Re: [PATCH v4 bpf-next 07/12] libbpf: add BPF static linker BTF and BTF.ext support

On Thu, Mar 18, 2021 at 12:40:31PM -0700, Andrii Nakryiko wrote:

SNIP

+
+	return NULL;
+}
+
+static int linker_fixup_btf(struct src_obj *obj)
+{
+	const char *sec_name;
+	struct src_sec *sec;
+	int i, j, n, m;
+
+	n = btf__get_nr_types(obj->btf);

hi,
I'm getting bpftool crash when building tests,

looks like above obj->btf can be NULL:

	(gdb) r gen object /home/jolsa/linux/tools/testing/selftests/bpf/btf_data2.linked1.o /home/jolsa/linux/tools/testing/selftests/bpf/btf_data2.o
	Starting program: /home/jolsa/linux/tools/testing/selftests/bpf/tools/sbin/bpftool gen object /home/jolsa/linux/tools/testing/selftests/bpf/btf_data2.linked1.o /home/jolsa/linux/tools/testing/selftests/bpf/btf_data2.o

	Program received signal SIGSEGV, Segmentation fault.
	btf__get_nr_types (btf=0x0) at btf.c:425

	425             return btf->start_id + btf->nr_types - 1;
	Missing separate debuginfos, use: dnf debuginfo-install elfutils-libelf-0.182-1.fc33.x86_64 libcap-2.48-2.fc33.x86_64 zlib-1.2.11-23.fc33.x86_64
	(gdb) 
	(gdb) bt
	#0  btf__get_nr_types (btf=0x0) at btf.c:425
	#1  0x000000000043c4ba in linker_fixup_btf (obj=obj@entry=0x7fffffffda50) at linker.c:1316
	#2  0x000000000043caf7 in linker_load_obj_file (linker=linker@entry=0x8612a0, filename=filename@entry=0x7fffffffe0db "/home/jolsa/linux/tools/testing/selftests/bpf/btf_data2.o", 
	    obj=obj@entry=0x7fffffffda50) at linker.c:653
	#3  0x000000000043df43 in bpf_linker__add_file (linker=linker@entry=0x8612a0, filename=filename@entry=0x7fffffffe0db "/home/jolsa/linux/tools/testing/selftests/bpf/btf_data2.o") at linker.c:412
	#4  0x000000000040d710 in do_object (argc=0, argv=0x7fffffffdce0) at gen.c:639
	#5  0x000000000040efd7 in cmd_select (cmds=cmds@entry=0x50a600 <cmds>, argc=3, argv=0x7fffffffdcc8, help=help@entry=0x40c6f7 <do_help>) at main.c:134
	#6  0x000000000040d7c4 in do_gen (argc=<optimized out>, argv=<optimized out>) at gen.c:686
	#7  0x000000000040efd7 in cmd_select (cmds=cmds@entry=0x50b400 <cmds>, argc=4, argv=0x7fffffffdcc0, help=help@entry=0x40ee93 <do_help>) at main.c:134
	#8  0x000000000040f88e in main (argc=<optimized out>, argv=<optimized out>) at main.c:469

I'm on current bpf-next/master


jirka

+	for (i = 1; i <= n; i++) {
+		struct btf_var_secinfo *vi;
+		struct btf_type *t;
+
+		t = btf_type_by_id(obj->btf, i);
+		if (btf_kind(t) != BTF_KIND_DATASEC)
+			continue;
+
+		sec_name = btf__str_by_offset(obj->btf, t->name_off);
+		sec = find_src_sec_by_name(obj, sec_name);
+		if (sec) {
+			/* record actual section size, unless ephemeral */
+			if (sec->shdr)
+				t->size = sec->shdr->sh_size;
+		} else {
+			/* BTF can have some sections that are not represented
+			 * in ELF, e.g., .kconfig and .ksyms, which are used
+			 * for special extern variables.  Here we'll
+			 * pre-create "section shells" for them to be able to
+			 * keep track of extra per-section metadata later
+			 * (e.g., BTF variables).
+			 */
+			sec = add_src_sec(obj, sec_name);
+			if (!sec)
+				return -ENOMEM;
+
+			sec->ephemeral = true;
+			sec->sec_idx = 0; /* will match UNDEF shndx in ELF */
+		}
+
+		/* remember ELF section and its BTF type ID match */
+		sec->sec_type_id = i;
+
+		/* fix up variable offsets */
+		vi = btf_var_secinfos(t);
+		for (j = 0, m = btf_vlen(t); j < m; j++, vi++) {
+			const struct btf_type *vt = btf__type_by_id(obj->btf, vi->type);
+			const char *var_name = btf__str_by_offset(obj->btf, vt->name_off);
+			int var_linkage = btf_var(vt)->linkage;
+			Elf64_Sym *sym;
+
+			/* no need to patch up static or extern vars */
+			if (var_linkage != BTF_VAR_GLOBAL_ALLOCATED)
+				continue;
+
+			sym = find_sym_by_name(obj, sec->sec_idx, STT_OBJECT, var_name);
+			if (!sym) {
+				pr_warn("failed to find symbol for variable '%s' in section '%s'\n", var_name, sec_name);
+				return -ENOENT;
+			}
+
+			vi->offset = sym->st_value;
+		}
+	}
+
+	return 0;
+}

SNIP