bpf: stop setting precise in current state

bpf: stop setting precise in current state

Setting reg->precise to true in current state is not necessary from
correctness standpoint, but it does pessimise the whole precision (or
rather "imprecision", because that's what we want to keep as much as
possible) tracking. Why is somewhat subtle and my best attempt to
explain this is recorded in an extensive comment for __mark_chain_precise()
function. Some more careful thinking and code reading is probably required
still to grok this completely, unfortunately. Whiteboarding and a bunch
of extra handwaiving in person would be even more helpful, but is deemed
impractical in Git commit.

Next patch pushes this imprecision property even further, building on top of
the insights described in this patch.

End results are pretty nice, we get reduction in number of total instructions
and states verified due to a better states reuse, as some of the states are now
more generic and permissive due to less unnecessary precise=true requirements.

SELFTESTS RESULTS
=================

$ ./veristat -C -e file,prog,insns,states ~/subprog-precise-results.csv ~/imprecise-early-results.csv | grep -v '+0'
File                                     Program                 Total insns (A)  Total insns (B)  Total insns (DIFF)  Total states (A)  Total states (B)  Total states (DIFF)
---------------------------------------  ----------------------  ---------------  ---------------  ------------------  ----------------  ----------------  -------------------
bpf_iter_ksym.bpf.linked1.o              dump_ksym                           347              285       -62 (-17.87%)                20                19          -1 (-5.00%)
pyperf600_bpf_loop.bpf.linked1.o         on_event                           3678             3736        +58 (+1.58%)               276               285          +9 (+3.26%)
setget_sockopt.bpf.linked1.o             skops_sockopt                      4038             3947        -91 (-2.25%)               347               343          -4 (-1.15%)
test_l4lb.bpf.linked1.o                  balancer_ingress                   4559             2611     -1948 (-42.73%)               118               105        -13 (-11.02%)
test_l4lb_noinline.bpf.linked1.o         balancer_ingress                   6279             6268        -11 (-0.18%)               237               236          -1 (-0.42%)
test_misc_tcp_hdr_options.bpf.linked1.o  misc_estab                         1307             1303         -4 (-0.31%)               100                99          -1 (-1.00%)
test_sk_lookup.bpf.linked1.o             ctx_narrow_access                   456              447         -9 (-1.97%)                39                38          -1 (-2.56%)
test_sysctl_loop1.bpf.linked1.o          sysctl_tcp_mem                     1389             1384         -5 (-0.36%)                26                25          -1 (-3.85%)
test_tc_dtime.bpf.linked1.o              egress_fwdns_prio101                518              485        -33 (-6.37%)                51                46          -5 (-9.80%)
test_tc_dtime.bpf.linked1.o              egress_host                         519              468        -51 (-9.83%)                50                44         -6 (-12.00%)
test_tc_dtime.bpf.linked1.o              ingress_fwdns_prio101               842             1000      +158 (+18.76%)                73                88        +15 (+20.55%)
xdp_synproxy_kern.bpf.linked1.o          syncookie_tc                     405757           373173     -32584 (-8.03%)             25735             22882      -2853 (-11.09%)
xdp_synproxy_kern.bpf.linked1.o          syncookie_xdp                    479055           371590   -107465 (-22.43%)             29145             22207      -6938 (-23.81%)
---------------------------------------  ----------------------  ---------------  ---------------  ------------------  ----------------  ----------------  -------------------

Slight regression in test_tc_dtime.bpf.linked1.o/ingress_fwdns_prio101
is left for a follow up, there might be some more precision-related bugs
in existing BPF verifier logic.

CILIUM RESULTS
==============

$ ./veristat -C -e file,prog,insns,states ~/subprog-precise-results-cilium.csv ~/imprecise-early-results-cilium.csv | grep -v '+0'
File           Program                         Total insns (A)  Total insns (B)  Total insns (DIFF)  Total states (A)  Total states (B)  Total states (DIFF)
-------------  ------------------------------  ---------------  ---------------  ------------------  ----------------  ----------------  -------------------
bpf_host.o     cil_from_host                               762              556      -206 (-27.03%)                43                37         -6 (-13.95%)
bpf_host.o     tail_handle_nat_fwd_ipv4                  23541            23426       -115 (-0.49%)              1538              1537          -1 (-0.07%)
bpf_host.o     tail_nodeport_nat_egress_ipv4             33592            33566        -26 (-0.08%)              2163              2161          -2 (-0.09%)
bpf_lxc.o      tail_handle_nat_fwd_ipv4                  23541            23426       -115 (-0.49%)              1538              1537          -1 (-0.07%)
bpf_overlay.o  tail_nodeport_nat_egress_ipv4             33581            33543        -38 (-0.11%)              2160              2157          -3 (-0.14%)
bpf_xdp.o      tail_handle_nat_fwd_ipv4                  21659            20920       -739 (-3.41%)              1440              1376         -64 (-4.44%)
bpf_xdp.o      tail_handle_nat_fwd_ipv6                  17084            17039        -45 (-0.26%)               907               905          -2 (-0.22%)
bpf_xdp.o      tail_lb_ipv4                              73442            73430        -12 (-0.02%)              4370              4369          -1 (-0.02%)
bpf_xdp.o      tail_lb_ipv6                             152114           151895       -219 (-0.14%)              6493              6479         -14 (-0.22%)
bpf_xdp.o      tail_nodeport_nat_egress_ipv4             17377            17200       -177 (-1.02%)              1125              1111         -14 (-1.24%)
bpf_xdp.o      tail_nodeport_nat_ingress_ipv6             6405             6397         -8 (-0.12%)               309               308          -1 (-0.32%)
bpf_xdp.o      tail_rev_nodeport_lb4                      7126             6934       -192 (-2.69%)               414               402         -12 (-2.90%)
bpf_xdp.o      tail_rev_nodeport_lb6                     18059            17905       -154 (-0.85%)              1105              1096          -9 (-0.81%)
-------------  ------------------------------  ---------------  ---------------  ------------------  ----------------  ----------------  -------------------

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20221104163649.121784-5-andrii@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>