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.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 414a7c58b4a45345cd6372cf9ff779b4883dd2b6..17bbff33e0e8a06c5865bfcda0ed6a76b93e23a6 100644 (file)
--- a/kernel/bpf/verifier.c
+++ b/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.