On Fri, Mar 19, 2021 at 9:23 AM Jiri Olsa [off-list ref] wrote:

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

SNIP

quoted

+
+     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:

I lost if (!obj->btf) return 0; somewhere along the rebases. I'll send
a fix shortly. But how did you end up with selftests BPF objects built
without BTF?

        (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

quoted

+     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