Commit 53eddb5e authored by Yonghong Song's avatar Yonghong Song Committed by Alexei Starovoitov
Browse files

libbpf: Support subprog address relocation 



A new relocation RELO_SUBPROG_ADDR is added to capture
subprog addresses loaded with ld_imm64 insns. Such ld_imm64
insns are marked with BPF_PSEUDO_FUNC and will be passed to
kernel. For bpf_for_each_map_elem() case, kernel will
check that the to-be-used subprog address must be a static
function and replace it with proper actual jited func address.

Signed-off-by: default avatarYonghong Song <yhs@fb.com>
Signed-off-by: default avatarAlexei Starovoitov <ast@kernel.org>
Acked-by: default avatarAndrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20210226204930.3885367-1-yhs@fb.com
parent b8f871fa

tools/lib/bpf/libbpf.c

+61 −3
Original line number Diff line number Diff line
@@ -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,
@@ -2979,6 +2985,23 @@ static bool sym_is_extern(const GElf_Sym *sym) 
	       GELF_ST_TYPE(sym->st_info) == STT_NOTYPE;

}



static bool sym_is_subprog(const GElf_Sym *sym, int text_shndx)

{

	int bind = GELF_ST_BIND(sym->st_info);

	int type = GELF_ST_TYPE(sym->st_info);



	/* in .text section */

	if (sym->st_shndx != text_shndx)

		return false;



	/* local function */

	if (bind == STB_LOCAL && type == STT_SECTION)

		return true;



	/* global function */

	return bind == STB_GLOBAL && type == STT_FUNC;

}



static int find_extern_btf_id(const struct btf *btf, const char *ext_name)

{

	const struct btf_type *t;
@@ -3435,6 +3458,23 @@ static int bpf_program__record_reloc(struct bpf_program *prog, 
		return -LIBBPF_ERRNO__RELOC;

	}



	/* loading subprog addresses */

	if (sym_is_subprog(sym, obj->efile.text_shndx)) {

		/* global_func: sym->st_value = offset in the section, insn->imm = 0.

		 * local_func: sym->st_value = 0, insn->imm = offset in the section.

		 */

		if ((sym->st_value % BPF_INSN_SZ) || (insn->imm % BPF_INSN_SZ)) {

			pr_warn("prog '%s': bad subprog addr relo against '%s' at offset %zu+%d\n",

				prog->name, sym_name, (size_t)sym->st_value, insn->imm);

			return -LIBBPF_ERRNO__RELOC;

		}



		reloc_desc->type = RELO_SUBPROG_ADDR;

		reloc_desc->insn_idx = insn_idx;

		reloc_desc->sym_off = sym->st_value;

		return 0;

	}



	type = bpf_object__section_to_libbpf_map_type(obj, shdr_idx);

	sym_sec_name = elf_sec_name(obj, elf_sec_by_idx(obj, shdr_idx));
@@ -6172,6 +6212,10 @@ 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;

			/* will be handled as a follow up pass */

			break;

		case RELO_CALL:

			/* will be handled as a follow up pass */

			break;
@@ -6358,11 +6402,11 @@ 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 +6418,22 @@ 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 subprog addr relocation, the relo->sym_off + insn->imm is

			 * 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 + insn->imm) / BPF_INSN_SZ;

		} 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.

			 */

			pr_warn("prog '%s': missing subprog addr relo for insn #%zu\n",

				prog->name, insn_idx);

			return -LIBBPF_ERRNO__RELOC;

		} else {

			/* if subprogram call is to a static function within

			 * the same ELF section, there won't be any relocation