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: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Song Liu <songliubraving@fb.com>
Link: https://lore.kernel.org/bpf/20211122235733.634914-3-memxor@gmail.com

diff --git a/tools/lib/bpf/gen_loader.c b/tools/lib/bpf/gen_loader.c
index c7bc77f..5a2d6bf 100644 (file)
--- a/tools/lib/bpf/gen_loader.c
+++ b/tools/lib/bpf/gen_loader.c
@@ -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 */