	}
 
+	btf_elf__verbose = verbose;
+	btf_elf__force = force;
+	type_id_off = btf__get_nr_types(btfe->btf);
+
 	if (!has_index_type) {
 		/* cu__find_base_type_by_name() takes "type_id_t *id" */
 		type_id_t id;
 		if (cu__find_base_type_by_name(cu, "int", &id)) {
 			has_index_type = true;
-			array_index_id = id;
+			array_index_id = type_id_off + id;
 		} else {
 			has_index_type = false;
-			array_index_id = cu->types_table.nr_entries;
+			array_index_id = type_id_off + cu->types_table.nr_entries;
 		}
 	}
 
-	btf_elf__verbose = verbose;
-	btf_elf__force = force;
-	type_id_off = btf__get_nr_types(btfe->btf);
-
 	cu__for_each_type(cu, core_id, pos) {
 		int32_t btf_type_id = tag__encode_btf(cu, pos, core_id, btfe, array_index_id, type_id_off);
 
diff --git a/btf_loader.c b/btf_loader.c
index 6ea207ea65b4..ec286f413f36 100644
--- a/btf_loader.c
+++ b/btf_loader.c
@@ -534,7 +534,7 @@ struct debug_fmt_ops btf_elf__ops;
 int btf_elf__load_file(struct cus *cus, struct conf_load *conf, const char *filename)
 {
 	int err;
-	struct btf_elf *btfe = btf_elf__new(filename, NULL);
+	struct btf_elf *btfe = btf_elf__new(filename, NULL, base_btf);
 
 	if (btfe == NULL)
 		return -1;
diff --git a/libbtf.c b/libbtf.c
index babf4fe8cd9e..3c52aa0d482b 100644
--- a/libbtf.c
+++ b/libbtf.c
@@ -27,6 +27,7 @@
 #include "dwarves.h"
 #include "elf_symtab.h"
 
+struct btf *base_btf;
 uint8_t btf_elf__verbose;
 uint8_t btf_elf__force;
 
@@ -52,9 +53,9 @@ int btf_elf__load(struct btf_elf *btfe)
 	/* free initial empty BTF */
 	btf__free(btfe->btf);
 	if (btfe->raw_btf)
-		btfe->btf = btf__parse_raw(btfe->filename);
+		btfe->btf = btf__parse_raw_split(btfe->filename, btfe->base_btf);
 	else
-		btfe->btf = btf__parse_elf(btfe->filename, NULL);
+		btfe->btf = btf__parse_elf_split(btfe->filename, btfe->base_btf);
 
 	err = libbpf_get_error(btfe->btf);
 	if (err)
@@ -63,7 +64,7 @@ int btf_elf__load(struct btf_elf *btfe)
 	return 0;
 }
 
-struct btf_elf *btf_elf__new(const char *filename, Elf *elf)
+struct btf_elf *btf_elf__new(const char *filename, Elf *elf, struct btf *base_btf)
 {
 	struct btf_elf *btfe = zalloc(sizeof(*btfe));
 	GElf_Shdr shdr;
@@ -77,7 +78,8 @@ struct btf_elf *btf_elf__new(const char *filename, Elf *elf)
 	if (btfe->filename == NULL)
 		goto errout;
 
-	btfe->btf = btf__new_empty();
+	btfe->base_btf = base_btf;
+	btfe->btf = btf__new_empty_split(base_btf);
 	if (libbpf_get_error(btfe->btf)) {
 		fprintf(stderr, "%s: failed to create empty BTF.\n", __func__);
 		goto errout;
@@ -679,11 +681,11 @@ static int btf_elf__write(const char *filename, struct btf *btf)
 {
 	GElf_Shdr shdr_mem, *shdr;
 	GElf_Ehdr ehdr_mem, *ehdr;
-	Elf_Data *btf_elf = NULL;
+	Elf_Data *btf_data = NULL;
 	Elf_Scn *scn = NULL;
 	Elf *elf = NULL;
-	const void *btf_data;
-	uint32_t btf_size;
+	const void *raw_btf_data;
+	uint32_t raw_btf_size;
 	int fd, err = -1;
 	size_t strndx;
 
@@ -735,18 +737,18 @@ static int btf_elf__write(const char *filename, struct btf *btf)
 			continue;
 		char *secname = elf_strptr(elf, strndx, shdr->sh_name);
 		if (strcmp(secname, ".BTF") == 0) {
-			btf_elf = elf_getdata(scn, btf_elf);
+			btf_data = elf_getdata(scn, btf_data);
 			break;
 		}
 	}
 
-	btf_data = btf__get_raw_data(btf, &btf_size);
+	raw_btf_data = btf__get_raw_data(btf, &raw_btf_size);
 
-	if (btf_elf) {
+	if (btf_data) {
 		/* Exisiting .BTF section found */
-		btf_elf->d_buf = (void *)btf_data;
-		btf_elf->d_size = btf_size;
-		elf_flagdata(btf_elf, ELF_C_SET, ELF_F_DIRTY);
+		btf_data->d_buf = (void *)raw_btf_data;
+		btf_data->d_size = raw_btf_size;
+		elf_flagdata(btf_data, ELF_C_SET, ELF_F_DIRTY);
 
 		if (elf_update(elf, ELF_C_NULL) >= 0 &&
 		    elf_update(elf, ELF_C_WRITE) >= 0)
@@ -770,12 +772,21 @@ static int btf_elf__write(const char *filename, struct btf *btf)
 			goto out;
 		}
 
+		if (write(fd, raw_btf_data, raw_btf_size) != raw_btf_size) {
+			fprintf(stderr, "%s: write of %d bytes to '%s' failed: %d!\n",
+				__func__, raw_btf_size, tmp_fn, errno);
+			goto out;
+		}
+
 		snprintf(cmd, sizeof(cmd), "%s --add-section .BTF=%s %s",
 			 llvm_objcopy, tmp_fn, filename);
+		if (system(cmd)) {
+			fprintf(stderr, "%s: failed to add .BTF section to '%s': %d!\n",
+				__func__, tmp_fn, errno);
+			goto out;
+		}
 
-		if (write(fd, btf_data, btf_size) == btf_size && !system(cmd))
-			err = 0;
-
+		err = 0;
 		unlink(tmp_fn);
 	}
 
diff --git a/libbtf.h b/libbtf.h
index 887b5bc55c8e..71f6cecbea93 100644
--- a/libbtf.h
+++ b/libbtf.h
@@ -27,8 +27,10 @@ struct btf_elf {
 	uint32_t	  percpu_shndx;
 	uint64_t	  percpu_base_addr;
 	struct btf	  *btf;
+	struct btf	  *base_btf;
 };
 
+extern struct btf *base_btf;
 extern uint8_t btf_elf__verbose;
 extern uint8_t btf_elf__force;
 #define btf_elf__verbose_log(fmt, ...) { if (btf_elf__verbose) printf(fmt, __VA_ARGS__); }
@@ -39,7 +41,7 @@ struct cu;
 struct base_type;
 struct ftype;
 
-struct btf_elf *btf_elf__new(const char *filename, Elf *elf);
+struct btf_elf *btf_elf__new(const char *filename, Elf *elf, struct btf *base_btf);
 void btf_elf__delete(struct btf_elf *btf);
 
 int32_t btf_elf__add_base_type(struct btf_elf *btf, const struct base_type *bt,
diff --git a/pahole.c b/pahole.c
index bd9b993777ee..d18092c1212c 100644
--- a/pahole.c
+++ b/pahole.c
@@ -22,12 +22,15 @@
 #include "dutil.h"
 #include "ctf_encoder.h"
 #include "btf_encoder.h"
+#include "libbtf.h"
+#include "lib/bpf/src/libbpf.h"
 
 static bool btf_encode;
 static bool ctf_encode;
 static bool first_obj_only;
 static bool skip_encoding_btf_vars;
 static bool btf_encode_force;
+static const char *base_btf_file;
 
 static uint8_t class__include_anonymous;
 static uint8_t class__include_nested_anonymous;
@@ -820,6 +823,7 @@ ARGP_PROGRAM_VERSION_HOOK_DEF = dwarves_print_version;
 #define ARGP_skip_encoding_btf_vars 317
 #define ARGP_btf_encode_force	   318
 #define ARGP_just_packed_structs   319
+#define ARGP_btf_base		   320
 
 static const struct argp_option pahole__options[] = {
 	{
@@ -1093,6 +1097,12 @@ static const struct argp_option pahole__options[] = {
 		.key  = ARGP_hex_fmt,
 		.doc  = "Print offsets and sizes in hexadecimal",
 	},
+	{
+		.name = "btf_base",
+		.key  = ARGP_btf_base,
+		.arg  = "SIZE",
+		.doc  = "Path to the base BTF file",
+	},
 	{
 		.name = "btf_encode",
 		.key  = 'J',
@@ -1234,6 +1244,9 @@ static error_t pahole__options_parser(int key, char *arg,
 		skip_encoding_btf_vars = true;		break;
 	case ARGP_btf_encode_force:
 		btf_encode_force = true;		break;
+	case ARGP_btf_base:
+		base_btf_file = arg;
+		break;
 	default:
 		return ARGP_ERR_UNKNOWN;
 	}
@@ -2682,6 +2695,15 @@ int main(int argc, char *argv[])
 		goto out;
 	}
 
+	if (base_btf_file) {
+		base_btf = btf__parse(base_btf_file, NULL);
+		if (libbpf_get_error(base_btf)) {
+			fprintf(stderr, "Failed to parse base BTF '%s': %ld\n",
+				base_btf_file, libbpf_get_error(base_btf));
+			goto out;
+		}
+	}
+
 	struct cus *cus = cus__new();
 	if (cus == NULL) {
 		fputs("pahole: insufficient memory\n", stderr);
@@ -2766,6 +2788,7 @@ out_cus_delete:
 #ifdef DEBUG_CHECK_LEAKS
 	cus__delete(cus);
 	structures__delete();
+	btf__free(base_btf);
 #endif
 out_dwarves_exit:
 #ifdef DEBUG_CHECK_LEAKS
-- 
2.24.1