Re: [PATCH bpf-next v2 08/11] libbpf: support local function pointer relocation
From: Andrii Nakryiko <hidden>
Date: 2021-02-23 08:04:44
On Wed, Feb 17, 2021 at 12:56 PM Yonghong Song [off-list ref] wrote:
quoted hunk ↗ jump to hunk
A new relocation RELO_SUBPROG_ADDR is added to capture local (static) function pointers loaded with ld_imm64 insns. 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 <redacted> --- tools/lib/bpf/libbpf.c | 40 +++++++++++++++++++++++++++++++++++++--- 1 file changed, 37 insertions(+), 3 deletions(-)diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c index 21a3eedf070d..772c7455f1a2 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_SUBPROG_ADDR, }; struct reloc_desc {@@ -579,6 +580,11 @@ static bool is_ldimm64(struct bpf_insn *insn) return insn->code == (BPF_LD | BPF_IMM | BPF_DW); } +static bool insn_is_pseudo_func(struct bpf_insn *insn) +{ + return is_ldimm64(insn) && 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,@@ -3406,6 +3412,16 @@ static int bpf_program__record_reloc(struct bpf_program *prog, return -LIBBPF_ERRNO__RELOC; } + if (GELF_ST_BIND(sym->st_info) == STB_LOCAL && + GELF_ST_TYPE(sym->st_info) == STT_SECTION &&
STB_LOCAL + STT_SECTION is a section symbol. But STT_FUNC symbol could be referenced as well, no? So this is too strict.
+ (!shdr_idx || shdr_idx == obj->efile.text_shndx) &&
this doesn't look right, shdr_idx == 0 is a bad condition and should be rejected, not accepted.
+ !(sym->st_value % BPF_INSN_SZ)) {
+ reloc_desc->type = RELO_SUBPROG_ADDR;
+ reloc_desc->insn_idx = insn_idx;
+ reloc_desc->sym_off = sym->st_value;
+ return 0;
+ }
+
So see code right after sym_is_extern(sym) check. It checks for valid
shrd_idx, which is good and would be good to use that. After that we
can assume shdr_idx is valid and we can make a simple
obj->efile.text_shndx check then and use that as a signal that this is
SUBPROG_ADDR relocation (instead of deducing that from STT_SECTION).
And !(sym->st_value % BPF_INSN_SZ) should be reported as an error, not
silently skipped. Again, just how BPF_JMP | BPF_CALL does it. That way
it's more user-friendly, if something goes wrong. So it will look like
this:
if (shdr_idx == obj->efile.text_shndx) {
/* check sym->st_value, pr_warn(), return error */
reloc_desc->type = RELO_SUBPROG_ADDR;
...
return 0;
}
quoted hunk ↗ jump to hunk
if (sym_is_extern(sym)) { int sym_idx = GELF_R_SYM(rel->r_info); int i, n = obj->nr_extern;@@ -6172,6 +6188,10 @@ bpf_object__relocate_data(struct bpf_object *obj, struct bpf_program *prog) } relo->processed = true; break; + case RELO_SUBPROG_ADDR: + insn[0].src_reg = BPF_PSEUDO_FUNC;
BTW, doesn't Clang emit instruction with BPF_PSEUDO_FUNC set properly already? If not, why not?
quoted hunk ↗ jump to hunk
+ /* will be handled as a follow up pass */ + break; 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_SUBPROG_ADDR) { pr_warn("prog '%s': unexpected relo for insn #%zu, type %d\n", prog->name, insn_idx, relo->type); return -LIBBPF_ERRNO__RELOC;@@ -6374,8 +6394,22 @@ 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. + */ + 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; + } else if (insn_is_pseudo_func(insn)) { + /* + * RELO_SUBPROG_ADDR relo is always emitted even if both + * functions are in the same section, so it shouldn't reach here. */ - sub_insn_idx = relo->sym_off / BPF_INSN_SZ + insn->imm + 1; + pr_warn("prog '%s': missing relo for insn #%zu, type %d\n",
nit: "missing subprog addr relo" to make it clearer?
+ prog->name, insn_idx, relo->type);
+ return -LIBBPF_ERRNO__RELOC;
} else {
/* if subprogram call is to a static function within
* the same ELF section, there won't be any relocation
--
2.24.1