On Wed, Jan 3, 2024 at 3:26 PM Yonghong Song <yonghong.song@xxxxxxxxx> wrote: > > 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> > --- > kernel/bpf/verifier.c | 21 +++++++++++++++++-- > .../selftests/bpf/progs/verifier_spill_fill.c | 16 +++++++------- > 2 files changed, 27 insertions(+), 10 deletions(-) > > Changelogs: > v1 -> v2: > - Preserve with-const-zero spill if writing is also zero > in check_stack_write_var_off(). > - Add a test with not-8-byte-aligned BPF_ST store. > > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c > index d4e31f61de0e..cfe7a68d90a5 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); > @@ -4613,11 +4613,28 @@ static int check_stack_write_var_off(struct bpf_verifier_env *env, > > /* Variable offset writes destroy any spilled pointers in range. */ > for (i = min_off; i < max_off; i++) { > + struct bpf_reg_state *spill_reg; > u8 new_type, *stype; > - int slot, spi; > + int slot, spi, j; > > slot = -i - 1; > spi = slot / BPF_REG_SIZE; > + > + /* If writing_zero and the the spi slot contains a spill of value 0, > + * maintain the spill type. > + */ > + if (writing_zero && !(i % BPF_REG_SIZE) && is_spilled_scalar_reg(&state->stack[spi])) { > + spill_reg = &state->stack[spi].spilled_ptr; > + if (tnum_is_const(spill_reg->var_off) && spill_reg->var_off.value == 0) { here we assume that *spilled* register is zero and will stay zero, even if it's imprecise. This is wrong, because imprecise SCALAR 0 is actually an unknown scalar when doing state pruning. So we need to either force the spilled register to be precise, or overwrite it with STACK_MISC. > + for (j = BPF_REG_SIZE; j > 0; j--) { > + if (state->stack[spi].slot_type[j - 1] != STACK_SPILL) > + break; > + } > + i += BPF_REG_SIZE - j - 1; > + continue; > + } > + } > + > stype = &state->stack[spi].slot_type[slot % BPF_REG_SIZE]; > mark_stack_slot_scratched(env, spi); > [...]
