Commit 0270090d authored by Kumar Kartikeya Dwivedi's avatar Kumar Kartikeya Dwivedi Committed by Andrii Nakryiko
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libbpf: Avoid double stores for success/failure case of ksym relocations 



Instead, jump directly to success case stores in case ret >= 0, else do
the default 0 value store and jump over the success case. This is better
in terms of readability. Readjust the code for kfunc relocation as well
to follow a similar pattern, also leads to easier to follow code now.

Suggested-by: default avatarAlexei Starovoitov <ast@kernel.org>
Signed-off-by: default avatarKumar Kartikeya Dwivedi <memxor@gmail.com>
Signed-off-by: default avatarAndrii Nakryiko <andrii@kernel.org>
Acked-by: default avatarSong Liu <songliubraving@fb.com>
Link: https://lore.kernel.org/bpf/20211122235733.634914-3-memxor@gmail.com
parent d4efb170

tools/lib/bpf/gen_loader.c

+21 −16
Original line number Diff line number Diff line
@@ -674,27 +674,29 @@ static void emit_relo_kfunc_btf(struct bpf_gen *gen, struct ksym_relo_desc *relo 
		return;

	}

	kdesc->off = btf_fd_idx;

	/* set a default value for imm */

	/* jump to success case */

	emit(gen, BPF_JMP_IMM(BPF_JSGE, BPF_REG_7, 0, 3));

	/* set value for imm, off as 0 */

	emit(gen, BPF_ST_MEM(BPF_W, BPF_REG_8, offsetof(struct bpf_insn, imm), 0));

	/* skip success case store if ret < 0 */

	emit(gen, BPF_JMP_IMM(BPF_JSLT, BPF_REG_7, 0, 1));

	emit(gen, BPF_ST_MEM(BPF_H, BPF_REG_8, offsetof(struct bpf_insn, off), 0));

	/* skip success case for ret < 0 */

	emit(gen, BPF_JMP_IMM(BPF_JA, 0, 0, 10));

	/* store btf_id into insn[insn_idx].imm */

	emit(gen, BPF_STX_MEM(BPF_W, BPF_REG_8, BPF_REG_7, offsetof(struct bpf_insn, imm)));

	/* obtain fd in BPF_REG_9 */

	emit(gen, BPF_MOV64_REG(BPF_REG_9, BPF_REG_7));

	emit(gen, BPF_ALU64_IMM(BPF_RSH, BPF_REG_9, 32));

	/* jump to fd_array store if fd denotes module BTF */

	emit(gen, BPF_JMP_IMM(BPF_JNE, BPF_REG_9, 0, 2));

	/* set the default value for off */

	emit(gen, BPF_ST_MEM(BPF_H, BPF_REG_8, offsetof(struct bpf_insn, off), 0));

	/* skip BTF fd store for vmlinux BTF */

	emit(gen, BPF_JMP_IMM(BPF_JA, 0, 0, 4));

	/* load fd_array slot pointer */

	emit2(gen, BPF_LD_IMM64_RAW_FULL(BPF_REG_0, BPF_PSEUDO_MAP_IDX_VALUE,

					 0, 0, 0, blob_fd_array_off(gen, btf_fd_idx)));

	/* skip store of BTF fd if ret < 0 */

	emit(gen, BPF_JMP_IMM(BPF_JSLT, BPF_REG_7, 0, 3));

	/* store BTF fd in slot */

	emit(gen, BPF_MOV64_REG(BPF_REG_9, BPF_REG_7));

	emit(gen, BPF_ALU64_IMM(BPF_RSH, BPF_REG_9, 32));

	emit(gen, BPF_STX_MEM(BPF_W, BPF_REG_0, BPF_REG_9, 0));

	/* set a default value for off */

	emit(gen, BPF_ST_MEM(BPF_H, BPF_REG_8, offsetof(struct bpf_insn, off), 0));

	/* skip insn->off store if ret < 0 */

	emit(gen, BPF_JMP_IMM(BPF_JSLT, BPF_REG_7, 0, 2));

	/* skip if vmlinux BTF */

	emit(gen, BPF_JMP_IMM(BPF_JEQ, BPF_REG_9, 0, 1));

	/* store index into insn[insn_idx].off */

	emit(gen, BPF_ST_MEM(BPF_H, BPF_REG_8, offsetof(struct bpf_insn, off), btf_fd_idx));

log:
@@ -803,17 +805,20 @@ static void emit_relo_ksym_btf(struct bpf_gen *gen, struct ksym_relo_desc *relo, 
	emit_bpf_find_by_name_kind(gen, relo);

	if (!relo->is_weak)

		emit_check_err(gen);

	/* set default values as 0 */

	/* jump to success case */

	emit(gen, BPF_JMP_IMM(BPF_JSGE, BPF_REG_7, 0, 3));

	/* set values for insn[insn_idx].imm, insn[insn_idx + 1].imm as 0 */

	emit(gen, BPF_ST_MEM(BPF_W, BPF_REG_8, offsetof(struct bpf_insn, imm), 0));

	emit(gen, BPF_ST_MEM(BPF_W, BPF_REG_8, sizeof(struct bpf_insn) + offsetof(struct bpf_insn, imm), 0));

	/* skip success case stores if ret < 0 */

	emit(gen, BPF_JMP_IMM(BPF_JSLT, BPF_REG_7, 0, 4));

	/* skip success case for ret < 0 */

	emit(gen, BPF_JMP_IMM(BPF_JA, 0, 0, 4));

	/* store btf_id into insn[insn_idx].imm */

	emit(gen, BPF_STX_MEM(BPF_W, BPF_REG_8, BPF_REG_7, offsetof(struct bpf_insn, imm)));

	/* store btf_obj_fd into insn[insn_idx + 1].imm */

	emit(gen, BPF_ALU64_IMM(BPF_RSH, BPF_REG_7, 32));

	emit(gen, BPF_STX_MEM(BPF_W, BPF_REG_8, BPF_REG_7,

			      sizeof(struct bpf_insn) + offsetof(struct bpf_insn, imm)));

	/* skip src_reg adjustment */

	emit(gen, BPF_JMP_IMM(BPF_JSGE, BPF_REG_7, 0, 3));

clear_src_reg:

	/* clear bpf_object__relocate_data's src_reg assignment, otherwise we get a verifier failure */