[PATCH RFC bpf-next 1/2] bpf: propagate nullness information for reg to reg comparisons

[Eduard Zingerman <eddyz87@xxxxxxxxx>]

Propagate nullness information for branches of register to register
equality compare instructions. The following rules are used:
- suppose register A maybe null
- suppose register B is not null
- for JNE A, B, ... - A is not null in the false branch
- for JEQ A, B, ... - A is not null in the true branch

E.g. for program like below:

  r6 = skb->sk;
  r7 = sk_fullsock(r6);
  r0 = sk_fullsock(r6);
  if (r0 == 0) return 0;    (a)
  if (r0 != r7) return 0;   (b)
  *r7->type;                (c)
  return 0;

It is safe to dereference r7 at point (c), because of (a) and (b).

Signed-off-by: Eduard Zingerman <eddyz87@xxxxxxxxx>
---
 kernel/bpf/verifier.c | 39 ++++++++++++++++++++++++++++++++++++++-
 1 file changed, 38 insertions(+), 1 deletion(-)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 2c1f8069f7b7..c48d34625bfd 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -472,6 +472,11 @@ static bool type_may_be_null(u32 type)
 	return type & PTR_MAYBE_NULL;
 }
 
+static bool type_is_pointer(enum bpf_reg_type type)
+{
+	return type != NOT_INIT && type != SCALAR_VALUE;
+}
+
 static bool is_acquire_function(enum bpf_func_id func_id,
 				const struct bpf_map *map)
 {
@@ -10046,6 +10051,7 @@ static int check_cond_jmp_op(struct bpf_verifier_env *env,
 	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, *src_reg = NULL;
+	struct bpf_reg_state *eq_branch_regs;
 	u8 opcode = BPF_OP(insn->code);
 	bool is_jmp32;
 	int pred = -1;
@@ -10155,7 +10161,7 @@ static int check_cond_jmp_op(struct bpf_verifier_env *env,
 	/* detect if we are comparing against a constant value so we can adjust
 	 * our min/max values for our dst register.
 	 * this is only legit if both are scalars (or pointers to the same
-	 * object, I suppose, but we don't support that right now), because
+	 * object, I suppose, see the next if block), because
 	 * otherwise the different base pointers mean the offsets aren't
 	 * comparable.
 	 */
@@ -10199,6 +10205,37 @@ static int check_cond_jmp_op(struct bpf_verifier_env *env,
 					opcode, is_jmp32);
 	}
 
+	/* if one pointer register is compared to another pointer
+	 * register check if PTR_MAYBE_NULL could be lifted.
+	 * E.g. register A - maybe null
+	 *      register B - not null
+	 * for JNE A, B, ... - A is not null in the false branch;
+	 * for JEQ A, B, ... - A is not null in the true branch.
+	 */
+	if (!is_jmp32 &&
+	    BPF_SRC(insn->code) == BPF_X &&
+	    type_is_pointer(src_reg->type) && type_is_pointer(dst_reg->type) &&
+	    type_may_be_null(src_reg->type) != type_may_be_null(dst_reg->type)) {
+		eq_branch_regs = NULL;
+		switch (opcode) {
+		case BPF_JEQ:
+			eq_branch_regs = other_branch_regs;
+			break;
+		case BPF_JNE:
+			eq_branch_regs = regs;
+			break;
+		default:
+			/* do nothing */
+			break;
+		}
+		if (eq_branch_regs) {
+			if (type_may_be_null(src_reg->type))
+				mark_ptr_not_null_reg(&eq_branch_regs[insn->src_reg]);
+			else
+				mark_ptr_not_null_reg(&eq_branch_regs[insn->dst_reg]);
+		}
+	}
+
 	if (dst_reg->type == SCALAR_VALUE && dst_reg->id &&
 	    !WARN_ON_ONCE(dst_reg->id != other_branch_regs[insn->dst_reg].id)) {
 		find_equal_scalars(this_branch, dst_reg);
-- 
2.37.1