With patch set [1], precision backtracing supports register spill/fill to/from the stack. The patch [2] allows initial imprecise register spill with content 0. This is a common case for cpuv3 and lower for initializing the stack variables with pattern r1 = 0 *(u64 *)(r10 - 8) = r1 and the [2] has demonstrated good verification improvement. For cpuv4, the initialization could be *(u64 *)(r10 - 8) = 0 The current verifier marks the r10-8 contents with STACK_ZERO. Similar to [2], let us permit the above insn to behave like imprecise register spill which can reduce number of verified states. I checked cpuv3 and cpuv4 with and without this patch. There is no change for cpuv3 since '*(u64 *)(r10 - 8) = 0' is only generated with cpuv4. For cpuv4: $ ../veristat -C old.cpuv4.csv new.cpuv4.csv -e file,prog,insns,states -s '|insns_diff|' File Program Insns (A) Insns (B) Insns (DIFF) States (A) States (B) States (DIFF) ----------------------------------------------------- ---------------------------------------------------- --------- --------- --------------- ---------- ---------- ------------- pyperf600_bpf_loop.bpf.linked3.o on_event 6066 4889 -1177 (-19.40%) 403 321 -82 (-20.35%) xdp_synproxy_kern.bpf.linked3.o syncookie_tc 12412 11719 -693 (-5.58%) 345 330 -15 (-4.35%) xdp_synproxy_kern.bpf.linked3.o syncookie_xdp 12478 11794 -684 (-5.48%) 346 331 -15 (-4.34%) test_cls_redirect.bpf.linked3.o cls_redirect 35483 35387 -96 (-0.27%) 2179 2177 -2 (-0.09%) local_storage_bench.bpf.linked3.o get_local 228 168 -60 (-26.32%) 17 14 -3 (-17.65%) test_l4lb_noinline.bpf.linked3.o balancer_ingress 4494 4522 +28 (+0.62%) 217 219 +2 (+0.92%) test_l4lb_noinline_dynptr.bpf.linked3.o balancer_ingress 1432 1455 +23 (+1.61%) 92 94 +2 (+2.17%) verifier_iterating_callbacks.bpf.linked3.o widening 52 41 -11 (-21.15%) 4 3 -1 (-25.00%) test_xdp_noinline.bpf.linked3.o balancer_ingress_v6 3462 3458 -4 (-0.12%) 216 216 +0 (+0.00%) ... test_l4lb_noinline and test_l4lb_noinline_dynptr has minor regression, but pyperf600_bpf_loop and local_storage_bench gets pretty good improvement. [1] https://lore.kernel.org/all/20231205184248.1502704-1-andrii@xxxxxxxxxx/ [2] https://lore.kernel.org/all/20231205184248.1502704-9-andrii@xxxxxxxxxx/ Cc: Kuniyuki Iwashima <kuniyu@xxxxxxxxxx> Cc: Martin KaFai Lau <kafai@xxxxxx> Signed-off-by: Yonghong Song <yonghong.song@xxxxxxxxx> --- kernel/bpf/verifier.c | 2 +- tools/testing/selftests/bpf/progs/verifier_spill_fill.c | 4 ++-- 2 files changed, 3 insertions(+), 3 deletions(-) diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c index a376eb609c41..17ad0228270e 100644 --- a/kernel/bpf/verifier.c +++ b/kernel/bpf/verifier.c @@ -4491,7 +4491,7 @@ static int check_stack_write_fixed_off(struct bpf_verifier_env *env, if (fls64(reg->umax_value) > BITS_PER_BYTE * size) state->stack[spi].spilled_ptr.id = 0; } else if (!reg && !(off % BPF_REG_SIZE) && is_bpf_st_mem(insn) && - insn->imm != 0 && env->bpf_capable) { + env->bpf_capable) { struct bpf_reg_state fake_reg = {}; __mark_reg_known(&fake_reg, insn->imm); diff --git a/tools/testing/selftests/bpf/progs/verifier_spill_fill.c b/tools/testing/selftests/bpf/progs/verifier_spill_fill.c index 39fe3372e0e0..05de3de56e79 100644 --- a/tools/testing/selftests/bpf/progs/verifier_spill_fill.c +++ b/tools/testing/selftests/bpf/progs/verifier_spill_fill.c @@ -496,13 +496,13 @@ SEC("raw_tp") __log_level(2) __success /* make sure fp-8 is all STACK_ZERO */ -__msg("2: (7a) *(u64 *)(r10 -8) = 0 ; R10=fp0 fp-8_w=00000000") +__msg("2: (7a) *(u64 *)(r10 -8) = 0 ; R10=fp0 fp-8_w=0") /* but fp-16 is spilled IMPRECISE zero const reg */ __msg("4: (7b) *(u64 *)(r10 -16) = r0 ; R0_w=0 R10=fp0 fp-16_w=0") /* validate that assigning R2 from STACK_ZERO doesn't mark register * precise immediately; if necessary, it will be marked precise later */ -__msg("6: (71) r2 = *(u8 *)(r10 -1) ; R2_w=0 R10=fp0 fp-8_w=00000000") +__msg("6: (71) r2 = *(u8 *)(r10 -1) ; R2_w=0 R10=fp0 fp-8_w=0") /* similarly, when R2 is assigned from spilled register, it is initially * imprecise, but will be marked precise later once it is used in precise context */ -- 2.34.1
