From: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Date: Tue, 23 Nov 2021 05:27:32 +0530
Subject: libbpf: Avoid double stores for success/failure case of ksym
relocations
Patch-mainline: v5.17-rc1
Git-commit: 0270090d396a8e7e7f42adae13fdfa48ffb85144
References: jsc#PED-1368
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
Acked-by: Shung-Hsi Yu <shung-hsi.yu@suse.com>
---
tools/lib/bpf/gen_loader.c | 37 +++++++++++++++++++++----------------
1 file changed, 21 insertions(+), 16 deletions(-)
--- a/tools/lib/bpf/gen_loader.c
+++ b/tools/lib/bpf/gen_loader.c
@@ -687,27 +687,29 @@ static void emit_relo_kfunc_btf(struct b
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:
@@ -816,17 +818,20 @@ static void emit_relo_ksym_btf(struct bp
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 */