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libbpf: Avoid joining .BTF.ext data with BPF programs by section name

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Instead of using ELF section names as a joining key between .BTF.ext and
corresponding BPF programs, pre-build .BTF.ext section number to ELF
section index mapping during bpf_object__open() and use it later for
matching .BTF.ext information (func/line info or CO-RE relocations) to
their respective BPF programs and subprograms.

This simplifies corresponding joining logic and let's libbpf do
manipulations with BPF program's ELF sections like dropping leading '?'
character for non-autoloaded programs. Original joining logic in
bpf_object__relocate_core() (see relevant comment that's now removed)
was never elegant, so it's a good improvement regardless. But it also
avoids unnecessary internal assumptions about preserving original ELF
section name as BPF program's section name (which was broken when
SEC("?abc") support was added).

Fixes: a3820c481112 ("libbpf: Support opting out from autoloading BPF programs declaratively")
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20220426004511.2691730-5-andrii@kernel.org
This commit is contained in:
Andrii Nakryiko
2022-04-25 17:45:05 -07:00
committed by Andrii Nakryiko
parent eb22de1f7d
commit f2e994e0b7
3 changed files with 65 additions and 29 deletions
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78
src/libbpf.c
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@@ -2765,6 +2765,9 @@ static int bpf_object__init_btf(struct bpf_object *obj,
btf__set_pointer_size(obj->btf, 8);
}
if (btf_ext_data) {
struct btf_ext_info *ext_segs[3];
int seg_num, sec_num;
if (!obj->btf) {
pr_debug("Ignore ELF section %s because its depending ELF section %s is not found.\n",
BTF_EXT_ELF_SEC, BTF_ELF_SEC);
@@ -2778,6 +2781,43 @@ static int bpf_object__init_btf(struct bpf_object *obj,
obj->btf_ext = NULL;
goto out;
}
/* setup .BTF.ext to ELF section mapping */
ext_segs[0] = &obj->btf_ext->func_info;
ext_segs[1] = &obj->btf_ext->line_info;
ext_segs[2] = &obj->btf_ext->core_relo_info;
for (seg_num = 0; seg_num < ARRAY_SIZE(ext_segs); seg_num++) {
struct btf_ext_info *seg = ext_segs[seg_num];
const struct btf_ext_info_sec *sec;
const char *sec_name;
Elf_Scn *scn;
if (seg->sec_cnt == 0)
continue;
seg->sec_idxs = calloc(seg->sec_cnt, sizeof(*seg->sec_idxs));
if (!seg->sec_idxs) {
err = -ENOMEM;
goto out;
}
sec_num = 0;
for_each_btf_ext_sec(seg, sec) {
/* preventively increment index to avoid doing
* this before every continue below
*/
sec_num++;
sec_name = btf__name_by_offset(obj->btf, sec->sec_name_off);
if (str_is_empty(sec_name))
continue;
scn = elf_sec_by_name(obj, sec_name);
if (!scn)
continue;
seg->sec_idxs[sec_num - 1] = elf_ndxscn(scn);
}
}
}
out:
if (err && libbpf_needs_btf(obj)) {
@@ -5642,7 +5682,7 @@ bpf_object__relocate_core(struct bpf_object *obj, const char *targ_btf_path)
struct bpf_program *prog;
struct bpf_insn *insn;
const char *sec_name;
int i, err = 0, insn_idx, sec_idx;
int i, err = 0, insn_idx, sec_idx, sec_num;
if (obj->btf_ext->core_relo_info.len == 0)
return 0;
@@ -5663,33 +5703,18 @@ bpf_object__relocate_core(struct bpf_object *obj, const char *targ_btf_path)
}
seg = &obj->btf_ext->core_relo_info;
sec_num = 0;
for_each_btf_ext_sec(seg, sec) {
sec_idx = seg->sec_idxs[sec_num];
sec_num++;
sec_name = btf__name_by_offset(obj->btf, sec->sec_name_off);
if (str_is_empty(sec_name)) {
err = -EINVAL;
goto out;
}
/* bpf_object's ELF is gone by now so it's not easy to find
* section index by section name, but we can find *any*
* bpf_program within desired section name and use it's
* prog->sec_idx to do a proper search by section index and
* instruction offset
*/
prog = NULL;
for (i = 0; i < obj->nr_programs; i++) {
if (strcmp(obj->programs[i].sec_name, sec_name) == 0) {
prog = &obj->programs[i];
break;
}
}
if (!prog) {
pr_warn("sec '%s': failed to find a BPF program\n", sec_name);
return -ENOENT;
}
sec_idx = prog->sec_idx;
pr_debug("sec '%s': found %d CO-RE relocations\n",
sec_name, sec->num_info);
pr_debug("sec '%s': found %d CO-RE relocations\n", sec_name, sec->num_info);
for_each_btf_ext_rec(seg, sec, i, rec) {
if (rec->insn_off % BPF_INSN_SZ)
@@ -5873,14 +5898,13 @@ static int adjust_prog_btf_ext_info(const struct bpf_object *obj,
void *rec, *rec_end, *new_prog_info;
const struct btf_ext_info_sec *sec;
size_t old_sz, new_sz;
const char *sec_name;
int i, off_adj;
int i, sec_num, sec_idx, off_adj;
sec_num = 0;
for_each_btf_ext_sec(ext_info, sec) {
sec_name = btf__name_by_offset(obj->btf, sec->sec_name_off);
if (!sec_name)
return -EINVAL;
if (strcmp(sec_name, prog->sec_name) != 0)
sec_idx = ext_info->sec_idxs[sec_num];
sec_num++;
if (prog->sec_idx != sec_idx)
continue;
for_each_btf_ext_rec(ext_info, sec, i, rec) {