From: Yonghong Song <yonghong.song@xxxxxxxxx> Date: Tue, 9 Jan 2024 21:13:48 -0800 > 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. > The change is in function check_stack_write_fixed_off(). > > Before this patch, spilled zero will be marked as STACK_ZERO > which can provide precise values. In check_stack_write_var_off(), > STACK_ZERO will be maintained if writing a const zero > so later it can provide precise values if needed. > > The above handling of '*(u64 *)(r10 - 8) = 0' as a spill > will have issues in check_stack_write_var_off() as the spill > will be converted to STACK_MISC and the precise value 0 > is lost. To fix this issue, if the spill slots with const > zero and the BPF_ST write also with const zero, the spill slots > are preserved, which can later provide precise values > if needed. Without the change in check_stack_write_var_off(), > the test_verifier subtest 'BPF_ST_MEM stack imm zero, variable offset' > will fail. > > I checked cpuv3 and cpuv4 with and without this patch with veristat. > There is no state 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 -f 'insns_diff!=0' > File Program Insns (A) Insns (B) Insns (DIFF) States (A) States (B) States (DIFF) > ------------------------------------------ ------------------- --------- --------- --------------- ---------- ---------- ------------- > local_storage_bench.bpf.linked3.o get_local 228 168 -60 (-26.32%) 17 14 -3 (-17.65%) > pyperf600_bpf_loop.bpf.linked3.o on_event 6066 4889 -1177 (-19.40%) 403 321 -82 (-20.35%) > test_cls_redirect.bpf.linked3.o cls_redirect 35483 35387 -96 (-0.27%) 2179 2177 -2 (-0.09%) > 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%) > test_xdp_noinline.bpf.linked3.o balancer_ingress_v6 3462 3458 -4 (-0.12%) 216 216 +0 (+0.00%) > verifier_iterating_callbacks.bpf.linked3.o widening 52 41 -11 (-21.15%) 4 3 -1 (-25.00%) > 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_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> Tested-by: Kuniyuki Iwashima <kuniyu@xxxxxxxxxx> Thanks!
