libbpf: Make btf_parse_elf process .BTF.base transparently

Update btf_parse_elf() to check if .BTF.base section is present.
The logic is as follows:

  if .BTF.base section exists:
     distilled_base := btf_new(.BTF.base)
  if distilled_base:
     btf := btf_new(.BTF, .base_btf=distilled_base)
     if base_btf:
        btf_relocate(btf, base_btf)
  else:
     btf := btf_new(.BTF)
  return btf

In other words:
- if .BTF.base section exists, load BTF from it and use it as a base
  for .BTF load;
- if base_btf is specified and .BTF.base section exist, relocate newly
  loaded .BTF against base_btf.
Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Alan Maguire <alan.maguire@oracle.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20240613095014.357981-6-alan.maguire@oracle.com

tools/lib/bpf/btf.c

@@ -116,6 +116,9 @@ struct btf {
 	/* whether strings are already deduplicated */
 	bool strs_deduped;
 
+	/* whether base_btf should be freed in btf_free for this instance */
+	bool owns_base;
+
 	/* BTF object FD, if loaded into kernel */
 	int fd;
 
@@ -969,6 +972,8 @@ void btf__free(struct btf *btf)
 	free(btf->raw_data);
 	free(btf->raw_data_swapped);
 	free(btf->type_offs);
+	if (btf->owns_base)
+		btf__free(btf->base_btf);
 	free(btf);
 }
 
@@ -1084,53 +1089,38 @@ struct btf *btf__new_split(const void *data, __u32 size, struct btf *base_btf)
 	return libbpf_ptr(btf_new(data, size, base_btf));
 }
 
-static struct btf *btf_parse_elf(const char *path, struct btf *base_btf,
-				 struct btf_ext **btf_ext)
+struct btf_elf_secs {
+	Elf_Data *btf_data;
+	Elf_Data *btf_ext_data;
+	Elf_Data *btf_base_data;
+};
+
+static int btf_find_elf_sections(Elf *elf, const char *path, struct btf_elf_secs *secs)
 {
-	Elf_Data *btf_data = NULL, *btf_ext_data = NULL;
-	int err = 0, fd = -1, idx = 0;
-	struct btf *btf = NULL;
 	Elf_Scn *scn = NULL;
-	Elf *elf = NULL;
+	Elf_Data *data;
 	GElf_Ehdr ehdr;
 	size_t shstrndx;
+	int idx = 0;
 
-	if (elf_version(EV_CURRENT) == EV_NONE) {
-		pr_warn("failed to init libelf for %s\n", path);
-		return ERR_PTR(-LIBBPF_ERRNO__LIBELF);
-	}
-
-	fd = open(path, O_RDONLY | O_CLOEXEC);
-	if (fd < 0) {
-		err = -errno;
-		pr_warn("failed to open %s: %s\n", path, strerror(errno));
-		return ERR_PTR(err);
-	}
-
-	err = -LIBBPF_ERRNO__FORMAT;
-
-	elf = elf_begin(fd, ELF_C_READ, NULL);
-	if (!elf) {
-		pr_warn("failed to open %s as ELF file\n", path);
-		goto done;
-	}
 	if (!gelf_getehdr(elf, &ehdr)) {
 		pr_warn("failed to get EHDR from %s\n", path);
-		goto done;
+		goto err;
 	}
 
 	if (elf_getshdrstrndx(elf, &shstrndx)) {
 		pr_warn("failed to get section names section index for %s\n",
 			path);
-		goto done;
+		goto err;
 	}
 
 	if (!elf_rawdata(elf_getscn(elf, shstrndx), NULL)) {
 		pr_warn("failed to get e_shstrndx from %s\n", path);
-		goto done;
+		goto err;
 	}
 
 	while ((scn = elf_nextscn(elf, scn)) != NULL) {
+		Elf_Data **field;
 		GElf_Shdr sh;
 		char *name;
 
@@ -1138,42 +1128,102 @@ static struct btf *btf_parse_elf(const char *path, struct btf *base_btf,
 		if (gelf_getshdr(scn, &sh) != &sh) {
 			pr_warn("failed to get section(%d) header from %s\n",
 				idx, path);
-			goto done;
+			goto err;
 		}
 		name = elf_strptr(elf, shstrndx, sh.sh_name);
 		if (!name) {
 			pr_warn("failed to get section(%d) name from %s\n",
 				idx, path);
-			goto done;
+			goto err;
 		}
-		if (strcmp(name, BTF_ELF_SEC) == 0) {
-			btf_data = elf_getdata(scn, 0);
-			if (!btf_data) {
-				pr_warn("failed to get section(%d, %s) data from %s\n",
-					idx, name, path);
-				goto done;
-			}
-			continue;
-		} else if (btf_ext && strcmp(name, BTF_EXT_ELF_SEC) == 0) {
-			btf_ext_data = elf_getdata(scn, 0);
-			if (!btf_ext_data) {
-				pr_warn("failed to get section(%d, %s) data from %s\n",
-					idx, name, path);
-				goto done;
-			}
+
+		if (strcmp(name, BTF_ELF_SEC) == 0)
+			field = &secs->btf_data;
+		else if (strcmp(name, BTF_EXT_ELF_SEC) == 0)
+			field = &secs->btf_ext_data;
+		else if (strcmp(name, BTF_BASE_ELF_SEC) == 0)
+			field = &secs->btf_base_data;
+		else
 			continue;
+
+		data = elf_getdata(scn, 0);
+		if (!data) {
+			pr_warn("failed to get section(%d, %s) data from %s\n",
+				idx, name, path);
+			goto err;
 		}
+		*field = data;
+	}
+
+	return 0;
+
+err:
+	return -LIBBPF_ERRNO__FORMAT;
+}
+
+static struct btf *btf_parse_elf(const char *path, struct btf *base_btf,
+				 struct btf_ext **btf_ext)
+{
+	struct btf_elf_secs secs = {};
+	struct btf *dist_base_btf = NULL;
+	struct btf *btf = NULL;
+	int err = 0, fd = -1;
+	Elf *elf = NULL;
+
+	if (elf_version(EV_CURRENT) == EV_NONE) {
+		pr_warn("failed to init libelf for %s\n", path);
+		return ERR_PTR(-LIBBPF_ERRNO__LIBELF);
+	}
+
+	fd = open(path, O_RDONLY | O_CLOEXEC);
+	if (fd < 0) {
+		err = -errno;
+		pr_warn("failed to open %s: %s\n", path, strerror(errno));
+		return ERR_PTR(err);
 	}
 
-	if (!btf_data) {
+	elf = elf_begin(fd, ELF_C_READ, NULL);
+	if (!elf) {
+		pr_warn("failed to open %s as ELF file\n", path);
+		goto done;
+	}
+
+	err = btf_find_elf_sections(elf, path, &secs);
+	if (err)
+		goto done;
+
+	if (!secs.btf_data) {
 		pr_warn("failed to find '%s' ELF section in %s\n", BTF_ELF_SEC, path);
 		err = -ENODATA;
 		goto done;
 	}
-	btf = btf_new(btf_data->d_buf, btf_data->d_size, base_btf);
-	err = libbpf_get_error(btf);
-	if (err)
+
+	if (secs.btf_base_data) {
+		dist_base_btf = btf_new(secs.btf_base_data->d_buf, secs.btf_base_data->d_size,
+					NULL);
+		if (IS_ERR(dist_base_btf)) {
+			err = PTR_ERR(dist_base_btf);
+			dist_base_btf = NULL;
+			goto done;
+		}
+	}
+
+	btf = btf_new(secs.btf_data->d_buf, secs.btf_data->d_size,
+		      dist_base_btf ?: base_btf);
+	if (IS_ERR(btf)) {
+		err = PTR_ERR(btf);
 		goto done;
+	}
+	if (dist_base_btf && base_btf) {
+		err = btf__relocate(btf, base_btf);
+		if (err)
+			goto done;
+		btf__free(dist_base_btf);
+		dist_base_btf = NULL;
+	}
+
+	if (dist_base_btf)
+		btf->owns_base = true;
 
 	switch (gelf_getclass(elf)) {
 	case ELFCLASS32:
@@ -1187,11 +1237,12 @@ static struct btf *btf_parse_elf(const char *path, struct btf *base_btf,
 		break;
 	}
 
-	if (btf_ext && btf_ext_data) {
-		*btf_ext = btf_ext__new(btf_ext_data->d_buf, btf_ext_data->d_size);
-		err = libbpf_get_error(*btf_ext);
-		if (err)
+	if (btf_ext && secs.btf_ext_data) {
+		*btf_ext = btf_ext__new(secs.btf_ext_data->d_buf, secs.btf_ext_data->d_size);
+		if (IS_ERR(*btf_ext)) {
+			err = PTR_ERR(*btf_ext);
 			goto done;
+		}
 	} else if (btf_ext) {
 		*btf_ext = NULL;
 	}
@@ -1205,6 +1256,7 @@ static struct btf *btf_parse_elf(const char *path, struct btf *base_btf,
 
 	if (btf_ext)
 		btf_ext__free(*btf_ext);
+	btf__free(dist_base_btf);
 	btf__free(btf);
 
 	return ERR_PTR(err);
@@ -5598,5 +5650,9 @@ void btf_set_base_btf(struct btf *btf, const struct btf *base_btf)
 
 int btf__relocate(struct btf *btf, const struct btf *base_btf)
 {
-	return libbpf_err(btf_relocate(btf, base_btf, NULL));
+	int err = btf_relocate(btf, base_btf, NULL);
+
+	if (!err)
+		btf->owns_base = false;
+	return libbpf_err(err);
 }

tools/lib/bpf/btf.h

@@ -18,6 +18,7 @@ extern "C" {
 
 #define BTF_ELF_SEC ".BTF"
 #define BTF_EXT_ELF_SEC ".BTF.ext"
+#define BTF_BASE_ELF_SEC ".BTF.base"
 #define MAPS_ELF_SEC ".maps"
 
 struct btf;