From: Denis Ovsienko <denis@ovsienko.info>
Date: Fri, 28 Mar 2025 23:27:33 +0000 (+0000)
Subject: Mind all-zeroes IPv4 netmask in gen_host().
X-Git-Url: https://git.tcpdump.org/libpcap/commitdiff_plain/8620df13397e5f6f1dc23202f28c365d3071ca59

Mind all-zeroes IPv4 netmask in gen_host().

When the IPv4 netmask is /0, gen_mcmp() returns an equivalent of either
true or false.  In the context of gen_host() it is clear when it is
going to be true, so take a shortcut and return a block with fewer
statements.

Now for "ip dst net 0.0.0.0/0" the unoptimized and the optimized filter
programs are the same (without the always-true comparison).
---

diff --git a/gencode.c b/gencode.c
index d36abbeb..74780cdc 100644
--- a/gencode.c
+++ b/gencode.c
@@ -5136,18 +5136,33 @@ gen_host(compiler_state_t *cstate, bpf_u_int32 addr, bpf_u_int32 mask,
 
 	case Q_IP:
 		b0 = gen_linktype(cstate, ETHERTYPE_IP);
+		/*
+		 * Belt and braces: if other code works correctly, any host
+		 * bits are clear and mask == 0 means addr == 0.  In this case
+		 * the call to gen_hostop() would produce an "always true"
+		 * instruction block and ANDing it with the link type check
+		 * would be a no-op.
+		 */
+		if (mask == 0 && addr == 0)
+			return b0;
 		b1 = gen_hostop(cstate, addr, mask, dir, 12, 16);
 		gen_and(b0, b1);
 		return b1;
 
 	case Q_RARP:
 		b0 = gen_linktype(cstate, ETHERTYPE_REVARP);
+		// Same as for Q_IP above.
+		if (mask == 0 && addr == 0)
+			return b0;
 		b1 = gen_hostop(cstate, addr, mask, dir, 14, 24);
 		gen_and(b0, b1);
 		return b1;
 
 	case Q_ARP:
 		b0 = gen_linktype(cstate, ETHERTYPE_ARP);
+		// Same as for Q_IP above.
+		if (mask == 0 && addr == 0)
+			return b0;
 		b1 = gen_hostop(cstate, addr, mask, dir, 14, 24);
 		gen_and(b0, b1);
 		return b1;
diff --git a/testprogs/TESTrun b/testprogs/TESTrun
index d8836786..12704fc1 100755
--- a/testprogs/TESTrun
+++ b/testprogs/TESTrun
@@ -9181,22 +9181,13 @@ my @accept_blocks = (
 		DLT => 'RAW',
 		snaplen => 2000,
 		aliases => ['ip dst net 0.0.0.0/0'],
-		opt => '
+		unopt => '
 			(000) ldb      [0]
 			(001) and      #0xf0
 			(002) jeq      #0x40            jt 3	jf 4
 			(003) ret      #2000
 			(004) ret      #0
 			',
-		unopt => '
-			(000) ldb      [0]
-			(001) and      #0xf0
-			(002) jeq      #0x40            jt 3	jf 6
-			(003) ld       #0x0
-			(004) jeq      #0x0             jt 5	jf 6
-			(005) ret      #2000
-			(006) ret      #0
-			',
 	}, # ip_dst_net_addr_0
 	{
 		name => 'ip_dst_net_addr_8',