[PATCH v2 4.19 05/19] bpf: extend is_branch_taken to registers

[Ovidiu Panait <ovidiu.panait@xxxxxxxxxxxxx>]

From: Alexei Starovoitov <ast@xxxxxxxxxx>

commit fb8d251ee2a6bf4d7f4af5548e9c8f4fb5f90402 upstream

This patch extends is_branch_taken() logic from JMP+K instructions
to JMP+X instructions.
Conditional branches are often done when src and dst registers
contain known scalars. In such case the verifier can follow
the branch that is going to be taken when program executes.
That speeds up the verification and is essential feature to support
bounded loops.

Signed-off-by: Alexei Starovoitov <ast@xxxxxxxxxx>
Acked-by: Andrii Nakryiko <andriin@xxxxxx>
Signed-off-by: Daniel Borkmann <daniel@xxxxxxxxxxxxx>
[OP: drop is_jmp32 parameter from is_branch_taken() calls and
     adjust context]
Signed-off-by: Ovidiu Panait <ovidiu.panait@xxxxxxxxxxxxx>
---
 kernel/bpf/verifier.c | 32 ++++++++++++++++++--------------
 1 file changed, 18 insertions(+), 14 deletions(-)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index f49f84b71a6b..275c1078d80b 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -4127,8 +4127,9 @@ static int check_cond_jmp_op(struct bpf_verifier_env *env,
 	struct bpf_verifier_state *this_branch = env->cur_state;
 	struct bpf_verifier_state *other_branch;
 	struct bpf_reg_state *regs = this_branch->frame[this_branch->curframe]->regs;
-	struct bpf_reg_state *dst_reg, *other_branch_regs;
+	struct bpf_reg_state *dst_reg, *other_branch_regs, *src_reg = NULL;
 	u8 opcode = BPF_OP(insn->code);
+	int pred = -1;
 	int err;
 
 	if (opcode > BPF_JSLE) {
@@ -4152,6 +4153,7 @@ static int check_cond_jmp_op(struct bpf_verifier_env *env,
 				insn->src_reg);
 			return -EACCES;
 		}
+		src_reg = &regs[insn->src_reg];
 	} else {
 		if (insn->src_reg != BPF_REG_0) {
 			verbose(env, "BPF_JMP uses reserved fields\n");
@@ -4166,19 +4168,21 @@ static int check_cond_jmp_op(struct bpf_verifier_env *env,
 
 	dst_reg = &regs[insn->dst_reg];
 
-	if (BPF_SRC(insn->code) == BPF_K) {
-		int pred = is_branch_taken(dst_reg, insn->imm, opcode);
-
-		if (pred == 1) {
-			 /* only follow the goto, ignore fall-through */
-			*insn_idx += insn->off;
-			return 0;
-		} else if (pred == 0) {
-			/* only follow fall-through branch, since
-			 * that's where the program will go
-			 */
-			return 0;
-		}
+	if (BPF_SRC(insn->code) == BPF_K)
+		pred = is_branch_taken(dst_reg, insn->imm, opcode);
+	else if (src_reg->type == SCALAR_VALUE &&
+		 tnum_is_const(src_reg->var_off))
+		pred = is_branch_taken(dst_reg, src_reg->var_off.value,
+				       opcode);
+	if (pred == 1) {
+		/* only follow the goto, ignore fall-through */
+		*insn_idx += insn->off;
+		return 0;
+	} else if (pred == 0) {
+		/* only follow fall-through branch, since
+		 * that's where the program will go
+		 */
+		return 0;
 	}
 
 	other_branch = push_stack(env, *insn_idx + insn->off + 1, *insn_idx,
-- 
2.17.1