bpf: generalize is_branch_taken to handle all conditional jumps in one place

Make is_branch_taken() a single entry point for branch pruning decision
making, handling both pointer vs pointer, pointer vs scalar, and scalar
vs scalar cases in one place. This also nicely cleans up check_cond_jmp_op().
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20231102033759.2541186-15-andrii@kernel.orgSigned-off-by: Alexei Starovoitov <ast@kernel.org>

kernel/bpf/verifier.c

@@ -14188,6 +14188,19 @@ static void find_good_pkt_pointers(struct bpf_verifier_state *vstate,
 	}));
 }
 
+/* check if register is a constant scalar value */
+static bool is_reg_const(struct bpf_reg_state *reg, bool subreg32)
+{
+	return reg->type == SCALAR_VALUE &&
+	       tnum_is_const(subreg32 ? tnum_subreg(reg->var_off) : reg->var_off);
+}
+
+/* assuming is_reg_const() is true, return constant value of a register */
+static u64 reg_const_value(struct bpf_reg_state *reg, bool subreg32)
+{
+	return subreg32 ? tnum_subreg(reg->var_off).value : reg->var_off.value;
+}
+
 /*
  * <reg1> <op> <reg2>, currently assuming reg2 is a constant
  */
@@ -14429,12 +14442,20 @@ static int is_pkt_ptr_branch_taken(struct bpf_reg_state *dst_reg,
 static int is_branch_taken(struct bpf_reg_state *reg1, struct bpf_reg_state *reg2,
 			   u8 opcode, bool is_jmp32)
 {
-	struct tnum reg2_tnum = is_jmp32 ? tnum_subreg(reg2->var_off) : reg2->var_off;
 	u64 val;
 
-	if (!tnum_is_const(reg2_tnum))
+	if (reg_is_pkt_pointer_any(reg1) && reg_is_pkt_pointer_any(reg2) && !is_jmp32)
+		return is_pkt_ptr_branch_taken(reg1, reg2, opcode);
+
+	/* try to make sure reg2 is a constant SCALAR_VALUE */
+	if (!is_reg_const(reg2, is_jmp32)) {
+		opcode = flip_opcode(opcode);
+		swap(reg1, reg2);
+	}
+	/* for now we expect reg2 to be a constant to make any useful decisions */
+	if (!is_reg_const(reg2, is_jmp32))
 		return -1;
-	val = reg2_tnum.value;
+	val = reg_const_value(reg2, is_jmp32);
 
 	if (__is_pointer_value(false, reg1)) {
 		if (!reg_not_null(reg1))
@@ -14915,27 +14936,7 @@ static int check_cond_jmp_op(struct bpf_verifier_env *env,
 	}
 
 	is_jmp32 = BPF_CLASS(insn->code) == BPF_JMP32;
-
-	if (BPF_SRC(insn->code) == BPF_K) {
-		pred = is_branch_taken(dst_reg, src_reg, opcode, is_jmp32);
-	} else if (src_reg->type == SCALAR_VALUE &&
-		   is_jmp32 && tnum_is_const(tnum_subreg(src_reg->var_off))) {
-		pred = is_branch_taken(dst_reg, src_reg, opcode, is_jmp32);
-	} else if (src_reg->type == SCALAR_VALUE &&
-		   !is_jmp32 && tnum_is_const(src_reg->var_off)) {
-		pred = is_branch_taken(dst_reg, src_reg, opcode, is_jmp32);
-	} else if (dst_reg->type == SCALAR_VALUE &&
-		   is_jmp32 && tnum_is_const(tnum_subreg(dst_reg->var_off))) {
-		pred = is_branch_taken(src_reg, dst_reg, flip_opcode(opcode), is_jmp32);
-	} else if (dst_reg->type == SCALAR_VALUE &&
-		   !is_jmp32 && tnum_is_const(dst_reg->var_off)) {
-		pred = is_branch_taken(src_reg, dst_reg, flip_opcode(opcode), is_jmp32);
-	} else if (reg_is_pkt_pointer_any(dst_reg) &&
-		   reg_is_pkt_pointer_any(src_reg) &&
-		   !is_jmp32) {
-		pred = is_pkt_ptr_branch_taken(dst_reg, src_reg, opcode);
-	}
-
+	pred = is_branch_taken(dst_reg, src_reg, opcode, is_jmp32);
 	if (pred >= 0) {
 		/* If we get here with a dst_reg pointer type it is because
 		 * above is_branch_taken() special cased the 0 comparison.