bpf: Support 32-bit scalar spills in stacksafe()

bpf: Support 32-bit scalar spills in stacksafe()

v1->v2: updated comments
v1: https://lore.kernel.org/bpf/20260322225124.14005-1-alexei.starovoitov@gmail.com/

The commit 6efbde200bf3 ("bpf: Handle scalar spill vs all MISC in stacksafe()")
in stacksafe() only recognized full 64-bit scalar spills when
comparing stack states for equivalence during state pruning and
missed 32-bit scalar spill. When 32-bit scalar is spilled the
check_stack_write_fixed_off() -> save_register_state() calls
mark_stack_slot_misc() for slot[0-3], which preserves STACK_INVALID
and STACK_ZERO (on a fresh stack slot[0-3] remain STACK_INVALID),
sets slot[4-7] = STACK_SPILL, and updates spilled_ptr.

The im=4 path is only reached when im=0 fails: The loop at im=0 already
attempts the 64-bit scalar-spill/all-MISC check. If it matches, i advances
by 7, skipping the entire 8-byte slot. So im=4 is only reached when bytes
0-3 are neither a scalar spill nor all-MISC — they must pass individual
byte-by-byte comparison first. Then bytes 4-7 get the scalar-unit
treatment.

is_spilled_scalar_after(stack, 4): slot_type[4] == STACK_SPILL from a
64-bit spill would have been caught at im=0 (unless it's a pointer spill,
in which case spilled_ptr.type != SCALAR_VALUE -> returns false at im=4
too). A partial overwrite of a 64-bit spill invalidates the entire slot in
check_stack_write_fixed_off().

is_stack_misc_after(stack, 4): Only checks bytes 4-7 are MISC/INVALID,
returns &unbound_reg. Comparing two unbound regs via regsafe() is safe.

Changes to cilium programs:
File             Program                            Insns (A)  Insns (B)  Insns     (DIFF)
_______________  _________________________________  _________  _________  ________________
bpf_host.o       cil_host_policy                        49351      45811    -3540 (-7.17%)
bpf_host.o       cil_to_host                             2384       2270     -114 (-4.78%)
bpf_host.o       cil_to_netdev                         112051     100269  -11782 (-10.51%)
bpf_host.o       tail_handle_ipv4_cont_from_host        61175      60910     -265 (-0.43%)
bpf_host.o       tail_handle_ipv4_cont_from_netdev       9381       8873     -508 (-5.42%)
bpf_host.o       tail_handle_ipv4_from_host             12994       7066   -5928 (-45.62%)
bpf_host.o       tail_handle_ipv4_from_netdev           85015      59875  -25140 (-29.57%)
bpf_host.o       tail_handle_ipv6_cont_from_host        24732      23527    -1205 (-4.87%)
bpf_host.o       tail_handle_ipv6_cont_from_netdev       9463       8953     -510 (-5.39%)
bpf_host.o       tail_handle_ipv6_from_host             12477      11787     -690 (-5.53%)
bpf_host.o       tail_handle_ipv6_from_netdev           30814      30017     -797 (-2.59%)
bpf_host.o       tail_handle_nat_fwd_ipv4                8943       8860      -83 (-0.93%)
bpf_host.o       tail_handle_snat_fwd_ipv4              64716      61625    -3091 (-4.78%)
bpf_host.o       tail_handle_snat_fwd_ipv6              48299      30797  -17502 (-36.24%)
bpf_host.o       tail_ipv4_host_policy_ingress          21591      20017    -1574 (-7.29%)
bpf_host.o       tail_ipv6_host_policy_ingress          21177      20693     -484 (-2.29%)
bpf_host.o       tail_nodeport_nat_egress_ipv4          16588      16543      -45 (-0.27%)
bpf_host.o       tail_nodeport_nat_ingress_ipv4         39200      36116    -3084 (-7.87%)
bpf_host.o       tail_nodeport_nat_ingress_ipv6         50102      48003    -2099 (-4.19%)
bpf_lxc.o        tail_handle_ipv4_cont                 113092      96891  -16201 (-14.33%)
bpf_lxc.o        tail_handle_ipv6                        6727       6701      -26 (-0.39%)
bpf_lxc.o        tail_handle_ipv6_cont                  25567      21805   -3762 (-14.71%)
bpf_lxc.o        tail_ipv4_ct_egress                    28843      15970  -12873 (-44.63%)
bpf_lxc.o        tail_ipv4_ct_ingress                   16691      10213   -6478 (-38.81%)
bpf_lxc.o        tail_ipv4_ct_ingress_policy_only       16691      10213   -6478 (-38.81%)
bpf_lxc.o        tail_ipv4_policy                        6776       6622     -154 (-2.27%)
bpf_lxc.o        tail_ipv4_to_endpoint                   7523       7219     -304 (-4.04%)
bpf_lxc.o        tail_ipv6_ct_egress                    10275       9999     -276 (-2.69%)
bpf_lxc.o        tail_ipv6_ct_ingress                    6466       6438      -28 (-0.43%)
bpf_lxc.o        tail_ipv6_ct_ingress_policy_only        6466       6438      -28 (-0.43%)
bpf_lxc.o        tail_ipv6_policy                        6859       5159   -1700 (-24.78%)
bpf_lxc.o        tail_ipv6_to_endpoint                   7039       4427   -2612 (-37.11%)
bpf_lxc.o        tail_nodeport_ipv6_dsr                  1175       1033    -142 (-12.09%)
bpf_lxc.o        tail_nodeport_nat_egress_ipv4          16318      16292      -26 (-0.16%)
bpf_lxc.o        tail_nodeport_nat_ingress_ipv4         18907      18490     -417 (-2.21%)
bpf_lxc.o        tail_nodeport_nat_ingress_ipv6         14624      14556      -68 (-0.46%)
bpf_lxc.o        tail_nodeport_rev_dnat_ipv4             4776       4588     -188 (-3.94%)
bpf_overlay.o    tail_handle_inter_cluster_revsnat      15733      15498     -235 (-1.49%)
bpf_overlay.o    tail_handle_ipv4                      124682     105717  -18965 (-15.21%)
bpf_overlay.o    tail_handle_ipv6                       16201      15801     -400 (-2.47%)
bpf_overlay.o    tail_handle_snat_fwd_ipv4              21280      19323    -1957 (-9.20%)
bpf_overlay.o    tail_handle_snat_fwd_ipv6              20824      20822       -2 (-0.01%)
bpf_overlay.o    tail_nodeport_ipv6_dsr                  1175       1033    -142 (-12.09%)
bpf_overlay.o    tail_nodeport_nat_egress_ipv4          16293      16267      -26 (-0.16%)
bpf_overlay.o    tail_nodeport_nat_ingress_ipv4         20841      20737     -104 (-0.50%)
bpf_overlay.o    tail_nodeport_nat_ingress_ipv6         14678      14629      -49 (-0.33%)
bpf_sock.o       cil_sock4_connect                       1678       1623      -55 (-3.28%)
bpf_sock.o       cil_sock4_sendmsg                       1791       1736      -55 (-3.07%)
bpf_sock.o       cil_sock6_connect                       3641       3600      -41 (-1.13%)
bpf_sock.o       cil_sock6_recvmsg                       2048       1899     -149 (-7.28%)
bpf_sock.o       cil_sock6_sendmsg                       3755       3721      -34 (-0.91%)
bpf_wireguard.o  tail_handle_ipv4                       31180      27484   -3696 (-11.85%)
bpf_wireguard.o  tail_handle_ipv6                       12095      11760     -335 (-2.77%)
bpf_wireguard.o  tail_nodeport_ipv6_dsr                  1232       1094    -138 (-11.20%)
bpf_wireguard.o  tail_nodeport_nat_egress_ipv4          16071      16061      -10 (-0.06%)
bpf_wireguard.o  tail_nodeport_nat_ingress_ipv4         20804      20565     -239 (-1.15%)
bpf_wireguard.o  tail_nodeport_nat_ingress_ipv6         13490      12224    -1266 (-9.38%)
bpf_xdp.o        tail_lb_ipv4                           49695      42673   -7022 (-14.13%)
bpf_xdp.o        tail_lb_ipv6                          122683      87896  -34787 (-28.36%)
bpf_xdp.o        tail_nodeport_ipv6_dsr                  1833       1862      +29 (+1.58%)
bpf_xdp.o        tail_nodeport_nat_egress_ipv4           6999       6990       -9 (-0.13%)
bpf_xdp.o        tail_nodeport_nat_ingress_ipv4         28903      28780     -123 (-0.43%)
bpf_xdp.o        tail_nodeport_nat_ingress_ipv6        200361     197771    -2590 (-1.29%)
bpf_xdp.o        tail_nodeport_rev_dnat_ipv4             4606       4454     -152 (-3.30%)

Changes to sched-ext:
File                       Program           Insns (A)  Insns (B)  Insns    (DIFF)
_________________________  ________________  _________  _________  _______________
scx_arena_selftests.bpf.o  arena_selftest       236305     236251     -54 (-0.02%)
scx_chaos.bpf.o            chaos_dispatch        12282       8013  -4269 (-34.76%)
scx_chaos.bpf.o            chaos_enqueue         11398       7126  -4272 (-37.48%)
scx_chaos.bpf.o            chaos_init             3854       3828     -26 (-0.67%)
scx_flash.bpf.o            flash_init             1015        979     -36 (-3.55%)
scx_flatcg.bpf.o           fcg_dispatch           1143       1100     -43 (-3.76%)
scx_lavd.bpf.o             lavd_enqueue          35487      35472     -15 (-0.04%)
scx_lavd.bpf.o             lavd_init             21127      21107     -20 (-0.09%)
scx_p2dq.bpf.o             p2dq_enqueue          10210       7854  -2356 (-23.08%)
scx_p2dq.bpf.o             p2dq_init              3233       3207     -26 (-0.80%)
scx_qmap.bpf.o             qmap_init             20285      20230     -55 (-0.27%)
scx_rusty.bpf.o            rusty_select_cpu       1165       1148     -17 (-1.46%)
scxtop.bpf.o               on_sched_switch        2369       2355     -14 (-0.59%)

Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20260323022410.75444-1-alexei.starovoitov@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>