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Tue, 9 Feb 2021 06:56:12 +0000 Subject: Re: [PATCH bpf-next 5/8] libbpf: support local function pointer relocation To: Andrii Nakryiko CC: bpf , Alexei Starovoitov , Cong Wang , Daniel Borkmann , Kernel Team References: <20210204234827.1628857-1-yhs@fb.com> <20210204234832.1629393-1-yhs@fb.com> From: Yonghong Song Message-ID: <48fbc3ee-739e-8baf-9165-77b7e92ab1e0@fb.com> Date: Mon, 8 Feb 2021 22:56:09 -0800 User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:78.0) Gecko/20100101 Thunderbird/78.7.1 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit X-Originating-IP: [2620:10d:c090:400::5:e222] X-ClientProxiedBy: MW4PR03CA0290.namprd03.prod.outlook.com (2603:10b6:303:b5::25) To BYAPR15MB4088.namprd15.prod.outlook.com (2603:10b6:a02:c3::18) MIME-Version: 1.0 X-MS-Exchange-MessageSentRepresentingType: 1 Received: from [IPv6:2620:10d:c085:21d6::1610] (2620:10d:c090:400::5:e222) by MW4PR03CA0290.namprd03.prod.outlook.com (2603:10b6:303:b5::25) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.20.3825.21 via Frontend Transport; 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Such ld_imm64 insns are marked with >> BPF_PSEUDO_FUNC and will be passed to kernel so >> kernel can replace them with proper actual jited >> func addresses. >> >> Signed-off-by: Yonghong Song >> --- >> tools/lib/bpf/libbpf.c | 33 ++++++++++++++++++++++++++++++--- >> 1 file changed, 30 insertions(+), 3 deletions(-) >> >> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c >> index 2abbc3800568..a5146c9e3e06 100644 >> --- a/tools/lib/bpf/libbpf.c >> +++ b/tools/lib/bpf/libbpf.c >> @@ -188,6 +188,7 @@ enum reloc_type { >> RELO_CALL, >> RELO_DATA, >> RELO_EXTERN, >> + RELO_LOCAL_FUNC, > > libbpf internally is using SUBPROG notation. I think "LOCAL" part is > confusing, so I'd drop it. How about just RELO_SUBPROG? We can > separately refactor these names to distinguish RELO_CALL from the new > one. It would be more clear if RELO_CALL was called RELO_SUBPROG_CALL, > and the new one either RELO_SUBPROG_ADDR or RELO_SUBPROG_REF (as in > subprog reference) Yes, we can use RELO_SUBPROG_ADDR. > >> }; >> >> struct reloc_desc { >> @@ -574,6 +575,12 @@ static bool insn_is_subprog_call(const struct bpf_insn *insn) >> insn->off == 0; >> } >> >> +static bool insn_is_pseudo_func(const struct bpf_insn *insn) >> +{ >> + return insn->code == (BPF_LD | BPF_IMM | BPF_DW) && > > there is is_ldimm64() function for this check (just move it up here, > it's a single-liner) I did not know it. Will use in the next revision. > >> + insn->src_reg == BPF_PSEUDO_FUNC; >> +} >> + >> static int >> bpf_object__init_prog(struct bpf_object *obj, struct bpf_program *prog, >> const char *name, size_t sec_idx, const char *sec_name, >> @@ -3395,6 +3402,16 @@ static int bpf_program__record_reloc(struct bpf_program *prog, >> return 0; >> } >> >> + if (insn->code == (BPF_LD | BPF_IMM | BPF_DW) && > > just move this check below the next if that checks !is_ldimm64, no > need to do it here early. Okay. > >> + GELF_ST_BIND(sym->st_info) == STB_LOCAL && >> + GELF_ST_TYPE(sym->st_info) == STT_SECTION && >> + shdr_idx == obj->efile.text_shndx) { > > see above how RELO_CALL is handled: shdr_idx != 0 check is missing. We > also validate that sym->st_value is multiple of BPF_INSN_SZ. Okay. Will add additional checking. > >> + reloc_desc->type = RELO_LOCAL_FUNC; >> + reloc_desc->insn_idx = insn_idx; >> + reloc_desc->sym_off = sym->st_value; >> + return 0; >> + } >> + >> if (insn->code != (BPF_LD | BPF_IMM | BPF_DW)) { > > feel free to use is_ldimm64 here as well, thanks! > >> pr_warn("prog '%s': invalid relo against '%s' for insns[%d].code 0x%x\n", >> prog->name, sym_name, insn_idx, insn->code); >> @@ -6172,6 +6189,9 @@ bpf_object__relocate_data(struct bpf_object *obj, struct bpf_program *prog) >> } >> relo->processed = true; >> break; >> + case RELO_LOCAL_FUNC: >> + insn[0].src_reg = BPF_PSEUDO_FUNC; >> + /* fallthrough */ > > fallthrough into an empty break clause seems a bit weird... just break > and leave the same comment as below? Yes, "break" seems cleaner. > >> case RELO_CALL: >> /* will be handled as a follow up pass */ >> break; >> @@ -6358,11 +6378,11 @@ bpf_object__reloc_code(struct bpf_object *obj, struct bpf_program *main_prog, >> >> for (insn_idx = 0; insn_idx < prog->sec_insn_cnt; insn_idx++) { >> insn = &main_prog->insns[prog->sub_insn_off + insn_idx]; >> - if (!insn_is_subprog_call(insn)) >> + if (!insn_is_subprog_call(insn) && !insn_is_pseudo_func(insn)) >> continue; >> >> relo = find_prog_insn_relo(prog, insn_idx); >> - if (relo && relo->type != RELO_CALL) { >> + if (relo && relo->type != RELO_CALL && relo->type != RELO_LOCAL_FUNC) { >> pr_warn("prog '%s': unexpected relo for insn #%zu, type %d\n", >> prog->name, insn_idx, relo->type); >> return -LIBBPF_ERRNO__RELOC; >> @@ -6374,8 +6394,15 @@ bpf_object__reloc_code(struct bpf_object *obj, struct bpf_program *main_prog, >> * call always has imm = -1, but for static functions >> * relocation is against STT_SECTION and insn->imm >> * points to a start of a static function >> + * >> + * for local func relocation, the imm field encodes >> + * the byte offset in the corresponding section. >> */ >> - sub_insn_idx = relo->sym_off / BPF_INSN_SZ + insn->imm + 1; >> + if (relo->type == RELO_CALL) >> + sub_insn_idx = relo->sym_off / BPF_INSN_SZ + insn->imm + 1; >> + else >> + sub_insn_idx = relo->sym_off / BPF_INSN_SZ + >> + insn->imm / BPF_INSN_SZ + 1; > > nit: keep it on a single line, it still fits within 100 characters and > is easier to visually compare to RELO_CALL case. Okay. > >> } else { >> /* if subprogram call is to a static function within >> * the same ELF section, there won't be any relocation > > don't we have to adjust insn->imm for this case as well? Let's add > selftests to make sure this works. This is for relo == NULL. I think my code (RELO_LOCAL_FUNC or RELO_SUBPROG_ADDR) won't hit this since there always relocations. That is why I didn't do anything here. > >> -- >> 2.24.1 >>