We can derive some new information for BPF_JNE in regs_refine_cond_op().
Take following code for example:

  /* The type of "a" is u16 */
  if (a > 0 && a < 100) {
    /* the range of the register for a is [0, 99], not [1, 99],
     * and will cause the following error:
     *
     *   invalid zero-sized read
     *
     * as a can be 0.
     */
    bpf_skb_store_bytes(skb, xx, xx, a, 0);
  }

In the code above, "a > 0" will be compiled to "jmp xxx if a == 0". In the
TRUE branch, the dst_reg will be marked as known to 0. However, in the
fallthrough(FALSE) branch, the dst_reg will not be handled, which makes
the [min, max] for a is [0, 99], not [1, 99].

For BPF_JNE, we can reduce the range of the dst reg if the src reg is a
const and is exactly the edge of the dst reg.

Signed-off-by: Menglong Dong <menglong8.dong@gmail.com>
---
 kernel/bpf/verifier.c | 29 ++++++++++++++++++++++++++++-
 1 file changed, 28 insertions(+), 1 deletion(-)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 727a59e4a647..08ee0e02df96 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -14332,7 +14332,34 @@ static void regs_refine_cond_op(struct bpf_reg_state *reg1, struct bpf_reg_state
 		}
 		break;
 	case BPF_JNE:
-		/* we don't derive any new information for inequality yet */
+		if (!is_reg_const(reg2, is_jmp32))
+			swap(reg1, reg2);
+		if (!is_reg_const(reg2, is_jmp32))
+			break;
+
+		/* try to recompute the bound of reg1 if reg2 is a const and
+		 * is exactly the edge of reg1.
+		 */
+		val = reg_const_value(reg2, is_jmp32);
+		if (is_jmp32) {
+			if (reg1->u32_min_value == (u32)val)
+				reg1->u32_min_value++;
+			if (reg1->u32_max_value == (u32)val)
+				reg1->u32_max_value--;
+			if (reg1->s32_min_value == (s32)val)
+				reg1->s32_min_value++;
+			if (reg1->s32_max_value == (s32)val)
+				reg1->s32_max_value--;
+		} else {
+			if (reg1->umin_value == (u64)val)
+				reg1->umin_value++;
+			if (reg1->umax_value == (u64)val)
+				reg1->umax_value--;
+			if (reg1->smin_value == (s64)val)
+				reg1->smin_value++;
+			if (reg1->smax_value == (s64)val)
+				reg1->smax_value--;
+		}
 		break;
 	case BPF_JSET:
 		if (!is_reg_const(reg2, is_jmp32))
-- 
2.39.2

On Tue, 2023-12-12 at 21:10 +0800, Menglong Dong wrote:
> We can derive some new information for BPF_JNE in regs_refine_cond_op().
> Take following code for example:
> 
>   /* The type of "a" is u16 */
>   if (a > 0 && a < 100) {
>     /* the range of the register for a is [0, 99], not [1, 99],
>      * and will cause the following error:
>      *
>      *   invalid zero-sized read
>      *
>      * as a can be 0.
>      */
>     bpf_skb_store_bytes(skb, xx, xx, a, 0);
>   }
> 
> In the code above, "a > 0" will be compiled to "jmp xxx if a == 0". In the
> TRUE branch, the dst_reg will be marked as known to 0. However, in the
> fallthrough(FALSE) branch, the dst_reg will not be handled, which makes
> the [min, max] for a is [0, 99], not [1, 99].
> 
> For BPF_JNE, we can reduce the range of the dst reg if the src reg is a
> const and is exactly the edge of the dst reg.
> 
> Signed-off-by: Menglong Dong <menglong8.dong@gmail.com>
> ---

Acked-by: Eduard Zingerman <eddyz87@gmail.com>

>  kernel/bpf/verifier.c | 29 ++++++++++++++++++++++++++++-
>  1 file changed, 28 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 727a59e4a647..08ee0e02df96 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -14332,7 +14332,34 @@ static void regs_refine_cond_op(struct bpf_reg_state *reg1, struct bpf_reg_state
>  		}
>  		break;
>  	case BPF_JNE:
> -		/* we don't derive any new information for inequality yet */
> +		if (!is_reg_const(reg2, is_jmp32))
> +			swap(reg1, reg2);
> +		if (!is_reg_const(reg2, is_jmp32))
> +			break;
> +
> +		/* try to recompute the bound of reg1 if reg2 is a const and
> +		 * is exactly the edge of reg1.
> +		 */
> +		val = reg_const_value(reg2, is_jmp32);
> +		if (is_jmp32) {
> +			if (reg1->u32_min_value == (u32)val)
> +				reg1->u32_min_value++;

Nit: I spent an unreasonable amount of time trying to figure out if
     overflow might be an issue here. Would it be helpful to add a
     comment like below? (not sure, maybe it's obvious and I'm being slow)
     
     /* u32_min_value is not equal to 0xffffffff at this point,
      * because otherwise u32_max_value is 0xffffffff as well,
      * in such a case both reg1 and reg2 would be constants,
      * jump would be predicted and reg_set_min_max() won't
      * be called.
      * Same reasoning works for all {u,s}{min,max}{32,64} cases below.
      */

On Wed, Dec 13, 2023 at 7:23 AM Eduard Zingerman <eddyz87@gmail.com> wrote:
>
> On Tue, 2023-12-12 at 21:10 +0800, Menglong Dong wrote:
> > We can derive some new information for BPF_JNE in regs_refine_cond_op().
> > Take following code for example:
> >
> >   /* The type of "a" is u16 */
> >   if (a > 0 && a < 100) {
> >     /* the range of the register for a is [0, 99], not [1, 99],
> >      * and will cause the following error:
> >      *
> >      *   invalid zero-sized read
> >      *
> >      * as a can be 0.
> >      */
> >     bpf_skb_store_bytes(skb, xx, xx, a, 0);
> >   }
> >
> > In the code above, "a > 0" will be compiled to "jmp xxx if a == 0". In the
> > TRUE branch, the dst_reg will be marked as known to 0. However, in the
> > fallthrough(FALSE) branch, the dst_reg will not be handled, which makes
> > the [min, max] for a is [0, 99], not [1, 99].
> >
> > For BPF_JNE, we can reduce the range of the dst reg if the src reg is a
> > const and is exactly the edge of the dst reg.
> >
> > Signed-off-by: Menglong Dong <menglong8.dong@gmail.com>
> > ---
>
> Acked-by: Eduard Zingerman <eddyz87@gmail.com>
>
> >  kernel/bpf/verifier.c | 29 ++++++++++++++++++++++++++++-
> >  1 file changed, 28 insertions(+), 1 deletion(-)
> >
> > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> > index 727a59e4a647..08ee0e02df96 100644
> > --- a/kernel/bpf/verifier.c
> > +++ b/kernel/bpf/verifier.c
> > @@ -14332,7 +14332,34 @@ static void regs_refine_cond_op(struct bpf_reg_state *reg1, struct bpf_reg_state
> >               }
> >               break;
> >       case BPF_JNE:
> > -             /* we don't derive any new information for inequality yet */
> > +             if (!is_reg_const(reg2, is_jmp32))
> > +                     swap(reg1, reg2);
> > +             if (!is_reg_const(reg2, is_jmp32))
> > +                     break;
> > +
> > +             /* try to recompute the bound of reg1 if reg2 is a const and
> > +              * is exactly the edge of reg1.
> > +              */
> > +             val = reg_const_value(reg2, is_jmp32);
> > +             if (is_jmp32) {
> > +                     if (reg1->u32_min_value == (u32)val)
> > +                             reg1->u32_min_value++;
>
> Nit: I spent an unreasonable amount of time trying to figure out if
>      overflow might be an issue here. Would it be helpful to add a
>      comment like below? (not sure, maybe it's obvious and I'm being slow)
>
>      /* u32_min_value is not equal to 0xffffffff at this point,
>       * because otherwise u32_max_value is 0xffffffff as well,
>       * in such a case both reg1 and reg2 would be constants,
>       * jump would be predicted and reg_set_min_max() won't
>       * be called.
>       * Same reasoning works for all {u,s}{min,max}{32,64} cases below.
>       */

Okay, I'll add this comment in the next version.

Thanks!
Menglong Dong