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

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On 8/23/22 4:15 PM, John Fastabend wrote:
Eduard Zingerman wrote:
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).

I think the idea makes sense. Perhaps Yonhong can comment seeing he was active
on the LLVM thread. I just scanned the LLVM side for now will take a look
in more detail in a bit.

The issue is discovered when making some changes in llvm compiler.
I think it is good to add support in verifier so in the future
if compiler generates such code patterns, user won't get
surprised verification failure.



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;
+}
+

Instead of having another helper is_pointer_value() could work here?
Checking if we need NOT_INIT in that helper now.

  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






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