bpf: fix liveness marking

4ast · davem330 · commit 8fe2d6ccd52b · 2017-10-07T23:25:17.000+01:00
while processing Rx = Ry instruction the verifier does regs[insn->dst_reg] = regs[insn->src_reg] which often clears write mark (when Ry doesn't have it) that was just set by check_reg_arg(Rx) prior to the assignment. That causes mark_reg_read() to keep marking Rx in this block as REG_LIVE_READ (since the logic incorrectly misses that it's screened by the write) and in many of its parents (until lucky write into the same Rx or beginning of the program). That causes is_state_visited() logic to miss many pruning opportunities. Furthermore mark_reg_read() logic propagates the read mark for BPF_REG_FP as well (though it's readonly) which causes harmless but unnecssary work during is_state_visited(). Note that do_propagate_liveness() skips FP correctly, so do the same in mark_reg_read() as well. It saves 0.2 seconds for the test below program before after bpf_lb-DLB_L3.o 2604 2304 bpf_lb-DLB_L4.o 11159 3723 bpf_lb-DUNKNOWN.o 1116 1110 bpf_lxc-DDROP_ALL.o 34566 28004 bpf_lxc-DUNKNOWN.o 53267 39026 bpf_netdev.o 17843 16943 bpf_overlay.o 8672 7929 time ~11 sec ~4 sec Fixes: dc503a8 ("bpf/verifier: track liveness for pruning") Signed-off-by: Alexei Starovoitov <ast@kernel.org> Acked-by: Edward Cree <ecree@solarflare.com> Acked-by: Daniel Borkmann <daniel@iogearbox.net> Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
@@ -653,6 +653,10 @@ static void mark_reg_read(const struct bpf_verifier_state *state, u32 regno)
 {
 	struct bpf_verifier_state *parent = state->parent;
 
+	if (regno == BPF_REG_FP)
+		/* We don't need to worry about FP liveness because it's read-only */
+		return;
+
 	while (parent) {
 		/* if read wasn't screened by an earlier write ... */
 		if (state->regs[regno].live & REG_LIVE_WRITTEN)
@@ -2345,6 +2349,7 @@ static int check_alu_op(struct bpf_verifier_env *env, struct bpf_insn *insn)
 				 * copy register state to dest reg
 				 */
 				regs[insn->dst_reg] = regs[insn->src_reg];
+				regs[insn->dst_reg].live |= REG_LIVE_WRITTEN;
 			} else {
 				/* R1 = (u32) R2 */
 				if (is_pointer_value(env, insn->src_reg)) {
