bpf: improve verifier liveness marks

registers with pointers filled from stack were missing live_written marks
which caused liveness propagation to unnecessary mark more registers as
live_read and miss state pruning opportunities later on.

                     before  after
bpf_lb-DLB_L3.o       2285   2270
bpf_lb-DLB_L4.o       3723   3682
bpf_lb-DUNKNOWN.o     1110   1110
bpf_lxc-DDROP_ALL.o   27954  27876
bpf_lxc-DUNKNOWN.o    38954  38780
bpf_netdev.o          16943  16937
bpf_overlay.o         7929   7929

Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

kernel/bpf/verifier.c

@@ -795,6 +795,11 @@ static int check_stack_read(struct bpf_verifier_env *env,
 		if (value_regno >= 0) {
 			/* restore register state from stack */
 			state->regs[value_regno] = state->stack[spi].spilled_ptr;
+			/* mark reg as written since spilled pointer state likely
+			 * has its liveness marks cleared by is_state_visited()
+			 * which resets stack/reg liveness for state transitions
+			 */
+			state->regs[value_regno].live |= REG_LIVE_WRITTEN;
 			mark_stack_slot_read(state, spi);
 		}
 		return 0;