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libbpf: Support CO-RE relocations for multi-prog sections

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Fix up CO-RE relocation code to handle relocations against ELF sections
containing multiple BPF programs. This requires lookup of a BPF program by its
section name and instruction index it contains. While it could have been done
as a simple loop, it could run into performance issues pretty quickly, as
number of CO-RE relocations can be quite large in real-world applications, and
each CO-RE relocation incurs BPF program look up now. So instead of simple
loop, implement a binary search by section name + insn offset.

Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Link: https://lore.kernel.org/bpf/20200903203542.15944-4-andriin@fb.com
This commit is contained in:
Andrii Nakryiko 2020-09-03 13:35:31 -07:00 committed by Alexei Starovoitov
parent c112239272
commit db2b8b0642
1 changed files with 74 additions and 8 deletions
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82
tools/lib/bpf/libbpf.c
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@ -2753,6 +2753,18 @@ static bool ignore_elf_section(GElf_Shdr *hdr, const char *name)
return false;
}
static int cmp_progs(const void *_a, const void *_b)
{
const struct bpf_program *a = _a;
const struct bpf_program *b = _b;
if (a->sec_idx != b->sec_idx)
return a->sec_idx < b->sec_idx ? -1 : 1;
/* sec_insn_off can't be the same within the section */
return a->sec_insn_off < b->sec_insn_off ? -1 : 1;
}
static int bpf_object__elf_collect(struct bpf_object *obj)
{
Elf *elf = obj->efile.elf;
@ -2887,6 +2899,11 @@ static int bpf_object__elf_collect(struct bpf_object *obj)
pr_warn("elf: symbol strings section missing or invalid in %s\n", obj->path);
return -LIBBPF_ERRNO__FORMAT;
}
/* sort BPF programs by section name and in-section instruction offset
* for faster search */
qsort(obj->programs, obj->nr_programs, sizeof(*obj->programs), cmp_progs);
return bpf_object__init_btf(obj, btf_data, btf_ext_data);
}
@ -3415,6 +3432,37 @@ static int bpf_program__record_reloc(struct bpf_program *prog,
return 0;
}
static bool prog_contains_insn(const struct bpf_program *prog, size_t insn_idx)
{
return insn_idx >= prog->sec_insn_off &&
insn_idx < prog->sec_insn_off + prog->sec_insn_cnt;
}
static struct bpf_program *find_prog_by_sec_insn(const struct bpf_object *obj,
size_t sec_idx, size_t insn_idx)
{
int l = 0, r = obj->nr_programs - 1, m;
struct bpf_program *prog;
while (l < r) {
m = l + (r - l + 1) / 2;
prog = &obj->programs[m];
if (prog->sec_idx < sec_idx ||
(prog->sec_idx == sec_idx && prog->sec_insn_off <= insn_idx))
l = m;
else
r = m - 1;
}
/* matching program could be at index l, but it still might be the
* wrong one, so we need to double check conditions for the last time
*/
prog = &obj->programs[l];
if (prog->sec_idx == sec_idx && prog_contains_insn(prog, insn_idx))
return prog;
return NULL;
}
static int
bpf_program__collect_reloc(struct bpf_program *prog, GElf_Shdr *shdr,
Elf_Data *data, struct bpf_object *obj)
@ -5229,6 +5277,11 @@ static int bpf_core_patch_insn(struct bpf_program *prog,
if (relo->insn_off % BPF_INSN_SZ)
return -EINVAL;
insn_idx = relo->insn_off / BPF_INSN_SZ;
/* adjust insn_idx from section frame of reference to the local
* program's frame of reference; (sub-)program code is not yet
* relocated, so it's enough to just subtract in-section offset
*/
insn_idx = insn_idx - prog->sec_insn_off;
insn = &prog->insns[insn_idx];
class = BPF_CLASS(insn->code);
@ -5619,7 +5672,7 @@ bpf_object__relocate_core(struct bpf_object *obj, const char *targ_btf_path)
struct bpf_program *prog;
struct btf *targ_btf;
const char *sec_name;
int i, err = 0;
int i, err = 0, insn_idx, sec_idx;
if (obj->btf_ext->core_relo_info.len == 0)
return 0;
@ -5646,24 +5699,37 @@ bpf_object__relocate_core(struct bpf_object *obj, const char *targ_btf_path)
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].section_name, sec_name)) {
prog = &obj->programs[i];
prog = &obj->programs[i];
if (strcmp(prog->section_name, sec_name) == 0)
break;
}
}
if (!prog) {
pr_warn("failed to find program '%s' for CO-RE offset relocation\n",
sec_name);
err = -EINVAL;
goto out;
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);
for_each_btf_ext_rec(seg, sec, i, rec) {
insn_idx = rec->insn_off / BPF_INSN_SZ;
prog = find_prog_by_sec_insn(obj, sec_idx, insn_idx);
if (!prog) {
pr_warn("sec '%s': failed to find program at insn #%d for CO-RE offset relocation #%d\n",
sec_name, insn_idx, i);
err = -EINVAL;
goto out;
}
err = bpf_core_apply_relo(prog, rec, i, obj->btf,
targ_btf, cand_cache);
if (err) {